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Offline rhb

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Testing RF connectors and cables
« on: August 24, 2019, 10:43:54 pm »
I was testing a bunch of SMA-F to N-F bulkhead adapters from ebay yesterday.  The neat thing about the SD-24 is you can test such things as fast as you can make the connections.  I was putting a very good Inmet N-M load on the N-F side.  The adapters came from wifi_expert.

Bear in mind In the time domain I'm testing to 20 GHz, but in the frequency domain I can only sweep to 2.9
GHz.  Notice that the TDR lets you see the SMA connection and the N connection separately.

TDR using a Tek 11801 w/ SD-24 20 GHz head

   Top is unmarked, but thought to be Suhner
   Middle is Astrolab
   Bottom is Chinese

819090-0

Next is a thru cal on my 8560A from 500 KHz to 500 MHz at 1 dB/div using an Astrolab part

819096-1

Chinese with same settings and using the Astrolab as the cal reference.

819102-2

Next is a thru cal on my 8560A from 500 KHz to 2.9 GHz at 1 dB/div using a Suhner (?) part

819114-3

Chinese with same settings using the Suhner as reference cal

819108-4

I'm building an RF switch deck to automate a full annual cal run of my bench.  I'll be using AR488 and Arduino Mega 2560s to control ST6P Radiall SMA relays for RF  and 44421A relay cards from a 3497A for DC, AC & resistance measurements. 

The Chinese SMA-F to N-F bulkhead adapters won't cut it for the cal system, but I'm also building an antenna and radio switch deck using another pair of the Radial relays.  Except for playing with an SDR and an abandoned Direct TV dish, I won't be doing anything above the 70 cm band.  I might use better grade adapters for those, but for HF/VHF/UHF the Chinese are just fine.

For those for whom an 11801 is out of reach, Leo Bodnar will provide his excellent BNC <40 ps square wave generator with a 1 MHz instead of 10 MHz period.  Naturally that limits you to the BW of your scope, but  it will test any connectors and cables you use with your scope to to full BW.

Just as you can do TDR on modern VNAs, you can do VNA by TDR with a DSO and computer.  I plan on dong some examples and writing software to do that. 

But I'm a severe ADD case, so no idea when that will happen. I've got a 7x12 Chinese mini-lathe awaiting further attention for making high precision RF stuff.  But first I have to rebuild the lathe to work to those tolerances.  That's a *lot* of work.  The machines are quite impressive, but it's really just a complete kit of parts which has been assembled to make sure they didn't forget something.  So for serious work they have to be completely disassembled, cleaned, critical parts replaced and then scraped by hand into alignment.  The hard part is learning to measure things accurately enough.
« Last Edit: August 24, 2019, 11:11:20 pm by rhb »
 
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Offline radiolistener

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Re: Testing RF connectors and cables
« Reply #1 on: August 24, 2019, 11:26:41 pm »
Usually Chinese connectors works good when it's new, but after some time it is oxidized and it leads to weak connection. I'm cleaning it with ethanol, after that it works better.
 

Online bitseeker

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Re: Testing RF connectors and cables
« Reply #2 on: August 24, 2019, 11:31:23 pm »
I've had some cheap BNC tee connectors fall apart. :palm:

As Dave says, "Built down to a price."
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Offline joeqsmith

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Re: Testing RF connectors and cables
« Reply #3 on: August 25, 2019, 12:38:43 am »
I've had some cheap BNC tee connectors fall apart. :palm:

As Dave says, "Built down to a price."

Same.  I have had the lock fall off of the male ones.
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #4 on: August 25, 2019, 01:05:31 am »
BNCs are particularly problematic.  I'm finding that it's easy to induce large reflections just by pressing on the male part.  The serious problem is that it doesn't always go away when you let go.

After my post to the FY600 thread I did some more testing and do *not* like the results.  I'm not sure what is going on yet.  I suspect it is just crappy BNCs, but I've got to find some that don't exhibit the problem to be sure it's not just inherent in the design of the BNC.

FWIW the little "w" at the end of the reflections in the TDR trace is the Inmet load.  On the Suhner and Astrolab adapters, there is a short flat spot separating the SMA and N connections where the adapter is purely resistive, but the Chinese is capacitive.   The level of detail you can see in the plots is amazing.  I'm tempted to cut a Chinese adapter in half lengthwise to compare it to the TDR trace.
 

Offline radiolistener

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Re: Testing RF connectors and cables
« Reply #5 on: August 25, 2019, 01:09:26 am »
I also tried Chinese BNC connectors, but they almost unusable. They have very weak connection. So, now I'm using mainly SMA connectors with SMA-to-BNC adapters.
 

Offline xrunner

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Re: Testing RF connectors and cables
« Reply #6 on: August 25, 2019, 01:24:55 am »
I've heard hams say over the years that putting an adapter in to extend your coax run was a bad idea. They'd say that it would cause 0.5 dB or 1 dB loss in the HF bands. Where they got the number I had no idea. So a few weeks ago I decided to test their claims. I put 7 various adapters (not just one) in series and ran some tests.

As you can see from the charts it was all an old wives tale.  :-DD Even in the 2 meter band it's not a big deal.

Now I have the data to call them out.
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Online 0culus

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Re: Testing RF connectors and cables
« Reply #7 on: August 25, 2019, 02:09:25 am »
At those frequencies I'd be very surprised if it made any noticeable difference in operation. UHF would probably be a different story though.
 

Offline TheSteve

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Re: Testing RF connectors and cables
« Reply #8 on: August 25, 2019, 02:19:49 am »
Think of old school PL-259's with an SO-239 barrel between them, crappy crimps to the coax or terrible soldering, poorly installed and not super tight, been outside in the weather for years without proper sealing. Then it isn't too hard to lose half a dB even at HF frequencies.

rhb - I like the TDR pics, do you have a nice way to translate the images to return loss in dB?
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Online 0culus

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Re: Testing RF connectors and cables
« Reply #9 on: August 25, 2019, 02:53:55 am »
Think of old school PL-259's with an SO-239 barrel between them, crappy crimps to the coax or terrible soldering, poorly installed and not super tight, been outside in the weather for years without proper sealing. Then it isn't too hard to lose half a dB even at HF frequencies.

rhb - I like the TDR pics, do you have a nice way to translate the images to return loss in dB?

Fair enough...I'm spoiled I guess.  :-DD
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #10 on: August 25, 2019, 05:54:28 am »
Give me digits and it's easy.  The hard part is getting digits.

I *think* I know how to do it by inspection.  But I haven't  convinced myself I've got it right yet.

It's a couple of morning sessions of serious math,   Not hard, but I've got to be absolutely running at 100%.  I've just not been up to that level of late because of the vicissitudes of getting older.  Time eventually gets us all.

PL-259s are an abomination.
 

Online bitseeker

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Re: Testing RF connectors and cables
« Reply #11 on: August 26, 2019, 12:03:57 am »
Shhh. Don’t say that stuff too loudly.  :-DD
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Offline David Hess

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Re: Testing RF connectors and cables
« Reply #12 on: August 26, 2019, 04:05:19 am »
BNCs are particularly problematic.  I'm finding that it's easy to induce large reflections just by pressing on the male part.  The serious problem is that it doesn't always go away when you let go.

After my post to the FY600 thread I did some more testing and do *not* like the results.  I'm not sure what is going on yet.  I suspect it is just crappy BNCs, but I've got to find some that don't exhibit the problem to be sure it's not just inherent in the design of the BNC.

BNC quality varies considerably but bayonet style connectors all wear more quickly than threaded connectors.  TNCs are amazingly better than BNCs but unfortunately never really caught on for use in test instruments and would not be suitable for oscilloscope probes anyway.

Some companies did make clamp and thread style BNC male connectors for use with standard female BNC connectors on test instruments but they are cumbersome.  They were used where a more secure mechanical connection or more reliable long term high frequency performance was required.  Some active probes used them.

For smaller connectors which are not SMA, I really like SMB as an easy to use push-on connector but they never caught on either.

Quote
FWIW the little "w" at the end of the reflections in the TDR trace is the Inmet load.  On the Suhner and Astrolab adapters, there is a short flat spot separating the SMA and N connections where the adapter is purely resistive, but the Chinese is capacitive.   The level of detail you can see in the plots is amazing.  I'm tempted to cut a Chinese adapter in half lengthwise to compare it to the TDR trace.

VNAs provide more electrical detail but TDR is easier to relate to physical construction and faster.  You cannot do it with an intact connector but a trick for use with TDR on exposed circuits is to run a pencil along the circuit to relate time to position.
« Last Edit: August 26, 2019, 04:09:09 am by David Hess »
 
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #13 on: August 26, 2019, 03:47:05 pm »

VNAs provide more electrical detail but TDR is easier to relate to physical construction and faster.  You cannot do it with an intact connector but a trick for use with TDR on exposed circuits is to run a pencil along the circuit to relate time to position.

A minor quibble, both have the same information content assuming appropriate acquisition parameters.  Depending upon the task one may be easier to interpret than the other.

In seismic processing one goes between time and frequency so often that most experienced people know what the other domain looks like by inspection.  Knowing the major transform pairs in the pictorial dictionary in Bracewell is very useful.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #14 on: August 26, 2019, 04:55:18 pm »
First some fooling around with the nifty 0.1-3000 MHz RF bridges sold by 60dB.com.  First the setup.  I've got an SD-24 feeding a ~20 ps rise time square wave to the INP terminal.

820374-0

The DUT and REF terminals are connected to the same 20 GHz SD-26 head (an SD-24 w/o the pulser).   The OUT is connected to another SD-26.  The DUT and REF ports match very closely at 118 mV.  The OUT port pulse is around 900 uV.   So if I haven't botched my math, the OUT isolation is -42 dB from the REF port.  Note that I have not time aligned the OUT port head with the DUT & REF port head.  I should have thought to do that.

820380-1

Finally a sweep on my 8560A from 1-2900 MHz at 2 dB/div

820386-2
« Last Edit: August 26, 2019, 05:00:30 pm by rhb »
 

Offline bd139

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Re: Testing RF connectors and cables
« Reply #15 on: August 26, 2019, 05:05:09 pm »
I've heard hams say over the years that putting an adapter in to extend your coax run was a bad idea. They'd say that it would cause 0.5 dB or 1 dB loss in the HF bands. Where they got the number I had no idea. So a few weeks ago I decided to test their claims. I put 7 various adapters (not just one) in series and ran some tests.

As you can see from the charts it was all an old wives tale.  :-DD Even in the 2 meter band it's not a big deal.

Now I have the data to call them out.

The PL259 HF insertion loss myth probably came about from the average ham's inability to put a PL259 together without fucking it up plus the fact that nearly all PL259 connectors, apart from the really high end Amphenol ones are complete garbage, plus they probably used rotten mouldy water soaked coax they got from a hamfest as a "bargain".

Literally yesterday I saw someone eyeing up a roll of coax which had rodent nibblings on it  :palm: ... couple of hundred watts up that and I bet the new rat piss dielectric smelled marvelous.

At HF / VHF I am measuring around 0.3dB loss worst case per 10m on milspec RG58U (foam dielectric + really thick braid) with Amphenol clamp on BNCs. Above VHF, I do not do. Yet.
« Last Edit: August 26, 2019, 05:08:20 pm by bd139 »
 

Offline David Hess

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Re: Testing RF connectors and cables
« Reply #16 on: August 26, 2019, 05:11:53 pm »
VNAs provide more electrical detail but TDR is easier to relate to physical construction and faster.  You cannot do it with an intact connector but a trick for use with TDR on exposed circuits is to run a pencil along the circuit to relate time to position.

A minor quibble, both have the same information content assuming appropriate acquisition parameters.  Depending upon the task one may be easier to interpret than the other.

A few years ago one of the RF or electronic trade magazines did a comparison of state of the art TDR and VNA instruments.  VNAs had an edge in bandwidth, dynamic range, and accuracy but TDRs were faster to use.  The instruments they reviewed could display results in either format.  HP made some sampling based instruments which combined oscilloscope, VNA, and TDR capabilities but they remained a specialized instrument.
 

Offline xrunner

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Re: Testing RF connectors and cables
« Reply #17 on: August 26, 2019, 05:29:36 pm »
Think of old school PL-259's with an SO-239 barrel between them, crappy crimps to the coax or terrible soldering, poorly installed and not super tight, been outside in the weather for years without proper sealing. Then it isn't too hard to lose half a dB even at HF frequencies.

The PL259 HF insertion loss myth probably came about from the average ham's inability to put a PL259 together without fucking it up plus the fact that nearly all PL259 connectors, apart from the really high end Amphenol ones are complete garbage, plus they probably used rotten mouldy water soaked coax they got from a hamfest as a "bargain".

Literally yesterday I saw someone eyeing up a roll of coax which had rodent nibblings on it  :palm: ... couple of hundred watts up that and I bet the new rat piss dielectric smelled marvelous.

At HF / VHF I am measuring around 0.3dB loss worst case per 10m on milspec RG58U (foam dielectric + really thick braid) with Amphenol clamp on BNCs. Above VHF, I do not do. Yet.

Absolutely, the quality and condition of the adapters makes a big difference. The next test would be to do it again with some crappy and old adapters. Unfortunately (for the testing not for myself), I only have good stuff on hand.  8)

However, I do know some hams whereby I could possibly obtain the needed crappy connectors.  :-DD



In case anyone's interested, the adapters used in series -

BNC-F | N-M--> N-F| N-F--> N-M | U-F--> U-M | U-M--> U-F | N-M--> N-F | N-F--> N-M | BNC-F
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #18 on: August 26, 2019, 05:57:04 pm »
Here's TDR on an Anritsu SOL N-M calibrator. 

Top image is a comparison of the Inmet load I used in the first post on top and the Anritsu load on the bottom.  The load is the hardest part of a cal kit to get right.  There really is a difference.

820422-0

Next is the rise time of the open

820410-1

And the fall time of the short

820416-2

One of the really nice things about TDR is you  can see the signal as it passes through the transmission line.  So you see the SMA-M cable and the SMA-F to N-F connector mirror reversed either side of the reflections from the open and short.

FWIW I weaseled my statement about information equivalence with "appropriate acquisition parameters".  My point simply being that a *properly* done Fourier transform will get the same result in the other domain.  For a wide range of reasons the engineering of a TDR and VNA would be rather different and I'd be surprised if there was a one to one match in what any comparison of a TDR and VNA showed.

As for connectors in coax runs, I think it's really more a comment on connector aging which has been subjected to the "telephone game".  I'd like to note that when I redid the mast connections on a friend's sailboat I coated the connectors with silicone grease before applying  a couple of layers of heat shrink.  The boat's been sold at least twice since then and last I heard was headed for Tahiti.  I'd love to know how the connections held up as it's been over 15 years.

I'll try to do some TDR work with a DSO and one of Leo's 1 MHz square wave pulsers here shortly, followed then by VNA via pulser and DSO.  Admittedly, the appearance of the nanoVNA makes that less important, but I've got an Owon XDS2102A which collects 20 Mpts of 12 bit data.  So with a bit of slicing and dicing I should be able to get  very good dynamic range.  But I got a shiny new toy on Thursday, so I've got to play with it a bit ;-)

Have Fun!
Reg
« Last Edit: August 26, 2019, 05:59:48 pm by rhb »
 
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Offline TheUnnamedNewbie

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Re: Testing RF connectors and cables
« Reply #19 on: August 27, 2019, 04:49:20 am »

FWIW I weaseled my statement about information equivalence with "appropriate acquisition parameters".  My point simply being that a *properly* done Fourier transform will get the same result in the other domain.  For a wide range of reasons the engineering of a TDR and VNA would be rather different and I'd be surprised if there was a one to one match in what any comparison of a TDR and VNA showed.


This is actually something certain VNAs use a lot. If you are doing 'simple' one-port measurements, you can sometimes get away with almost no cal by doing a time-domain de-embed. What it will do is simply convert the f-dom trace to t-dom, chop off the first bit (as that would be your DUT connector you want to de-embed) and then go back to the f-dom.

Pretty cool stuff. Would be neat to see some things on cables, but I think that is hard to see with a TDR? I've done some measurements of cheap coax at the lab, and it was not pretty... But don't have any real results saved. I should re-do that some time.
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #20 on: August 28, 2019, 12:27:05 am »
This is an entirely random series of experiments.  A lot of it is driven by what I have on hand or some task such as checking the quality of the SMA-F to N-F adapters in the first post.

I did some tests with one of Leo Bodnar's pulsers and a 200 MHz DSO which I'll post eventually.  It's not as impressive as having 20 GHz BW, but still useful.  Though the appearance of the nanoVNA may well change things.  I've ordered one.  At $50 delivered with tax it was impossible not to try one out.
 

Offline bd139

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Re: Testing RF connectors and cables
« Reply #21 on: August 29, 2019, 11:30:08 pm »
Random experiment I did this evening. I ran out of continuous lengths of RG58U greater than 10m and I needed a 20m chunk so I figured I'd try something stupid.

I went "full CB troll", stripped 2 inches off each end and rammed a choc block in the middle of the two 10m lengths, wrapped it up in cheap insulation tape and chucked it on my RigExpert analyser.

VSWR was flat to 22MHz and at 55MHz it was still 1.07 (~29dB RL) compared to a baseline measurement of 1.0 on a properly terminated patch of the same length.

Ergo YMMV, at least if you don't push into VHF.

And no I'm not leaving it like that. I've got a 50m roll turning up on Monday :D

I shall leave the real measurements to those who can afford car priced VNA's :D
« Last Edit: August 29, 2019, 11:32:53 pm by bd139 »
 
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #22 on: August 31, 2019, 03:20:04 am »
You need to start following the nanoVNA as it develops.  I got one today and am about to start beating on it.  But initial take is it's pretty cool even if it is a bit rough around the edges.  But for $50 delivered in 3 days it's quite impressive.

I should note that the SMA PCB connector snapped all 4 pins when I was tightening the short to do a cal using a Suhner torque wrench.  So if you get one, repackage it with more robust connections.

Have Fun!
Reg
 

Offline OwO

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Re: Testing RF connectors and cables
« Reply #23 on: August 31, 2019, 05:13:31 am »
just got info that nanoVNA V2 will have frequency coverage to 2.5GHz and retail for the same price
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Offline bd139

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Re: Testing RF connectors and cables
« Reply #24 on: August 31, 2019, 09:06:57 am »
Interesting. I looked at some of the "amateur radio grade" VNAs on the market a while back and was unimpressed for the price. microVNA looks interesting though.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #25 on: September 07, 2019, 11:55:56 pm »
I got an APC-7 to N-F adapter from China today.  These are about $22-23 each on ebay from besttrade360 .

I've got 3x used Wiltron APC-7 to N-M and a couple of APC-7 to SMA-M adapters.  So I Calibrated my 8560A using an RG402 N-M to N-F jumper and an N-M to N-M jumper.  I then made up N-M to N-F adapters using various bits.  I  inserted them in the cable and swept them from 1 MHz to 2.9 GHz.

The 8560A is set to 2 dB/div, so I read the ripples as about 0.125 dB.  That seems quite reasonable for a stack of connectors.  For an 8753B/85046A system, it looks "good enough" and I've ordered a 2nd one.

Spectrum for first stack

829566-0

What the connector stacks look like:

829560-1

Second spectrum

829572-2

APC-7 to N-F spectrum

829584-3

Connector stack

829578-4

With typical Chinese prices at $45, used name brand at $100 and new UK made at $135 I thought it worth trying one.

I'll post more detailed TDR results for all the items once I get my 11801 set up again.

Reg
« Last Edit: September 08, 2019, 12:00:43 am by rhb »
 
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Offline TheUnnamedNewbie

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Re: Testing RF connectors and cables
« Reply #26 on: September 08, 2019, 07:43:42 am »
Wonder how they age, because unless the problem is in the aging, these show good performance.

That is nice performance though.
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Offline _Wim_

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Re: Testing RF connectors and cables
« Reply #27 on: September 08, 2019, 12:24:36 pm »
Wonder how they age, because unless the problem is in the aging, these show good performance.

That is nice performance though.

But even is aging is not so good, typical use for these is to leave them on the unit to act as a connector saver/adaptor, so I think this is a very useful result. Would be interesting to test them up to their rated spec (12Ghz) if someone has the proper gear for this.
 

Offline radiolistener

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Re: Testing RF connectors and cables
« Reply #28 on: September 08, 2019, 12:47:00 pm »
typical use for these is to leave them on the unit to act as a connector saver/adaptor

$45 for a connector cap? looks like too expensive  :D
 

Offline _Wim_

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Re: Testing RF connectors and cables
« Reply #29 on: September 08, 2019, 01:01:40 pm »
$45 for a connector cap? looks like too expensive  :D

it "only" 22$, so you can put one at both ends  ::)
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #30 on: September 08, 2019, 03:25:46 pm »
Several of you need to understand the post before commenting.  This test is mostly relevant only to owners of 85046A and 85047A S parameter test sets for the 8753x VNA.  I don't know of other gear that uses an APC-7 connector.

The 85046A is a 3 GHz S parameter test set for the 8753x series of VNAs and uses APC-7 connectors.  Unfortunately, APC-7 never caught on so connectors and cables are generally quite hard to find and usually expensive when you do.

As a practical matter, putting APC-7 to N-F adapters on the 85046A & 85047A (6 GHz version) so that standard N cables and connectors can be used is the norm.  The APC-7 is sexless, so wear is not an issue as it is with other connectors.

Because the SOL calibration of the VNA corrects all errors to the reference plane of the calibration port, the little bit of ripple seen in the SA sweep is of no consequence.  And it is likely that the ripple is caused by all the other connectors as much as by the APC-7 to N-F.  That will be easy to determine by TDR with the 11801/SD-24.

« Last Edit: September 08, 2019, 03:55:09 pm by rhb »
 

Offline TheSteve

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Re: Testing RF connectors and cables
« Reply #31 on: September 08, 2019, 06:25:35 pm »
I saw those cheaper(under $25) APC7 to N adapters a few months back and considered trying a pair but I had an offered accepted on a pair of Maury ones so I never did buy them. It is nice to see the performance doesn't look bad at all.
I could sweep mine to 26.5 GHz but they would mode before that. It would be nice to see how the cheap ones perform out to 18 GHz. Lower cost N connectors often start to perform much worst past 11 GHz or so.
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #32 on: September 08, 2019, 11:44:12 pm »
I made a deliberate decision to limit most of my gear to 3 GHz.  I did buy an 8566B so I can check for harmonics from my 8648C, but  good RF connectors and cables get so expensive above 3 GHz that I think that was a good place to draw the line.  And there is a *lot* of very fine gear that goes to 3 GHz.
 

Offline tkamiya

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Re: Testing RF connectors and cables
« Reply #33 on: September 09, 2019, 02:19:27 am »
I should note that the SMA PCB connector snapped all 4 pins when I was tightening the short to do a cal using a Suhner torque wrench.  So if you get one, repackage it with more robust connections.

Have Fun!
Reg

I have busted SMA connectors of various kinds.  Then I dawned on me....  these are made of brass....  My Suhner wrench is calibrated for stainless steel torque.  I'd check that to make sure.
 

Offline tkamiya

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Re: Testing RF connectors and cables
« Reply #34 on: September 09, 2019, 02:21:31 am »
One thing I'm very interested in knowing (and I lack proper equipment to conduct the test myself) is sample variation in various makes.  I'd expect well known brand name connectors to be fairly consistent but cheap junk will be all over the place.  But this is only an assumption.  Has anyone done this kind of test?
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #35 on: September 09, 2019, 03:28:47 pm »
The example I posted of SMA-F to N-F adapters was representative of a lot of 24.  One unit had the N thread major diameter  under size by 0.03 mm,  enough that the plastic protector fell off when I put it back on after testing it with the 11801.  However, electrically I did not notice a difference among them. They all had a similar reflection response.

From my testing, BNC connectors are the most variable.  If the male locking collar is not a tight fit on the body, the connectors will give *very* variable results every time they are used or even jiggled in use. I'll post an example later.

I suggest getting a nanoVNA for $30-60 US.  There are 3 different versions on the market, but considerable debate whether there are significant differences among them.  There is a very active mailing list on groups.io with lots of interesting things going on.  You could  use one of those to test RF connectors to 900 MHz.  There is a python script to do TDR using it.  I'm hoping to modify the FW to do SWR and TDR only on the STM32F072CBT6 based units.  I think if I delete all the other features that there will be room.    A completely portable unit  would be very  handy for antenna and feedline work, especially for field day operations.

There is a console USB serial port so you can get magnitude and phase data back as ASCII text.  That would make it very easy to scan a bunch of connectors in the frequency domain and plot the results with gnuplot.  You could also calculate mean and standard deviation at each frequency.

Because of the very high cost of RF connectors I decided that I'd buy cheap units and simply test them all.  I'm building an RF switching system to automate full annual calibrations of my test bench.  The cheap SMA-F to N-F would not be satisfactory for that, but for ham radio work below 1 GHz they are fine.  So I'll use them for a remote antenna switch.

I plan to continue this thread for a long time.  But I only have so much time.  I'd like to urge you to get a nanoVNA, order a bunch of connectors suited to your needs, test them and post your results along with a link to the seller.  That would help a lot of people.

Have Fun!
Reg
 
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Offline tkamiya

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Re: Testing RF connectors and cables
« Reply #36 on: September 09, 2019, 05:50:14 pm »
Great suggestion!

TDR has been on my list for some time.   I'll have to make one soon.  I think there is an already made module with very fast rise pulse - which name escapes me.  I do the same on connectors.  I buy bulk cheap.  What's obviously not good goes to garbage.  My main frustration is thin-ness of gold plating.  Use few times and I can see corrosion forming.  Oh well.

Those mini-vna looks fun.  I need little higher frequency but for that kind of money, it'll be nice to play with. 

Thanks for your effort and also a very nice reply.
 

Offline jadew

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Re: Testing RF connectors and cables
« Reply #37 on: September 09, 2019, 08:54:58 pm »
One thing I'm very interested in knowing (and I lack proper equipment to conduct the test myself) is sample variation in various makes.  I'd expect well known brand name connectors to be fairly consistent but cheap junk will be all over the place.  But this is only an assumption.  Has anyone done this kind of test?

I have tested lots of SMA, BNC, TNC and N-type cables and connector assemblies, because I wanted to find a balance between price and performance for a product I'm making. What I found was that both connectors and cables (of the same make and PN) have usually very little variation in the specified bandwidth, regardless of price.

Of course the specified bandwidth varies with price, but generally speaking, within that bandwidth they seemed to be roughly the same.

After the specified bandwidth it's a different story: you'll start seeing a bit more variation, as well as intermittent flaws, and all this gets worse the cheaper the connector is.

What I found interesting was that those flaws were also consistent. They were always there for that particular type of connector.



I made a deliberate decision to limit most of my gear to 3 GHz.  I did buy an 8566B so I can check for harmonics from my 8648C, but  good RF connectors and cables get so expensive above 3 GHz that I think that was a good place to draw the line.  And there is a *lot* of very fine gear that goes to 3 GHz.

That is indeed a good place to draw the line. You can do 3-4 GHz on the cheap, but after that you can't rely on your measurements if it didn't hurt you a little when you bought those connectors. The higher the frequency, the more it has to hurt.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #38 on: September 09, 2019, 09:32:07 pm »
Great suggestion!

TDR has been on my list for some time.   I'll have to make one soon.  I think there is an already made module with very fast rise pulse - which name escapes me.  I do the same on connectors.  I buy bulk cheap.  What's obviously not good goes to garbage.  My main frustration is thin-ness of gold plating.  Use few times and I can see corrosion forming.  Oh well.

Those mini-vna looks fun.  I need little higher frequency but for that kind of money, it'll be nice to play with. 

Thanks for your effort and also a very nice reply.

Leo Bodnar sells an excellent fast rise pulse generator at leobodnar.com.  The standard unit is a 10 MHz square wave, however, Leo will provide a unit with a 1 MHz square wave which works better for TDR.  I'll post some screen shots using a 200 MHz DSO.  I have three,  a 1 MHz & 10 MHz square wave and a 100 ps pulse version.

The nanoVNA *should* do better for TDR than the pulser unless you have a 1 GHz scope or design a low cost sampling scope.  That would be a very cool OSSW/OSHW project.  Use a GPSDO for timing and an MCU with a 12-14 bit ADC.  It would be a serious challenge to design the sampling strobe circuit layout.

But for *real* TDR fun nothing beats a Tek 11801 or CSA803 and SD-24 TDR head.  20 GHz BW and femtosecond time resolution.  I got an 11801 with a bad NVRAM for $145 delivered.  It was $40 for the replacement NVRAM and I picked up a pair of SD-22, 12.5 GHz dual channel heads for $150 delivered.  The 11801 calibrator has a19-20 ps rise time.   I recently got an SD-24 for $250 from Tektronix via ebay which has a pair of the same pulsers.

The downside of the 11801/CSA803 is there is no component level service data available.  So other than replacing bad NVRAM chips, all you can do is swap boards from a parts mule.

Reg
 

Offline tkamiya

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Re: Testing RF connectors and cables
« Reply #39 on: September 10, 2019, 12:03:39 am »
"Leo Boner":  That's the name I was looking for!

I have a CSA602A with 1 GHz plugins.  Or....  I could use a sweep generator up to 26.5GHz and SpecAn, or a detector and an XY scope.  I'll have to do some research.  On later, I tried it recently.  It was rather difficult to assert problems because flatness is not quite there.  Since I have to use MaxHold, I can't use normalize function either. 
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #40 on: September 10, 2019, 02:16:08 am »
"Leo Boner":  That's the name I was looking for!

I have a CSA602A with 1 GHz plugins.  Or....  I could use a sweep generator up to 26.5GHz and SpecAn, or a detector and an XY scope.  I'll have to do some research.  On later, I tried it recently.  It was rather difficult to assert problems because flatness is not quite there.  Since I have to use MaxHold, I can't use normalize function either.

It's "Bodnar".  He's a great guy with a fantastic product at a bargain price.

To do TDR from frequency domain data you have to have the phase information. So I can't think of any way to do TDR from an SA & TG.

An impulse at any time delay is constant amplitude in frequency.  The only thing that changes is the exp(i*2*pi*f*t) term) for the shift of the spike in time.  That is a pure phase shift.

Get a nanoVNA and join the party.  It's a lot of fun and there is a lot of stuff that needs to be developed.  The more the merrier.  It's the RF equivalent of the $20 LCR tester.  I predict that 1-2 years from now *everyone* who is serious about RF will have one even if they have a $20K+ VNA.  It fills the "no money for T&M kit" niche and the "need something convenient for field work"  niche.  David Kirkby talked me into getting one and I'm glad he did.  It is really neat.

Have Fun!
Reg
 

Offline OwO

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Re: Testing RF connectors and cables
« Reply #41 on: September 10, 2019, 05:16:43 am »
Or wait for V2 to get 3GHz upper frequency range and reduced noise because of no longer using the third harmonic. I just got info that V2 will be priced the same as V1. It will use ADF4350 synthesizers in addition to the si5351 so in theory it can go up to 4.4GHz.
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #42 on: September 10, 2019, 07:21:18 pm »
There will still be a slew of this version that people have.  If someone gets a V 2, the V 1 can go in the field day kit.

What really matters is software which makes full use of the capabilities of either model, user documentation, etc.

edy555 and hugen79 have provided us with a very nice instrument which almost any radio amateur can afford.  There are a lot of better units, but this one is about $35 delivered from China.  My xaVNA, VNWA and 8753B cost many times more.

Things like python scripts which test RF connectors across all the ham bands or other ranges and compute statistics about uniformity, losses, etc have long term value.  Most of the work is in creating a good presentation of the information.

And if, like me, you have no experience using a VNA, the nanoVNA is an excellent learning tool.  Set yourself a task such as testing feedlines and antennas and figure out how to do it so that it is entirely in the nanoVNA FW.  Treat it as a device with a changeable personality and develop different personalities for different use cases.  But most of all.....

Have Fun!
Reg
 
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Offline FriedLogic

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Re: Testing RF connectors and cables
« Reply #43 on: September 14, 2019, 07:53:18 am »
From my testing, BNC connectors are the most variable.  If the male locking collar is not a tight fit on the body, the connectors will give *very* variable results every time they are used or even jiggled in use.

I think that the shield connection is one of the main contributors to how well they work, and is something that many cheap connectors and adaptors get wrong:

https://www.eevblog.com/forum/metrology/distributing-lab-standards/msg2670798/#msg2670798

I've had less problems with the BNC(f) connectors, but they might still damage whatever is plugged into them if the dimensions are wrong or the machining is bad. I tried some BNC(f) to SMA(m) adaptors on one of Leo Bodnar's BNC pulsers which had about a 32ps rise time, and the cheap one worked not too bad, although it had the slowest rise time.

https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/msg1372481/#msg1372481

Unfortunately the scope I was using limited the rise time a bit.
 

Offline TheSteve

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Re: Testing RF connectors and cables
« Reply #44 on: September 17, 2019, 04:57:56 pm »
I see APC7 to N out of China for 16.99 each shipped now. For anyone who need some at what can only be described as cheap check out ebay.
VE7FM
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #45 on: September 17, 2019, 10:42:01 pm »
Sigh...  I already ordered the 2nd one I needed for my 85046A S parameter set for around $23.

Might be worth getting a couple extra though at that price.    I just put the one I have on my 11801 with an SD-24.  It looks very good.

First scope image is the reflection from an Astrolab SMA-F to N-F and an Anritsu SOL load.  This is a very good quality RG402 SMA cable and adapter.

838371-0

838377-1

Second image is the reflection from a stack of the Astrolab SMA-F to N-F  to a Wiltron N-M to APC-7 to the Chinese APC-7 to N-F to the Anritsu load.  The SD-24 pulser is a 250 mV step.

838383-2

838389-3

The cheap Chinese APC-7 to N-F adapter is not quite as good as the used ebay Wiltron APC-7 to N-M adapter, but it's not bad and very acceptable for the price.  It's certainly good out past 7 GHz.

Have Fun!
Reg

FYI search ebay with "RF Coaxial APC7 to N Female Connector Adapter" to find the $17 ones.  I just bought 2.
« Last Edit: September 18, 2019, 05:25:00 pm by rhb »
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #46 on: September 20, 2019, 12:57:36 am »
Here's some more TDR porn.

I've set up so I'm looking at the rise time of the transmitted step from the SD-24 as it passes through a 30 cm Chinese RG402 SMA-M jumper, Astrolab SMA-F/N-F, Wiltron N-M/APC-7, Chinese APC-7/N-F, N-M/N-M,Astrolab N-F/SMA-F and another 30 cm Chinese RG402.  The N-M/N-M doesn't have a name, just model designation, but is silverplated and in NOS condition.

839650-0

The highlighted trace is the through step which has a 35 ps rise time.  The other trace is the reflection trace.  The large bumps at far left and right are the Astrolab SMA-F/N-F adapters.

839646-1

If I substitute another Wiltron APC-7/M in place of the Chinese APC-&/N-F and  N-M/N-M I also get 35 ps rise time, but much smaller reflections.   al the visible reflections are from the SMA-F/N-F adapters.

839642-2

839638-3

 A cheap ebay SMA-F/SMA-F bulkhead connector has a 33 ps rise time and a single 30 cm piece of RG402 has a 31 ps rise time.  A 3.5 mm short at the input has a 28 ps rise and fall.
« Last Edit: September 20, 2019, 12:59:39 am by rhb »
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #47 on: September 20, 2019, 01:09:02 am »
Here's the SMA-F/SMA-F adapter that came with my nanoVNA.

First hand tightened as recommended to me after I broke the PCB side ground pins of the SMA port on the nanoVNA which had not been soldered on both sides.  I soldered bulkhead mount SMA-F on in place of the originals.

839656-0

Now here's what it looks like after tightening with my Suhner torque wrench.

839660-1

So tell me, which do you think is better?

Have Fun!
Reg
 
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Online 0culus

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Re: Testing RF connectors and cables
« Reply #48 on: September 20, 2019, 01:15:55 am »
Definitely needs to be torqued haha. I'm considering getting one, as a guy at work got one and I'm super impressed with what it does for the form factor and price.  :-DD
 

Offline xrunner

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Re: Testing RF connectors and cables
« Reply #49 on: September 20, 2019, 01:37:26 am »
Clear results.  ;)

I guess another tool I need to buy.  :clap:
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #50 on: September 20, 2019, 02:31:16 am »
I think I should note that most of the problem is a matter of not being able to grip the SMA male.   A knurled round wrench the diameter of an N male with a slot cut in it would get them tight enough.

I have tightened a *lot* of different connections while watching the 11801 display.  It's given me a very good sense of how much force is needed to get a good connection.  Lacking a lathe you could cut a small square of metal, drill a hole in the center and then saw to the hole to create a slot and clean up with a file.  While not elegant, it *is* good enough.

However, if you're playing above 6 GHz, using SMA you definitely need a wrench.  But the price tag for playing up there is so high I have to ask "Why?".  It's interesting to be sure, but do you *really* want to spend $50-100 for a *single* connector?

I first encountered the SMA in my HP 8601A sweeper.  I thought it was the cat's meow, but also assumed it was only intended for infrequent connections.  The life cycle connection limit reinforces that.

I consider it a great way to connect shielded modules.  I'm not real keen on them beyond that.  But they are cheap and good enough if you stay below 6 GHz and much better than BNC.  I've got some BNC test examples coming next.

Have Fun!
Reg
 

Online syau

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Re: Testing RF connectors and cables
« Reply #51 on: September 20, 2019, 02:46:14 am »
Planning to buy one, which torque value should I use for these brass based connectors ?
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #52 on: September 20, 2019, 04:19:41 am »
I don't know the standards from memory.  Look it up on the Internet.

I don't even know what mine is.  I need to check it and may need to buy another if it is set for stainless steel connector torque.

Have Fun!
Reg
 
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Offline TheUnnamedNewbie

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Re: Testing RF connectors and cables
« Reply #53 on: September 20, 2019, 05:39:59 am »
Planning to buy one, which torque value should I use for these brass based connectors ?

In my experience, it doesn't matter too much what torque exactly you use. The most important thing is that you use a torque wrench - a fraction of a turn with these connectors can be the difference between tight fit and damaged connector face.

From memory, all our 3.5/2.92 mm wrenches are 1 Nm, and our 2.4/1.85mm wrenches are 0.9 Nm, 1mm connectors uses 0.7 Nm? I am not 100% sure. I've seen 1.1 Nm as well for 3.5/SMA.

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Offline _Wim_

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Re: Testing RF connectors and cables
« Reply #54 on: September 21, 2019, 08:21:30 am »
« Last Edit: September 21, 2019, 08:23:13 am by _Wim_ »
 
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Offline rhb

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Re: Testing RF connectors and cables
« Reply #55 on: September 21, 2019, 05:48:47 pm »
FWIW the 2nd APC-7/N-F adapter from besttrade360 arrived today.  The first time I connected it, I saw a large reflection at the N-F interface which raised concerns about QC.  However, when I connected it a 2nd time after hooking up the 1st one and saving the TDR trace I got comparable results to the 1st one.

Measured rise time was 33-34 ps on the thru connection and the largest reflections were from the Astrolab SMA-F/N-F adapters.

I'll report on the $16.50 each pair when they arrive from China.  But certainly these are entirely satisfactory for use on an 85046A or 85047A S parameter test set.

With all the RF connectors one needs and the high prices of the name brands, getting a pair for under $50 vs a pair for $270+  makes a huge difference.  It makes buying Chinese connectors and simply returning those which fail testing worth the trouble.  But testing *is* mandatory.

Have Fun!
Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #56 on: September 26, 2019, 06:19:31 am »
Well, too of the APC7-N Female arrived here today.

The first thing I noticed was the APC7 nut was really loose.  Then I noticed the lack of the threaded sleeve that enables them to mate with one-another, probably explaining the looseness.  And then the nail in the coffin was the surface finish.  The mating surface is not smooth - like it wasn't machined at all.  Unfortunately, the focus leaves a lot to be desired, but it's quite visible on the first attached picture.  The surrounding surface isn't smooth either, but it's not critical.

The N barrels and ends in contrast are nicely machined, but both are blemished.

I'm not going to use them.  Given the APC7 end, I'm not too keen to be mating my N standards with the N ends.

rhb: Do yours have the threaded sleeve that's clearly visible on the second attachment?

73, Orin KJ7JQ.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #57 on: September 26, 2019, 04:34:09 pm »
The male threaded sleeve  can get stuck down in the female threaded nut.

You can take a pin spanner and unscrew the APC-7 end.  You'll find 4 pieces.

843154-0

The problem is that the pin that is supposed to keep the male threaded sleeve from rotating is too short and doesn't engage the slot properly.  If you turn it too far, the pin gets pushed out of the slot and the male sleeve gets stuck in the bottom of the female nut.

843158-1

So here they are put back together as they should be.

843162-2

If you look closely, you can see that the brass pin is not flush.  If you try to drive it in you run the risk of knocking it all the way through.  then it's a *real* nuisance.

Then use the pin spanner (I used some heavy tweezers) to screw the central bushing back on the body.

If you're constantly remaking the APC-7 connection it's worth fixing.  But if you're going to only attach it to an 85046A or 85047A and then leave it there it's not a huge problem.

NB:  After knocking the too short brass pin out, I was able to properly  fix it with a piece of 1/16" gas welding rod.  It took a good bit of time, so I suggest *not* doing that.  However, the male sleeve now operates reliably.
« Last Edit: September 26, 2019, 05:42:53 pm by rhb »
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #58 on: September 27, 2019, 06:57:53 am »
So, I disassembled the adapters and indeed, the sleeve had escaped.

A Park SPA-2 pin wrench will work, though you do have to squeeze it rather tight.  I wouldn't recommend it.  I suspect the cheap snap-ring pliers the auto parts stores sell would work better.

I'll see how they do back to back up to 3GHz on the 8753C at work...

 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #59 on: September 27, 2019, 02:55:21 pm »
Please post the results.  Once I get my 2nd pair I should be able to do the same with my 8753B.  Meanwhile I'll sweep the current pair back to back on my 8560A.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #60 on: October 10, 2019, 12:01:14 am »
I just got the 2nd pair of APC-7/N-F adapters which I ordered on ebay from hualun1988 for $16.49 each.  I made them up with Wiltron APC-7/N-M adapters, did a thru cal on my 8560A using an N-M/N-M and an N-M/N-F cable and then inserted them. The ripple is approximately trace width on all of them at 2 dB/div. I judge it as no more than 0.15 dB from 1 MHz to 2.9 GHz.  The cables alone had ~1 dB of ripple around 40 MHz and 2.5 GHz before normalization and are of good quality.

Both of the new adapters had to be taken apart and the pin put back in the slot,  but the pins are long enough to stay engaged.  At least if you don't get ham handed.

I don't have a way to test these above 2.9 GHz except by TDR with the 11801 & SD-24.  I'll add that and the 8560A photos later.

I also got a Chinese N-M SOL cal kit for $17.99, US stock.  I'll be testing those against my Anritsu SOL with the Wiltron APC-7/N-M & Chinese APC-7/N-F and also a probably Chinese APC-7/SMA-M.  The TDR will allow seeing which interfaces have the most mismatch.

I can see no reason these won't serve on an 85046A just fine.  With the ebay asking prices for an 85047A, I don't see myself getting one any time soon.  And no real need for one anyway.  Going above 3 GHz just makes *everything* cost more.

Have Fun!
Reg
 

Offline rastro

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Re: Testing RF connectors and cables
« Reply #61 on: October 10, 2019, 05:54:13 pm »
Has anyone used an indicator/gauge on any of these bargan connectors?

The ACP and N connectors are probably a little more robust/forgiving. 
I'd be particularly concerned with SMA adapters since they can be connected to 3.5mm connectors and cause potential damage.

rastro
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #62 on: October 10, 2019, 06:59:41 pm »
Sadly, I missed a Maury A-027A set on ebay because I set the clock wrong.  I'm planning to make a set of GO/NO-GO gauges myself.  But the 7" x 14" Chinese mini-lathe I bought to do the work is just a semi-finished  kit of parts assembled at the factory to make sure they didn't leave anything out.  Serious work required to finish it.

A big problem with the 3.5 mm compatible connectors is the tolerances required.
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #63 on: October 10, 2019, 11:49:35 pm »
Please post the results.  Once I get my 2nd pair I should be able to do the same with my 8753B.  Meanwhile I'll sweep the current pair back to back on my 8560A.


I finally got around to playing a little.

Calibrated the 8753C for S11 with an old 85032B N calibration kit using an existing APC7-NM adapter on Port 1.

I put the two APC7-NF adapters from the 85032B back to back and added a 909F load.  This is the lower trace below, then did the same with the Chinese adapters.  The resulting upper trace speaks for itself:



To prove to myself nothing horrible was going on, I removed the adapters and attached the calibration load - (909F.gif).  The lower trace is now the calibration load.  It looks reasonable and is about what I'd expect measuring the load I'd used for calibration:



So I then tried one Chinese adapter with a 909C APC7 load - (SingleAPC7-909C.gif):



And finally, a 909C N-F load (909C-N-F.gif) - to see what we can expect out of a 909C compared to a 909F.  The 909C isn't as good as a 909F.  It's the flatter of the two traces.  (Unfortunately, the plot capture didn't draw the traces in different colors.)



I didn't do any S12 measurements...  I have two semi-reasonable Mini Circuits N-M to N-M cables.  One is intermittently bad.  I could only find one at work today and I think it's the bad one.  Let me know if there are any particular measurements you'd like.

Edit: Embedded the images and added final comment.
« Last Edit: October 12, 2019, 04:54:20 pm by orin »
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #64 on: October 11, 2019, 02:12:22 am »
Would you please edit the post to describe the plots in more detail?  I'm a bit unsure which trace is which.

Here are my results using the 8560A.  I forgot to take pictures of the logmagnitude response of the cables before normalization. 

The thru cal using  N-M/N-M & N-M/N-F cables:

852132-0

852136-1

The $16.49 Chinese APC-7/N-F & Wiltron APC-7/N-M inserted:

852140-2

852144-3

All of the 4 Chinese APC-7/N-F adapters I've bought produce comparable results.

As this is an insertion test normalized to the ends of the adapter stack, it seems to me a pretty good test.  I need to buy a name brand  N-M/N-F adapter for a comparison. But this looks pretty good to me.  Please bear in mind, I'm a complete novice at this stuff.  While there is a lot I do know, what I don't know is vastly larger.  It's infinite.

I spent the day trying to impose something resembling order around here.  A long way to go though.  I ordered 72 latch top bins today to help.  It will probably be a few days before I set up the 11801 & SD-24.  I suspect that will show that the poor finish on the APC-7 side of the Chinese adapter is the culprit for the ringing.  In which case I'll try smoothing that face with some 1200 grit wet or dry sandpaper one one adapter  and see what happens.

Have Fun!
Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #65 on: October 11, 2019, 06:27:38 am »
I linked to the images inline.

Basically, I put a good load at the end of the adapter stack and measured S11.  It was fine for the adapters that came with the HP cal kit, but pretty poor IMO for the Chinese adapters.  I also tried a single Chinese adapter with a not quite as good APC7 load.  It wasn't great, but not as bad as two of the adapters stacked.

Orin.
 

Offline rastro

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Re: Testing RF connectors and cables
« Reply #66 on: October 11, 2019, 01:52:21 pm »
Sadly, I missed a Maury A-027A set on ebay because I set the clock wrong.  I'm planning to make a set of GO/NO-GO gauges myself.  But the 7" x 14" Chinese mini-lathe I bought to do the work is just a semi-finished  kit of parts assembled at the factory to make sure they didn't leave anything out.  Serious work required to finish it.

A big problem with the 3.5 mm compatible connectors is the tolerances required.
That sounds like a good project.  I have an HP 3.5mm indicator/gauges but I would like to make something also for "N" and SMA connectors.

I've used my 3.5mm gauges to measure some SMA adapters/cables purchased from eBay and the results where concerning.  Quite a few male center pins extended too far out beyond specification.  This can clearly damage a quality 3.5mm connector and maybe even another mating SMA connector.  Until you have an indicator/gauge you're kinda flying blind.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #67 on: October 11, 2019, 05:37:29 pm »
@orin

There's something wrong with the plots.  20 dB is a large reflection change.  Other than theory, all I know about VNAs is a couple of hours playing with a nanoVNA.  But none of the curves look good to me.

S21 is simply incident minus S11 and I'm not seeing the level of difference you are.  Even without normalization I'm only seeing ~1 dB variations in S21 through the N-M/N-M & N-F/N-M cable connection.  And for the 4 cases I tested the mismatch at the cable connectors was 6-8x the mismatch of the Wiltron & Chinese N-M/N-F stack when I inserted them.

What do you have available to test with?  Can you assemble APC-7 loads from the various adapters and loads you have?  If so, normalizing on one of those and then substituting the others should provide a good picture of the variations among the APC-7 adapters.  Complicated of course, by using different loads.

What does S11 look like with each of the Chinese APC-7/N-F adapters and the same HPAK load?

If you normalize S11 from the HPAK APC-7 load and then substitute the Chinese APC-7 & an HPAK load that should give a reasonable good picture of how different they are.

Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #68 on: October 12, 2019, 12:42:15 am »

S21 is simply incident minus S11 and I'm not seeing the level of difference you are.  Even without normalization I'm only seeing ~1 dB variations in S21 through the N-M/N-M & N-F/N-M cable connection.  And for the 4 cases I tested the mismatch at the cable connectors was 6-8x the mismatch of the Wiltron & Chinese N-M/N-F stack when I inserted them.


Actually, 1dB variation in S21 is quite large according to the rule of thumb here:  https://www.edn.com/electronics-blogs/all-aboard-/4430355/How-much-return-loss-is-too-much--Rule-of-Thumb--12

Quote

What do you have available to test with?  Can you assemble APC-7 loads from the various adapters and loads you have?  If so, normalizing on one of those and then substituting the others should provide a good picture of the variations among the APC-7 adapters.  Complicated of course, by using different loads.

What I don't have is an APC7 Open or Short, so I can't calibrate to an APC7.  I suppose I could measure the APC7 load with the HP adapter... but I ran out of time today.

Quote

What does S11 look like with each of the Chinese APC-7/N-F adapters and the same HPAK load?

If you normalize S11 from the HPAK APC-7 load and then substitute the Chinese APC-7 & an HPAK load that should give a reasonable good picture of how different they are.

Reg


I spent the time to measure S21...

Upper trace is with two back to back HP adapters, lower trace with two back to back Chinese adapters.  Note the scale is 0.1 dB/div.

Orin.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #69 on: October 12, 2019, 03:45:19 am »
That's a 1% difference in S21 which is quite impressive.  There is no reason I'm aware of to expect random connectors and cables to match more closely outside of a metrology lab which routinely tests and discards stuff when it gets worn.  Much of which now gets sold on ebay as "used". Most of my gear is all 30 years old.  I'm quite sure that there have been more than 500 N connections made to my 8560A ports in 30 years.  This is the reason for doing an  insertion test after normalization if at all possible.

When you get some time, please look into your S11 measurement results.  S21 should be the difference of incident and S11.  This is just conservation of energy.  If I understand the plots, that doesn't appear to be the case, so it may be the directional bridge is damaged or else something was set improperly.  The wonderful thing about embedding a computer in a measurement system is it can now do things on its own without informing the user.

Have Fun!
Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #70 on: October 15, 2019, 06:05:29 am »

When you get some time, please look into your S11 measurement results.  S21 should be the difference of incident and S11.  This is just conservation of energy.  If I understand the plots, that doesn't appear to be the case, so it may be the directional bridge is damaged or else something was set improperly.  The wonderful thing about embedding a computer in a measurement system is it can now do things on its own without informing the user.

Have Fun!
Reg


I'll check the 85046 and redo the measurement, but I don't really see anything wrong... the 909C I measured came in at 45dB return loss and it's spec'd at 1.01 SWR to 2GHz which would be about 46dB return loss.  The HP adapters are spec'd at >= 30dB return loss, DC - 6GHz and my measurement is better than that.

The 18 or 19dB return loss of the Chinese adapters I measured is much better than the 0.3dB S21 measurement - if there is no insertion loss, I'd make that a return loss of 12dB: 10 * log(1 - 10**(-0.3/10))

...so I checked the bridges on the 85046A and they are better than 30dB directivity (I don't have an APC7 short so I have to measure it through an APC7-N adapter meaning the true directivity is better than what I see).  Calibration can easily handle 30db directivity as that is the spec at the 85046A's limits.

I did the same measurement using port 2/S22 and got similar results.  I also have a coupler from an 85047A where I can connect the coupled port to the 85046A port 2 and look at S21.  The results were similar, but noisier.

FWIW, I also tried a couple of N F-F adapters in place of the APC7-APC7 adapters.  One was even worse than the back to back Chinese adapters at 3GHz!  The other was 40-odd dB return loss - about the same as the HP APC7 adapters back to back.

 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #71 on: October 15, 2019, 08:44:50 pm »
What I don't yet understand is the magnitude of the changes in response with frequency.  That seems excessive to me.

When I started doing TDR on my RF connectors I discovered that I had several Pomona 6 dB BNC attenuators that were bad.  I've got all the bad parts I found segregated.  Eventually I'll apply red paint so I can safely use them for demonstrating various problems.  But I found enough bad name brand used stuff to not feel so confident about the difference between those and new Chinese, provided they are tested.

Thanks for contributing to the thread.  I'm in entropy reduction mode.  I bought 36x 15x11x3.5" and 36x 15x11x16" Sterilite containers with latching lids and am about to order a bunch of the half size containers in both depths.  I haven't gotten to putting labels on the tubs, but will eventually.  At the moment I'm emptying stuff out of the shipping boxes into the tubs.

Have Fun!
Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #72 on: October 15, 2019, 10:12:37 pm »
What I don't yet understand is the magnitude of the changes in response with frequency.  That seems excessive to me.

Well, I looked at the F-F adapter that I found to be worse than the Chinese APC7s.  It's a Huber+Suhner.  I remembered Dr Kirkby had commented on them when searching for an inexpensive N F-F adapter: https://www.febo.com/pipermail/time-nuts/2013-March/074871.html

So I went digging for a datasheet.  This appears to be the one:

https://ecatalog.hubersuhner.com/product/E-Catalog/Radio-frequency/Adapters/31_N-50-0-2-133_NE

Mouser want nearly $28 for them!

Return loss spec'd at 15dB 2 to 8GHz, 30dB 0 to 2GHz.  So such variations in S11 aren't unheard of!

Orin.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #73 on: October 15, 2019, 10:58:33 pm »
Wow!   I'd love to put one on my 11801/SD-24 for a few minutes.  It would *not* look good.

I'm increasingly convinced that stopping at 3 GHz was a good idea.  Connectors and cables just get too expensive when you go above 3 GHz.  And propagation gets thoroughly weird.

I was going to put a bunch of time into the nanoVNA FW, but there are so many people doing so much there I decided to it's better to wait for the dust to settle.

I think I can contribute more by doing a comprehensive  tutorial on TDR and VNA testing of cables and connectors.  Seismic work involves switching between time and frequency so much I can generally look at one domain and draw a cartoon of the other domain.  But I was not taught that.  I just learned it. I *should* have been taught to do it.  It's not hard.

One can do much better than what Joel Dunsmore presents in his book and it does *not* require proprietary software.  Just a thorough knowledge of the Fourier transform and the 1D wave equation.

Have Fun!
Reg
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #74 on: October 24, 2019, 01:37:32 am »
More TDR porn.

This is the 11801/SD-24 with a good quality SMA-M/N-M cable.  This particular channel has a 23 ps rise time on the open APC-3.5 connector so it is able to test to about 15 GHz accounting for the pulser rise time and the input rise time.

From the bottom going up:

SMA-M/N-M cable only.  As can be seen the cable and the N-M connector are not a very good match above 12 GHz.  No real surprise there.

Cable and Astrolab N-F/SMA-F open.

Cable, Astrolab 16301 N-F/SMA-F adapter and a Radiall SMA-M terminator.

Chinese APC-7/N-F adapter attached to the cable with the APC-7 side open.  The reflection from the open APC-7 connector shows precisely the plane of contact.

A pair of the Chinese APC-7/N-F adapters back to back with the N-F open

The top trace is a pair of the Chinese APC-7/N-F adapters back to back with an Inmet 64671 Model 3070M terminator. 

The opens show the reference plane for each of the connections, though with a slight delay because of the 23 ps rise time.

The  capacitive discontinuity at the cable/connector interface is frequencies above 12 GHz.  So the  Chinese APC-7/N-F adapters I received are certainly acceptable up to 3 or 6  GHz.  Which is not to say the ones @orin received are.

I ordered an HP APC-7 short and open, a couple of HP 900A APC-7 loads and an APC-7 jumper. When those arrive I'll set up my 8753B/85046A and compare the frequency domain for these devices.

These plots show that the N interface is a bigger issue than anything else.

Have Fun!
Reg

Edit:  Added additional trace and rewrote the annotation.
« Last Edit: October 24, 2019, 03:05:12 am by rhb »
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #75 on: October 24, 2019, 06:01:50 am »

The  capacitive discontinuity at the cable/connector interface is frequencies above 12 GHz.  So the  Chinese APC-7/N-F adapters I received are certainly acceptable up to 3 or 6  GHz.  Which is not to say the ones @orin received are.


I could always send them to you along with the F-F adapter that measured even worse.  My curiosity far out-weighs the cost of a priority mail box.

I have an N connector gauge arriving Monday, so they'll be available sometime next week after I get around to checking them.

Orin.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #76 on: October 24, 2019, 02:21:32 pm »

The  capacitive discontinuity at the cable/connector interface is frequencies above 12 GHz.  So the  Chinese APC-7/N-F adapters I received are certainly acceptable up to 3 or 6  GHz.  Which is not to say the ones @orin received are.


I could always send them to you along with the F-F adapter that measured even worse.  My curiosity far out-weighs the cost of a priority mail box.

I have an N connector gauge arriving Monday, so they'll be available sometime next week after I get around to checking them.

Orin.

Mine also.  I'll be glad to split the shipping cost and pay for the return postage.  I'm really looking forward to getting my 8753B/85046A set up so I can do S11 and S21 in both the time and frequency domains.

Once I can get digits out of the instruments I'm going to write a FOSS  TDR/VNA program.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #77 on: October 26, 2019, 11:54:23 pm »
I got an APC-7 cable today and an HP APC-7 O/S. My APC-7 loads are still in transit.  Here is TDR using the APC-7 cable and one of the $16.50 APC-7/N-F adapters.

I'm now using an SMA-M/APC-7 adapter and the APC-7 cable.  This gives me >11 GHz BW to the end of the cable and back.  I have 26 ps at the unterminated 3.5 mm connector and 31 ps at the cable end.

The first image shows the unterminated APC-7 at the left, next are the Chinese APC-7/N-F and  N-M O/S followed by the Anritsu O/S.  The electrical length of the APC-7/N-F is 172 ps.  The rising trace with slightly greater overshoot than the partially overlying highlighted trace is the APC-7/N-F unterminated.

861796-0

The second image shows the delay difference between the Chinese and Anritsu O/S reference planes which is 40 ps.  The reference plane for the Chinese O/S is the end of the N-F center conductor which I measure with calipers to be ~4 mm from the end of the center conductor.  The reference plane for the Anritsu is the face of the N-F outer shell.

Other than the reference plane location I don't see a dramatic difference between the  $18 Chinese O/S and the Anritsu which sells for 10x the price.  However, the load is another matter.  But I'll leave that for later when I have my APC-7 HP 909A loads.

861800-1


Have Fun!
Reg
« Last Edit: October 27, 2019, 12:00:57 am by rhb »
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #78 on: October 29, 2019, 07:13:00 pm »
The N connector gage arrived and I took the Chinese adapters home to measure...

The kit came with a .197/.223 master gage, so measurements are relative to it.

One adapter N-F measured +0.007 and the other +0.009.  So 0.002" difference in location of the F contact.  These measurements are repeatable so I'm confident in the 0.002" difference.

The uncertainty is 0.0024 according to the Maury datasheet, so it's possible that one protrudes too far... I'll have to see how the HP adapters measure in comparison.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #79 on: October 29, 2019, 11:12:04 pm »
Would you post some pictures of the gauge?  I've got several lathes, so making one is not a big issue.  And far cheaper than buying them.  Especially as I have several  0.0001" indicators, so all I'd need to do is make a piece to clamp on the stem.

I'd like to see how they did the dial indicator to RF connector interface.

Reg
 

Offline orin

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Re: Testing RF connectors and cables
« Reply #80 on: October 30, 2019, 12:09:38 am »
Would you post some pictures of the gauge?  I've got several lathes, so making one is not a big issue.  And far cheaper than buying them.  Especially as I have several  0.0001" indicators, so all I'd need to do is make a piece to clamp on the stem.

I'd like to see how they did the dial indicator to RF connector interface.

Reg

Page 22 here: https://www.maurymw.com/pdf/datasheets/2Y-051.pdf or ebay #254392757267 or #143315065493 will give you an idea of what they look like.  The master gage (Maury Microwave's spelling) is the knurled piece.  Mine has a .223" hole and .197" protrusion.  I'd have preferred .207 for both... Shouldn't be too hard to make myself one on the mini lathe though.

The end of the indicator shaft is bored (I assume) to accept a male pin though no doubt you could machine an extension for a regular indicator.

There is one piece stays on the dial gauge, then there are two other pieces that you have to swap for M/F connectors.  Small set screws hold the pieces to the indicator.

I'll try to take pics of mine tonight.

 

Offline orin

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Re: Testing RF connectors and cables
« Reply #81 on: October 30, 2019, 10:52:46 pm »
Pictures attached.

From the left - adapter for measuring F connectors, adapter for measuring M connectors, master gage (protrusion mimics the N F 'pin', the hole the shoulder of a male pin).

863990-0

Basically, you add the adapter to the indicator, push the master gage on and zero the indicator.  Then push the connector of interest on and read the pin position relative to the master gage.

I also took one of the F-F adapters that measured worse on the 8753C home.  It showed 0.000 and -0.0015 relative to the master gage, so the F pin is recessed by 0.010" from the N standard.  This is likely to avoid problems mating with low quality connectors... but the return loss isn't great when you get to 3GHz.
« Last Edit: October 31, 2019, 12:54:33 am by orin »
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #82 on: October 31, 2019, 05:37:59 pm »
Thanks for the pictures.  Those should be easy to make on the mini-lathe.  A gauge for checking the diameter of the male pins is more difficult.   A boring bar small enough is *very* fragile.  It might be easier to modify the tips of a caliper to allow reaching into a connector shell.

One can buy precision tapered hole gauges for a few bucks to check the female sockets.  So that is easy.

My HP 909A APC-7 loads came yesterday, so I'll try to set up the 8753B/85046A and learn to use it.  Once I  can get digits out of the 11801 and 8753B I'll be able to directly compare TDR and VNA measurements of the same devices via Fourier transform.

I've gotten quite interested in the possibility of measuring and correcting for the imperfections of cal kits by solving an inverse problem.  Solving an analogous problem of removing multiple  reflections in the water column is routine in seismic work.

Also while it is routine to acquire impulse responses for the instruments and source, typically those can't be found when reprocessing old data.  So the art of estimating those from the data is highly refined.
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #83 on: November 03, 2019, 05:29:00 pm »
I did a quick comparison of various SMA-M jumpers to answer a question the the TekScopes groups.io list.

First  some short high quality jumpers and a Chinese ebay jumper.  Top trace is the bent solid tube then the black 90 degree and the Chinese solder filled braid at the bottom.  In particular note the amplitude of the connection to the SD-24.  Note that the connector on the Chinese jumper is better than the black jumper, but not as good as the salvage jumper.

865818-0

Then a pair of longer RG402 soldered braid cables.  The blue plastic covered jumper is the bottom trace in both the reflected and the thru pairs.  While not shown, the rise time on both the thrus is ~31 ps, so slightly slower than the rise time of the SD-24 pulser and the SD-26 used for the thru.  The SD-24 is ~29 ps for the upper channel and ~25 ps for the lower channel and the inputs open.
865822-1

Here's what they all look like.

865814-2

Edit:  It turns out I had the speed of the channels backwards.  Here are photos showing the heads with the input connectors open.  The first is the full step response.  The 2nd is just the reflection from the open 3.5 mm connectors.  The upper input is highlighted.  The time between the steps is 2x how long it takes for the signal go go from the sampling point to the connector.
« Last Edit: November 03, 2019, 06:41:27 pm by rhb »
 

Offline rhb

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Re: Testing RF connectors and cables
« Reply #84 on: April 12, 2020, 06:51:41 pm »
 i finally got some APC-7/SMA-F connectors from China.  One has problems with the threaded sleeve not extending.  Both appear to have been assembled using something similar to Loctite, so I'm not able to fix the sleeve issue.

For comparison I have some new Weinschel APC-7/SMA-F adapters.

968446-0

from the bottom:

Chinese RG402 SMA-M to SMA-M open

with Wienschel adapter open

with HP 909A APC-7 50 ohm load.


The vertical lines on the lower two traces show the plane of the open. Obviously the adapter is better than the cable termination.

The Chinese APC-7/SMA-F are clearly not as good.  Though they look as if they would be usable on an 8753x up to 6 GHz.

968450-1

The traces are as the Weinschel case, just different APC-7/SMA-F adapter.

I got 4 Weinschels for $100 and 2 of the Chinese for $30.

It's a bit harder to grasp what is going on with the Chinese stuff as a capacitive bump appears when the load is connected which is not there when it is open.  The fingers on the center conductor stick out farther than on the 909A loads or the Weinschel adapters.

Once I get all the cabling for my 8510C I'll update the TDR displays with VNA sweeps.

Edit:  I just noticed that the photo for the Chinese adapter has the bottom trace at 10 mV/div.  The other traces are 20 mV/div.  The capacitive reflection is about 0.04 for both the SMA-M on the RG402 cable and the -0.04 for the adapter.  Not great, but usable below 6 GHz.

I was able to disassemble the adapter with the non-working sleeve and fix it.  They appear to have been epoxied.  So a hot air gun should simplify taking them apart.

These are obviously not a good deal relative to the Weinschels, but you might have to wait a long time to get the sort of deal I found.

Addendum:

I just measured the rise time going through the two Chinese AP-7/SMA-F adapters and two Weinschels and both have a thru rise time of 35 ps.  With a single20"  RG402 jumper I measure 30 ps.

Reg
« Last Edit: April 12, 2020, 11:42:44 pm by rhb »
 


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