Look at this 50 ohm terminator. This isn't right is it?

[OilsFan]

I bought this 50 ohm terminator today and noticed it looked awfully weird when I got home. Is this thing missing the internal insulation piece and center conductor?

[kise]

ohh, yes that is broken...
sad... ;(

[dentaku]

Is that an old MPC in the background of the photo?

[Richard Crowley]

Yes, the center pin and the insulator that holds it are both missing here.

Now if it were a cheap "F-connector" terminator, you might see the bare lead of the resistor used a the center "pin".
But certainly not for a proper connector like BNC or N, etc.

[OilsFan]

Is that an old MPC in the background of the photo?

Yep. MPC-2000.   

I guess I'll have to go back by the place and exchange the terminator for a good one...

[SL4P]

It also looks like it may have been a 75-ohm termination.

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

50-ohm connectors - often used incorrectly in crap video leads - DO have the nylon sleeve extending forward.

[Richard Crowley]

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

Right. I am careful to use true 75 ohm BNC hardware for my HD-SDI digital video cameras.

Note that the 75 ohm connector has the same center pin as the 50 ohm version, however.

The photo of the terminator in question appears to show not only the bare wire from the resistor, but also the resistor body itself (turquoise blue color). So the dielectric insulation is missing, also

[Electro Fan]

It also looks like it may have been a 75-ohm termination.

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

50-ohm connectors - often used incorrectly in crap video leads - DO have the nylon sleeve extending forward.

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

[miguelvp]

That's why I prefer mine color coded.

Purple 75 Ohms, Green 50 Ohms.

[Richard Crowley]

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

Yes, that is a normal 50-ohm BNC connector.  Exactly what we would expect to see on a 50-ohm cable.

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Tektronix didn't make much that could be properly characterized as "crap".

[Electro Fan]

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

Yes, that is a normal 50-ohm BNC connector.  Exactly what we would expect to see on a 50-ohm cable.

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Tektronix didn't make much that could be properly characterized as "crap".

This is what I was thinking  (didn't seem like if Tektronix put the nylon sleeve extending forward into the barrel that this attribute alone could cause a connector or cable to be considered crap).  Personally, I am a huge Tek fan.

[SL4P]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

[Richard Crowley]

Back in the analog age of baseband video, the mismatch caused by a 50-ohm connector on a 75-ohm cable was minimal to inconsequential.
But now in the digital age of HD-SDI digital video, it is much more critical to use proper 75-ohm connectors.
That is my first-hand experience dealing with analog and now digital video and RF for several decades.

[SL4P]

Richard  - Sounds like we come from the same workspace!

[tggzzz]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

[SeanB]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

OK, more info on this submarine cable then.

[tggzzz]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

OK, more info on this submarine cable then.

Thought you'd never ask http://en.wikipedia.org/wiki/TAT-7

Reason the impedance was fixed:

• polythene dielectric => fixed epsilon-sub-r, 2.25
• steel core strong enough to support cable on the 4km trip to the sea bed => fixed core diameter, ~10mm from memory
• cable vessel's hold has a certain volume, sufficient cable length required => fixed outer diameter, ~50mm from memory

There are no degrees of freedom left, and 61.8 ohms is what you get. (those figures give 64.4ohms, so they are in the right ballpark)

[SeanB]

ATT Techtrack on YT had a little series on those coax cables recently. It was very interesting on how they were made and shipped, along with the repeater design as well.

[Electro Fan]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

Hi SL4P, sorry - I should have been clearer in my post - I was just trying to show that a high quality provider like Tektronix used the particular cable/connector assembly technique under discussion without getting into a debate about what constitutes "crappy" (I tried to offset the word "crappy" with quote marks to help distinguish/differentiate it from Tektronix but I could have done a much better job of that.) Sorry again.  EF

[krivx]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

Hi SL4P, sorry - I should have been clearer in my post - I was just trying to show that a high quality provider like Tektronix used the particular cable/connector assembly technique under discussion.  EF

The use that assembly for a 50 ohm cable. Not 75.

[miguelvp]

I haven't had the need to get new RG-58 cables (50 Ohm) since I do use network cables for that purpose.

Is there any difference between using RG-58 intended for networking or RG-59 if you need a 75 Ohm cable but intended for video, equipment instead of using what the scope manufacturers supply?
For example using RG-58 instead of RG-142, a bit more attenuation and single instead of double braided, but does it matter that much?

I've only used monoprice for HDMI cables so far, but I do like the quality of their products so I bet they are a good source for competitive priced 50 and 75 ohms cables:
http://www.monoprice.com/Category?c_id=102&cp_id=10221&cs_id=1022101
http://www.monoprice.com/Category?c_id=102&cp_id=10203&cs_id=1020307

[Richard Crowley]

Baseband analog video tends to use 75-ohm as does digital video.
EDIT: And consumer RF (i.e. cable and satellite and antenna TV) uses 75-ohm.
Professional/scientific RF and the old 10Base2 coax ethernet used 50-ohm cable.

Actually, for very short jumper cables at baseband signals, there wasn't amy real problem wtih using 50-ohm coax for 75-ohm connections.
And back in the analog baseband video days, it was very common to see 50-ohm BNC connectors used everywhere, even for 75-ohm cable.
IME, it has only been in the era of HD digital signals (3G-SDI, etc.) that we are more conscientious about using 75-ohm BNC connectors.

[IconicPCB]

There is the questionof mechanical compatibility of 50  and 75 ohm hardware.

Precision connectors used to get made less precise if mated with incorrect impedance counterparts.

[Richard Crowley]

IME, 50-ohm and 75-ohm BNC connectors are mechanically interchangable.
I had been using (50-ohm) BNC connectors for several years before I even knew they were impedance-specific.
I had never even seen a 75-ohm variety BNC connector.

The only difference appears to be the dielectric plastic between the inner pin and the outer shell.
The 50-ohm variety appear to have the inner cavity nearly completely occupied by the white plastic dielectric.
While the 75-ohm variety appear to have almost no dielectric insulation in the "working cavity" of either the male or female BNC gender.

[IconicPCB]

They are almost compatible.

The centre conductor pin diameters are different and from memory the 50 Ohm pin is likely to damage the 75 ohm female part of the connector and deform it beyond 75 ohm spec.

[miguelvp]

I use the same T-connectors regardless if i'm using 75 Ohm cables and terminators, or 50 Ohm cables and terminators.
They don't make different T-connectors for different frequencies so they must be physically the same.

Any variation might be vendor specific.

[Richard Crowley]

Because of the absence of the white plastic dielectric mechanical support, the center contact of the female BNC connector is very fragile even when mating only with proper 75-ohm male BNC connectors.  I have had to use long-nose pliers to "reform" the contacts in many of my HD-SDI video gear.  75-ohm BNC connectors (of both genders) are significantly more fragile than the 50-ohm variety IME. 

I have three $300 lengths of special flexible RG-6 cable which I use between my cameras and the video switcher.  Alas, when we set up the mobile system last week, one of those expensive cables is dead.  Now I have to troubleshoot it to find out what went wrong.  The connectors are fragile to begin with, but any improper kinking of the cable will deform the eye pattern bad enough to kill HD-SDI digital signals dead.

I am ordering a 75-ohm version of the W2AEW pulse generator to use TDR to try to find the cable fault.
https://www.tindie.com/products/land_boards/cable-tester-fast-pulse-generator-50-ohm/

[miguelvp]

$300 cables? wow. And I complain when a coax is more than $1.50 per foot

[SL4P]

GEEK NOTE:
This is interesting to see a very minor problem that I had always been aware of - finally come into discussion.  It has always irked me that 'manufacturers' choose to use the wrong part - because it's cheaper or more convenient for them , and the dumb customers won't know any better... regardless of any chance of ''doing the job properly'' the first time. Finally !

This is often the reason first-world products are better made and more reliable than second- or third-world goods that are not specified or supervised by more experienced product teams.

The inferior products are not necessarily a bad idea or done wrongly - they are just made badly.

[SeanB]

If you want odd terminators look at waveguide terminators, which are basically a tapered waveguide filled with an epoxy and ferrite powder mix. They get nice and toasty in use with applied power.

[OilsFan]

OP here. Haven't been in the forum for a while but it seems some interesting discussion ensued. Just to clear up my case, that terminator in the photo I posted was 50ohm. It was stamped in the metal on the back and had a green label stating 50 ohms. I went and exchanged it for a new one and all seems fine now.

[macboy]

They are almost compatible.

The centre conductor pin diameters are different and from memory the 50 Ohm pin is likely to damage the 75 ohm female part of the connector and deform it beyond 75 ohm spec.

You may be thinking of N type RF connectors. Those are not mating compatible between 50 and 75 ohm versions.
BNC OTOH, differs only in the presence or absence of the dielectric in both the male and female ends. BNC was designed as 50 ohm and with the dielectric present, but it was discovered that leaving it out (using air instead) made it very nearly 75 ohms, so no other mechanical redesign such as pin diameter was necessary for the 75 ohm variant (unlike the N type).

Attached is a little picture I just took of a 75 ohm BNC coupler and cable with 75 ohm connector, plus 50 ohm variants for comparison. I have scores of 75 ohm BNC stuff because it tends to get dumpstered when Telcos redo their labs.

[IconicPCB]

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

[edavid]

Because of the absence of the white plastic dielectric mechanical support, the center contact of the female BNC connector is very fragile even when mating only with proper 75-ohm male BNC connectors.  I have had to use long-nose pliers to "reform" the contacts in many of my HD-SDI video gear.  75-ohm BNC connectors (of both genders) are significantly more fragile than the 50-ohm variety IME. 

This is why it's better to use 50 ohm connectors for non-critical 75 ohm applications, like analog video.

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

Do you have any evidence for this claim, like a manufacturer's statement?  As far as I know, it is not true.

[IconicPCB]

Personal experience.

I used to work in what was known as HP product type 01 ( analogue instrumentation maintenance and cal dc-40GHz) in early eighties .

Also Google listings point to this difference and use of different dielectric materials in 50 and 75 ohm fittings.

[macboy]

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

There is no physical difference between the mating surfaces of 75 ohm and 50 ohm. The slightly pointed vs. rounded tip of the pin seen in the photo is of no consequence, just a matter of preference of the manufacturer. This datasheet from Amphenol/Connex details all mechanicals of their 50 and 75 BNCs. There is no difference noted for any metal surfaces between 50 and 75 ohm variants. And this manufacturer states that "It is implicit in IEC 169-8 that 75 ohm BNCs made to comply with that standard will mate in a non-destructive manner with the 50 ohm BNC connectors described in IEC 169-8.... In over 15 years and a many million BNC connectors we have no first hand experience of incompatibility between 50 ohm and 75 ohm types, other than extremely rare (and very obvious) manufacturing faults."

Again, N type connectors are definitely not physically compatible between 50 and 75 ohm variants, and this leads a lot of people to believe, erroneously, that the same is true of BNC.

[IconicPCB]

Non destructive mateable does not mean identical.

While the catalogue listing may make that claim it does not support it with a dimensioned drawing of coaxial parts of the connector.

I would need to to see the dimensioned drawing in order to accept identical mechanical structures are involved.

[SL4P]

OK - now we have that out of the way...  make sure your connectors and cable are matched.    Solid core vs stranded, foil vs braid, diameter etc...

[flolic]

Ever heard of 5 ohm terminator? 

Few days ago I received SMA 50 ohm terminator from *bay (item no. 380319202322) which behave very strangely during some RF tests. Quick check with DMM showed it is actually 5.1 ohms resistor inside 

Words of wisdom, do not buy chinese crap, or if you do, triple check it...

[Howardlong]

Ever heard of 5 ohm terminator? 

Few days ago I received SMA 50 ohm terminator from *bay (item no. 380319202322) which behave very strangely during some RF tests. Quick check with DMM showed it is actually 5.1 ohms resistor inside 

Words of wisdom, do not buy chinese crap, or if you do, triple check it...

LOL! Do you have a picture of the mini tear down by any chance?

[flolic]

I will dissect it and post pictures when I get home 

[tautech]

I will dissect it and post pictures when I get home 

Hacksaw? Grinder? Big hammer would be my recomendation. 

Get some decent 50 Ohm Feed-throughs and be done with it.
It doesn't take much of a demanding waveform to demonstrate how poor the performance of tee's and terminators is. 
Compare and see for yourself.

Got my pair of Tek 1x & 10x many years ago, think I paid US$30 for them both off eBay and it sure hurt at the time.
But quality lasts and knowing that, the price is forgotten in time.

Or a scope with 50 Ohm inputs. 

[Howardlong]

Not sure if you'd be intending to use an SMA terminator for use as a scope terminator, more likely it's for an RF application.

[flolic]

Hacksaw? Grinder? Big hammer would be my recomendation. 

Haha, I would like to use hammer, but I also want to see what's inside.

Get some decent 50 Ohm Feed-throughs and be done with it.

I have one of these (http://uk.rs-online.com/web/p/rf-terminators/5464695/), it's enough for my scope needs.

[flolic]

As promised, tear down images 










To be honest, I expected worse than this. Center pin is soldered to resistor and plastic insulator is made of PTFE. It does not melt when heated with soldering iron.
Resistor ground connection is by contact pressure.
There is only one "small" problem, wrong resistor value 

[Richard Crowley]

There is only one "small" problem, wrong resistor value 

50 ohms is NOT a standard resistor value, even in the E192 series

[c4757p]

Richard: he said 5.1, not 51!

Also, SMA is usually used at rather high frequencies - no way in hell that cheapo axial is still a resistor above 100 MHz or so...

[Richard Crowley]

Richard: he said 5.1, not 51!

Also, SMA is usually used at rather high frequencies - no way in hell that cheapo axial is still a resistor above 100 MHz or so...

Looks like Green - Brown - Black to me.  = 51 ohms.  Who said anything about 5.1?  That would be absurd.
I would have expected better "termination" of the ground side of the resistor, though.   

If that resistor is 5.1 ohms, then is is marked incorrectly.

[c4757p]

Green-brown-black-silver-brown, dude! 510 * 10^-2 ± 1%

[flolic]

Yap, that's 5.1 ohm resistor 

[T3sl4co1l]

Richard: he said 5.1, not 51!

Also, SMA is usually used at rather high frequencies - no way in hell that cheapo axial is still a resistor above 100 MHz or so...

Nah, it's good up to several GHz at least.  Well, if it were a proper 50 ohm that is.

The coaxial construction means it's a lossy transmission line.

Tim

[c4757p]

Coaxial? What about the spiral cut?

[T3sl4co1l]

The spiral cut will act as a helical transmission line inside the wall.

It will have some dispersion due to being helical, worsening SWR at very high frequencies (wavelength on the order of the spiral length).  The spiral isn't very long, so it's in the GHz.

On second thought, the impedance is probably too low, causing a small hump in SWR probably in the low GHz, or maybe a bit below.  Mainly because the resistor end caps are right up in there.

Through hole resistors are frequently quoted as being terrible, which is absurd.  Many EE "rules of thumb" are.  Only a terrible engineer would take such statements at face value.

Tim

[c4757p]

Hmm. You're probably right about that. One irritating thing with axials is that the greater length often makes it inconvenient to minimize parasitics with them - was just building a pulse generator today for a scope calibration, and an axial resistor attenuator had just too much inductance for a clean pulse but an SMD one was fine. That, I suppose, was more due to the extra loop area caused by the shape of the part when applied across the transmission line than by the part itself.

[T3sl4co1l]

Yup.  It's all about how you apply it.  An axial 1/4W is probably a good match to 100 ohm transmission line over a ground plane, which is fine for thinner traces on a PCB, but not necessarily as practical for 50 ohm or other stuff.  A resistor floating in space (on some elevation, or suspended between terminal strips perhaps) can be even higher, which would be handy for, say, old Tek stuff which typically used ~1kohm system impedances (tube tech, at high voltages, for driving deflection plates).  High impedance, of course, is associated with inductivity against lower impedances, hence the 'rule of thumb'.

If you had really low impedance (e.g., some resistors in an attenuator, anything with a generally low system impedance -- very fast switching converters come to mind), larger, and sideways, resistors (e.g., 1218 vs. 1812) might be better.

Tim

[VK5RC]

I am surprised they didn't use a pair of 100 ohm smt resistors,  probably would have been cheaper!

[SeanB]

I am surprised they didn't use a pair of 100 ohm smt resistors,  probably would have been cheaper!

Harder to assemble though, it is hard to solder inside a hole, while the resistor is a push fit with no soldering needed.

[IconicPCB]

Back in the seventies HP used to make switchable attenuators using carbon composition resistors.

Resistors had adjustable capacitors around the body ( a technician would manualy form a sliver of foil around the body of the resistor  ) to peak the inductive components of the resistor construction.

No reason why a 50.1 ohm resistor inside the SMA shell would be any worse off than the resistor inside the attenuator assembly

[edavid]

Here's some data suggesting that an axial resistor is not good enough for an SMA terminator:  http://defenseelectronicsmag.com/site-files/defenseelectronicsmag.com/files/archive/rfdesign.com/mag/511RFDF3.pdf

[macboy]

Here's some data suggesting that an axial resistor is not good enough for an SMA terminator:  http://defenseelectronicsmag.com/site-files/defenseelectronicsmag.com/files/archive/rfdesign.com/mag/511RFDF3.pdf

The problem with that reference is that it talks about an axial leaded resistor on PCB, not used in a coaxial construction.

[T3sl4co1l]

Curious if the plot there is calculated from the assumed models, or actually measured (with smoothing or curve fit).

As mentioned, these things are all transmission lines on some level, so a lumped equivalent will only be reasonable in the asymptotic limit (frequencies below resonance, cutoff, 1/4 wave equivalent, or however you might describe it), and approximate near the first peak or valley.  A real transmission line structure exhibits harmonics or modes, which a first order lumped equivalent cannot represent.

So, you could spam more lumped components in there to make better and better equivalents, but... why?

I find it amusing that they propose a chip resistor's inductance is entirely due to the end contacts, and that this is three times greater than the inductance of a ball.

You can also place chip resistors upside-down, which is supposed to be of mild advantage over 10 or 20 GHz.

Here's a good modeling example which includes diffusion (skin effect) and one resonant mode (which should be a helical resonator mode of the winding).  The number of lumped components is much greater than the single physical component, as you can see.
http://seventransistorlabs.com/Images/CurveFit2.png
http://seventransistorlabs.com/Images/CurveFit1.png

Tim

[IconicPCB]

The "helix" is made up of resistive , lossy material and any resonance would have particularly low Q. I would not expect any significant salient features in the return loss measurement on a termination made from an axial resistor.
In fact I used to make termination resistors inside F connectors. My favourite tuning tool to improve return loss figures was a pair of combination pliers which I used to squash the connector ferrule.

On the question of lossy transmission line, used to use window putty loaded with iron powder to dampen potential instabilities on input and output ports of high gain broadband multichannel MATV amplifiers.

Do not get too stuck on the resistor construction, if it fits the bandwidth of interest.. You use it,... or try to nudge it into shape