Author Topic: Another ebay noise source  (Read 2877 times)

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Offline OwO

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Re: Another ebay noise source
« Reply #25 on: May 04, 2020, 03:58:00 am »
You really don't need a sine wave to accurately measure return loss at all. The harmonics of the RF and LO signals mix to create harmonics in the baseband signal, and this is all rejected by the DSP filter which has nulls exactly at the harmonics. Even with a shitty mixer like the SA612 the nano still achieves superior performance compared to anything you will get from a SA + TG + return loss bridge, below 200MHz at least. With the V2 I've checked absolute accuracy by measuring some 5ohm, 10ohm, 200ohm, 500ohm resistors. I may publish the results later but with a SA+TG you can not even hope to get +/-3dB accuracy of return loss because your measurements are going to be ripply (due to the cables) with no way of calibrating it out.
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Offline OwO

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Re: Another ebay noise source
« Reply #26 on: May 04, 2020, 04:26:01 am »
The nano (both -H and V2) easily does <0.1dB absolute return loss accuracy near the 0dB operating point, and can show return loss down to -30dB (subject only to calibration kit limitations). Try doing that with the TG or noise generator approach. If your goal is to measure return loss, a SA, noise generator, etc are simply wrong tools for the job. The purpose of a SA is to look at signal spectrums, for example to see what kind of spurs your transmitter has. A TG on a spectrum analyzer is useless when you have access to a VNA of any kind.
« Last Edit: May 04, 2020, 04:33:40 am by OwO »
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Offline rhb

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Re: Another ebay noise source
« Reply #27 on: May 04, 2020, 05:36:54 am »
This is the 8560A with a 60dB.com bridge and the noise source this thread is about and a small partially collapsed whip antenna on the DUT port of the bridge,

[attach=1]

Test setup:

[attach=2]

I've swept filters and crystals many times w/ my 8560A.  Hasn't seemed useless to me.  A VNA might be better for some things, but if all you want is the log magnitude transfer function an SA w/ TG  works quite well.  Log magnitude transfer function is scalar network analysis.  You are not using the phase angle information.  So any tool that provides that information will do just fine.  An SA & TG or noise source *are* scalar network analyzers.

Aside from the nanoVNA I've also got an SDRplay RSP2, VNWA xaVNA, 8753B and 99,99% of an 8510C.  I'm short one extension port jumper.  And a couple of RTL-SDR dongles on the way.  So I'll do a bunch of comparisons eventually.  I'm sort of curious what TDR of the antenna w/ the 11801 & SD-24 looks like.

Have Fun!
Reg
 

Offline rhb

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Re: Another ebay noise source
« Reply #28 on: May 05, 2020, 03:53:45 pm »
Well I have attempted using the noise source in lieu of a tracking generator to make return loss measurements.  No luck, even though a YouTube video shows differently.  I connected the noise source to a return loss bridge, HF antenna and used a spectrum analyzer and a SDR-dongle to view the response.  The noise source is definitely working but I see no differences when I attach the antenna.  I am using a 50 Ohm dummy load as a reference. 

I will try to take a picture of how I attached all the components together and will not rule out the fact that I messed up hi hi.  Thanks.

You need to attach the noise source to the IN port, the SDR to one of the two ports at that end of the RF bridge and a 50 ohm terminator at the other end.  The photo in the previous post shows the setup.

you can not even hope to get +/-3dB accuracy of return loss because your measurements are going to be ripply (due to the cables) with no way of calibrating it out.

If you have that big a ripple problem it is because you've got *very* bad connections and are getting reflections at the connections.  Those can certainly be calibrated out with an SA & TG.  Not quite sure how you'd do it with an 8560A and noise source.     With an SDR you'd save a sweep with a good load, sweep with the antenna and difference the two.  A VNA is not immune to the problem, though a proper cal will automagically correct it for you.

Have Fun!
Reg
 
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Offline rhb

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Re: Another ebay noise source
« Reply #29 on: May 09, 2020, 01:30:35 am »
OK here are screen shots using a V 3 RTL-SDR and Spektrum.  The antenna is not extended the same as when I did the test with the 8560A.  So the dip is at a slightly different frequency.

Procedure:

Connect RF bridge as shown in the 8560A test with a 50 ohm termination on the DUT port

Set relative mode per the Spektrum UI

[attach=1]

Remove 50 ohm terminator and attach antenna

[attach=2]

The ripple is reflections at the connection to the RTL-SDR and the RF bridge.  That is proved by the period of the ripples.  The length derived from the 2-way travel time in the cable from the RTL-SDR to the RF bridge is 2.8 m which  is the measured length of the cable. The RTL-SDR is not very close to the 50 ohms of the cable which is a good quality cable. As demonstrated in the 8560A case, the RF bridge is pretty good. 

The ripples were 2-3 times larger before I tightened the SMA connectors with a torque wrench.  I also put a 4 dB MiniCircuits SMA attenuator on the RTL-SDR to reduce the mismatch.

It's important to note that the amplitude response of the RTL-SDR is no where close to flat.  So normalization (aka relative mode) is critical to getting a usable result.

Have Fun!
Reg

Here are the same tests using the V2 RTL-SDR

[attach=3]

[attach=4]


I don't know why the V 3 dongle is a poorer match, but I'd guess it's the bias tee.
« Last Edit: May 09, 2020, 05:01:13 pm by rhb »
 

Offline galileo

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Re: Another ebay noise source
« Reply #30 on: June 25, 2020, 12:22:22 am »
Bridge: http://g3ynh.info/zdocs/bridges/appendix/self_eval_refbrig.pdf
Si5351 as "signal generator"
Linrad + HF radio or RTL SDR as a receiver.
~60dB repeatable RL measurements without issues.
Keep in mind that these types of bridges are calibrated to a load and common mode noise is a huge issue.
 

Offline fmkit

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Re: Another ebay noise source
« Reply #31 on: June 25, 2020, 11:25:09 am »
few posts bac seeing ebay   D6 38-4400 SA/TG I laughed about included  bridge,  it's not usable at all !
I have another similar  NTW4400 board  modified v=w/simple bridge using smallest 1:1 transformer and resistors, we have 3 unused TG RF-out so I used one for permanent  onboard bridge while keeping original functionality. The bridge almost flat to 1950MHz and here is test of UHF antenna using both bridges, red line is my DIY bridge while green is ebay gimmik (note upper SMA ground connection ;) )
 

Offline mio83

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Re: Another ebay noise source
« Reply #32 on: June 25, 2020, 02:58:04 pm »
Regarding a 'hobbyist' way to produce signals, the HackRF is not too bad.

See the following two pictures for a spectrum display of a 500Mhz and 1Ghz signals.

As you can see harmonics are -35dbc. Furthermore in can do all sorts of modulations (being a SDR) and it works from 1Mhz to 6Ghz.
I have done a few videos about the HackRF as a signal generator in my youtube channel (see, e.g., ) if you are interested.


@rhb : I am interested in the MoRFeus. I asked in their forum and they are going to sell a new batch this summer. Could you please use your very cool equipment to do a proper review of it and experiments?  :-+  :bullshit:
 

Offline rhb

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Re: Another ebay noise source
« Reply #33 on: June 25, 2020, 04:10:53 pm »
I *think* I posted some spectra made with my 8560A, but perhaps not. The MoRFeus produces a *lot* of odd order harmonics.  IIRC it's a single ADF4351 so to be useful you need an external filter and even then it has issues at the low end of the range with the closely spaced harmonics.  I'll run some tests this evening.

I've got a couple of graduate level monographs on analog filter design coming and will be building a bunch of filters and testing them.  No particular schedule for that.  I'm severely ADD and randomly wake up and do something completely different.  Just can't play and repair guitars, design, build, repair and test electronics and do precision metal working easily in the same week.  Something always gets on the back burner. for weeks or months.  Usually cleaning house and shop, though I'm taking advantage of the mild weather here to clean my shop.  Driven by the desire to get 4 5ft high stacks of test gear out of my dining room.  But to do that I've got to build a new lab.  Hence the priority on cleaning up the shop.

Have Fun!
Reg

Have Fun!
Reg
 

Offline mio83

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Re: Another ebay noise source
« Reply #34 on: June 25, 2020, 08:10:29 pm »
I *think* I posted some spectra made with my 8560A, but perhaps not. The MoRFeus produces a *lot* of odd order harmonics.

Yes, sorry. I meant an overall overview, especially of its mixer functionality (which is the main goal of the moRFeus).

But basically I wanted to motivate you do keep posting spectra screenshots and investigations, I enjoy them :)

Thanks!
 

Offline rhb

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Re: Another ebay noise source
« Reply #35 on: June 25, 2020, 08:27:17 pm »
OK, understood.  I've not fooled with the mixer yet.  I'll look into using it as a tracking generator with the 8650A using the 1st LO output.  That would be of considerable interest to people who don't have the TG option in their 856x.  IRC it has a built in bias tee which makes me nervous without a good DC block to protect my 8560A from confused user.

However, I just got a Hugen nanoVNA-H 4 in the mail so this evening's play time is spoken for ;-)  It's *really* impressive for under $65 delivered.  Very nice box and case.  The larger screen is a big improvement.

I came across this the other day by way of HackaDay:

https://www.elenco.com/product/amfm-radio-kitcombo-ic-transistor/

and ordered on.  I think that having the schematic on the PCB is really cool.  Not sure if/when I'll build it, but as a tool for teaching someone radio electronics it was just too good to pass up.  I've long wanted to have a set of boards with 2N3904 and 2N3906 transistors in all the various amplifier topologies with the schematic underneath so one could look at the circuit while making measurements.  I wouldn't go to that much trouble for a full AM/FM radio, but for $45 I'll surely take one just to have if I ever have a chance to teach someone radio electronics.

Have Fun!
Reg

BTW I'm eyeball deep in "Scope Wars"
 


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