Noise on VCO voltage ramp

Finding an asynchronous glitch is hard on an analog scope.

Cheers

Phil Hobbs

It is to a major extent reproduceable, though, Phil. I can manually override the ramp and set a voltage which will show up a glitch in the frequency domain, which is a useful plus. You reckon it would be better to use a DSO for this? I do have one.

Frequency domain glitch? You mean a discrete spur?

A glitch is a transient time-domain animal. A DSO can save a single instance, or (with averaging) ignore the noise and asynchronous background and just reproduce the glitch itself.

Cheers

Phil Hobbs

In other words, it's picking up garbage from something in your shop (or cave). I suggest you start by moving or removing all the switching power supplies, wall warts, light dimmers, desk lamps, or gizmos within a few feet that might have a switching power supply inside. Next, cover the network analyzer with some aluminum foil to see if the glitches are being delivered via RF. My desktop wi-fi access point does that sometimes. If that also fails, drag your network analyzer and scope to another part of your shop (or cave), that's away from all the gizmos, and see if the problem persists. In other words, determine if the garbage is internally generated, externally radiated, or conducted via the power lines, clip leads, cables, etc.

If you really want to see the noise, glitch, trash, garbage, or whatever signal, set you scope so that the horizontal trigger is off and the horizontal sweep is free running. Then, slooooooowly vary the horizontal sweep until you see fairly stable glitches. If the noise is periodic, you should be able to see something.

I could have put it better I guess. What I mean is I'm seeing thin spikes in the RF output spectrum of the VNA at other frequencies than the desired one. I'm guessing these spikes are the result of unwanted transient voltage spikes present on the DC VCO control voltage which sweeps the RF output frequency from 4Mhz through to 1.3Ghz.

Well, Jeff has posted some advice for me on that aspect which I intend to try out tomorrow using my Tek DSO which is probably much better suited to the task than the analogue one I've hithertofore been using..

Yeah, I did try all that and it didn't help, Jeff. Also there's no glitches or noise when I pull the RF signal away, so at least it's not something coming from the spectrum analyser.

I *suspect* noise is somehow getting onto the DC VCO ramp voltage and manifesting as spikes in the RF output of the VNA accordingly. Fortunately, there is a jack in the back of the VNA for the application of an external VCO control voltage, so I plan to avail myself of that with a big old linear bench supply and see if the problem goes away.

Thanks, Jeff. Duly noted!

Which is a discrete spur. Gotcha.

If there's a forest of them, spaced at equal intervals, they could be coming from a periodic time-domain source.

Yeah, I'm fonder of analog scopes than JL is, but my beautiful Tek 2467 hasn't been used in quite awhile, whereas I use the 1180x, TDS 694C and TDS 784As most days that I'm in the lab. For manual driving, the 2467 has the best triggering of the lot, but it's very hard to beat a DSO for general use.

Cheers

Phil Hobbs

Oh, and btw, I know you like details such as this, I'll be using my Tek 2232 DSO for this measurement. I don't really get on with digital scopes, but seems like there's little alternative for this job. :(

There *are* equally-spaced spikes at the lower end of the spectrum, up to about 100Mhz, but thereafter they assume a far more random appearance - although they are NOT random as they reappear at the same place on the spectrum and at the same amplitude with each sweep. Plus there *is*lower-level truly random noise around 500-600Mhz - so looks like 3 different and unrelated faults to fix!! I do love my vintage test gear, but I often spend more time fixing it than using it! :(

I only have one DSO out of 13 scopes in all: the Tek 2232. It's nothing special; it was okay in its day I guess, but I'm sure John's cheapo Chinese Rigol could piss all over it in a head-to-head "test-off" lol.

Bad idea. The Tek 2232 has both digital and analog storage: I suggest you use it as a conventional analog scope for the untriggered horizontal sweep. If the frequency of the glitch source is stable, you should be able to "tune" it in by adjusting the horizontal sweep (with the trigger turned off). That works much better in analog mode than in digital. Better yet, go back to using your analog Tek 2465A for looking at the VCO control voltage.

Incidentally, I just noticed that you screwed up in your original posting. You provided most of the details, a few of the numbers, but failed to describe the "weird problem with the network analyzer" and what "spurious spikes in the frequency domain on the RF output" might be. Is that like a spurious RF signal appearing on the SA swept display? If so, are you picking up some junk from nearby radio or TV stations? If you can see it, what's the frequency on the SA? I can also imagine that your test setup or cabling may be causing these problems. A probe on the VCO line could easily pickup RF junk from everywhere.

All this goes back to my three commandments for getting a sane answer to questions on Usenet:

1. What problem are you trying to solve? Just one or two lines and leave the details for later.
2. What do you have to work with? What's your level of expertise and what tools and test equipment do you have available?
3. Where are you stuck and what did you do to get there?

I would guess(tm) that they are mixer spurs produced by the two oscillators used to produce the network analyzer RF output signal. A clue as to their relative amplitude compared to the RF output would be helpful.

Look at the spurious stuff coming out of the network analyzer. Now, vary the network analyzer frequency and see how they move. You can tell the order (harmonic number) of the spurious components by how much they move. For example, if you change the network analyzer 1 MHz and the spur moves 3 MHz, than you are dealing with the 3rd harmonic of one of the two input signals to the mixer in the network analyzer.

I also suspect that you might be picking up RF from nearby radio and TV broadcast stations. Install a small antenna into the RF IN port of your spectrum analyzer and see if there are signals at the same frequencies that you are seeing them on the network analyzer RF output. If so, you might check the shielding and grounding in the network analyzer.

Yep. Failure to observe my advice will result in divine retribution.

I wouldn't dare, Jeff. Thanks again for the tips; they will be followed up....

I'm afraid I don't have that option at the moment. The 2465A blew up a couple of days ago and has now joined my pile of "awaiting attention" junk. :( I didn't blow it up, by the way; it managed it all by itself.

I think you may have missed the clarification I posted. Here it is again:

"I could have put it better I guess. What I mean is I'm seeing thin spikes in the RF output spectrum of the VNA at other frequencies than the desired one. I'm guessing these spikes are the result of unwanted transient voltage spikes present on the DC VCO control voltage which sweeps the RF output frequency from 4Mhz through to 1.3Ghz."

I think that's a very valuable point there. One possible 'infection vector' for noise is the method I'm using for sniffing the VNA. As you may know, spectrum analyzers of this vintage were very intolerant of DC on the input socket. There doesn't even seem to be a margin of error! So any DC riding on the signal you're trying to obtain a spectrum for and you blow up the front end. And I have a bit of a complex when it comes to blowing up test equipment. I'm very, very cautious these days, so I've just used a few turns of wire to couple the signal from the VNA to the SA. This removes the DC risk, but does mean there's no shielding around the sniffer coil. And right next to these items there's my linear power supply powering up my Yaesu 857d. Now the linear supply should be fine, being linear, but I'm just wondering if the Yaesu, even in receive mode, might be generating some stray spurious that the SA is picking up somehow. I'll switch those items off when I try it again. My working practices are a bit slapdash I must admit, but no worse than the average hobbyist I would imagine.

Am 13.01.20 um 13:09 schrieb Cursitor Doom:

Probably that hybrid that sends all those 24xx to their heaven. Runs too hot. Don't expect to get a replacement hybrid.

Now back to the weird spectra that my Agilent 89441A presents under remote control.

And while I'm at it, how does that Wehnelt cylinder stuff work in an analog scope? I have a 200 MHz Iwatsu since > 35 years that I kinda love and that has a trace sharpness / astigmatism problem.

cheers, Gerhard

No, it's not that chip that's gone. And those are perfectly happy to run hot, the problem comes when people accidentally block the air vents directly behind the chip, in which case yes, of course it will blow.

Cursitor Doom wrote in news: snipped-for-privacy@4ax.com:

Divine intervention is what happens when your design fails to impress, and God decides to "let the smoke out" of your circuit.

My 2445 lost it's trace. I replaced the HV module. $56 from a guy in Greece. Works fine now.

Divine retribution and intervention are quite different. Divine retribution is where a lightning bolt from out of the clear blue sky strikes the transgressor down following a chronic failure to RTFM and follow the instructions. Divine intervention is where the participants in some technical or political discussion are unable to reach a suitable conclusion or compromise, and both need to be reminded who is the boss.

Also, none of my better designs have impressed anyone. In most cases, they attracted far more criticism than praise. Whenever I provide something to the design review committee for target practice, I consider it a very good sign if they take the time to tear it apart and offer their collective misjudgment. However, if the committee is passive and offers little criticism, I'm certain that I've done something fundamentally wrong.

See my comments on divine retribution elsewhere in this thread.

Yep. I read your comments after I posted my comments. I often read Usenet threads starting from the most recent, rather than in chronological order. That saves me the embarrassment of answering questions that were previously answered, but also causes me to miss important added information.

Yep.