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Old 7th Jun 2015, 11:20 am   #61
Chris Wilson
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Default Re: HP8568B calibrator noise / spurious signals

OK, I believe I am following this. I am going to have to wait until some connectors come, I have not had to deal with squirting things in and out of SMC connectors, I don't have the right ones to link up to there. Hopefully by mid week i will have.

I just want to add that I have bypassed the front panel attenuator and relay by injecting direct to the mixers or whatever they are in the alloy cases at the side of the RF section. The problem remained.

What I do see, and that strikes me as odd, is this. Feed in the 20MHz test signal, to the N connector. Set centre frequency to 20MHz. Set span to 990kHz. The signal "hump peak" is rock solid, no jitter. Tap the connector, or front panel. The signal is still rock solid. Change span to 1.01MHz. The signal peak jitters from side to side, sometimes as much as half a graticule division, and at the same time, more often than not, the noise is visible too. tap the connector or front panel and the whole shape of the signal jitters and wavers. Sometimes the noise is created too. Switching back to the sub 1MHz span and back again a few times and I believe this jitter and microphony changes, and occasionally it disappears. With the bottom and side off the case tapping the plethora of cavity oscillator and filter boxes at the side shows its this area that is causing the microphony. Whether it's the cause of the jitter, too, I am unsure. I alos see that the lower the frequency of input signal, the worse the noise and jitter appears, so if I use my sig gen to inject say 500Hz the noise, when it appears, looks far worse than at 500MHz. Similarly the microphony is worse the lower the frequency. But I suspect that is normal due to bandwidth?

The machine is a lot more stable if run for a few hours, so maybe temperature related?

I found the feed through that takes the voltage input into the "whatever" that changes span from sub to over 1MHz. The voltage, from memory, changes from 18.6V to 0.7V. There was no AC component I could see. What is happening, electro mechanically at this span change? I read the manual but cannot find a simple description of what changes occur internally at this point.


The microphony seems FAR greater than I would expect. I realise anything with cavities and probes or whatever will be subject to some.

Jeremy, would you mind doing this test on your machine if you get a moment, and tell me if you see this microphony at 20MHz centre, and just over 1MHz span please?


Thanks to David for the explanation of the various noise forms!
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Last edited by Chris Wilson; 7th Jun 2015 at 11:41 am.
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Old 7th Jun 2015, 12:28 pm   #62
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Default Re: HP8568B calibrator noise / spurious signals

I've got a lot of stuff to do today but I'll try your tests later this afternoon

I think the biggest difference between <1MHz and >1MHz spans is that this is where the analyser swaps between 'Lock and Roll' sweep mode and the mode where LO1 gets held in lock and the sweep is done by other means.

So we may be getting closer to the answer... It could end up being as simple as a duff/intermittent capacitor in the YTO circuits.

Therefore, the IF2 injection test can be put on hold for now. I think we need to look at how well the YTO circuit is performing in lock and roll mode.
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Old 7th Jun 2015, 12:34 pm   #63
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Default Re: HP8568B calibrator noise / spurious signals

Quote:
Originally Posted by Radio Wrangler View Post
1) Does the noise show up on the zero-hertz response - the blip you see at 0Hz even with no input applied? If so this eliminates the stuff before your first mixer.

Thanks for the noise explanation!

I have centre frequency zero Hz. Span 1MHz and the noise is coming and going as per usual. I think I have it set as you are suggesting. Thanks for that test suggestion!
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Old 7th Jun 2015, 12:50 pm   #64
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Default Re: HP8568B calibrator noise / spurious signals

You can read about Lock and Roll mode in the V1 pdf of the service manual.

Look up A6 YIG OSCILLATOR and scroll down a few pages to see an explanation of Lock and Roll mode.


Here's one for David to enjoy...

I googled "Lock and Roll HP8568" and found an interesting doc written by Siegfried Linkwitz. David knows this chap from his days at HP.

I've attached it below
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File Type: doc 8566A Spectrum Analyzer-2.doc (59.0 KB, 132 views)
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Old 7th Jun 2015, 3:10 pm   #65
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Default Re: HP8568B calibrator noise / spurious signals

Quote:
The microphony seems FAR greater than I would expect.
There should be a visible difference in microphony between <1MHz and >1MHz spans because the system is not held in phase lock during the sweep when >1MHz spans are selected. i.e. in Lock and Roll mode.

When the span is <1MHz the PLL system is in lock all the time (closed loop)and it has a fairly wide loop bandwidth of just over 50kHz.

Both the 1st YTO LO and the 2nd LO (cavity) are inside the loop so the PLL can clean up any mild microphony from these oscillators reasonably well.

So the PLL can self correct for mild microphony on narrow spans because it sees the microphony as an error that it can deal with. i.e. the loop is fast enough to correct out the majority of the microphony that falls within its 50kHz loop bandwidth.

But for the 1.1MHz span the benefit of the PLL is lost because the LO system is running in open loop during the sweep. So there is no PLL to correct the microphony from LO1 and LO2. So any microphony you see is the raw uncorrected microphony of the 1st LO and the 2nd cavity oscillator combined.

However, if you don't deliberately cause microphony, do you still see a difference in the sporadic 'noise/jitter' problem when you are on a 990kHz span compared to a 1.01MHz span?
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Old 7th Jun 2015, 4:21 pm   #66
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Default Re: HP8568B calibrator noise / spurious signals

Below 1mHz spans can be noisy (vertical single spikes across part or all of the frequency), but never any jitter side to side. Over 1mHz spans I often see irregular jitter side by side, and I also see the same noise spikes. The side to side jitter is not microphonic, I am touching nothing. It just comes and goes, seemingly randomly but as I say, this jitter is NEVER seen in below 1mHz spans.

It may have 2 faults, the noise and the jitter

Tell me if you get fed up with this! Thanks Jeremy.
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Old 7th Jun 2015, 5:41 pm   #67
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Default Re: HP8568B calibrator noise / spurious signals

I tried doing the microphony tests on my HP8568B and I did a quick youtube video of it

The audio is very low because I had to set the camera microphone such that it didn't pick up the overpowering fan noise from the HP8568B. So you need plenty of volume to hear my commentary. Sorry about this...

Hope it proves useful


https://www.youtube.com/watch?v=fRYB...ature=youtu.be
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Old 7th Jun 2015, 7:31 pm   #68
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Default Re: HP8568B calibrator noise / spurious signals

Very useful indeed. I seem to have a similar noise floor, and my microphony is very similar, so that's one less worry. Pictures speak a thousand words and all that... Thanks for uploading to YouTube, that was the best possible way to show my how your machine responds.

I only have a very poor resolution web cam, but my effort to show the current state of play is at:

http://www.gatesgarth.com/noise.avi


I think you'll need to download and save the file to play it.

20MHz span, just over and then just under 1MHz span, rest at default settings. The noise is quite visible, the jitter not quite as obvious, but hopefully clear enough. Thanks.
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Old 7th Jun 2015, 8:28 pm   #69
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Default Re: HP8568B calibrator noise / spurious signals

I've watched your video and it was very useful too!

I think it shows that the problem is quite severe and is related to frequency instability. Below 1MHz spans the PLL is trying very hard to correct for the effects of the instability but it can't always succeed. Especially if the jitter has high frequency components to it that lie well above the 50kHz loop bandwidth (or correction bandwidth) of the PLL. This jitter is happening too quickly for it to track and correct and it will show up as random spikes on the display as the display isn't quick enough to display it properly. So the jitter here will appear broken up and spiky on the display as the display can only show sample fragments of what is really happening.

The problem could well be a dodgy capacitor in the frequency control board A22 or maybe the YIG board A6. But that's just a guess.

It would be nice to look on another analyser and see (and rule out) the 1748.6 MHz LO2 signal at its internal monitor point as this cavity oscillator should run freely in both PLL and L&R mode. So if it is healthy it should not show any jitter like this. It should just look like a reasonably stable, free running cavity oscillator with maybe 100kHz of drift over a few minutes.

It may also be possible to tune the YTO manually with a clean steering signal to see if the jitter is still there. I'll reread the manual later as there may be a service link/mode that permits this easily.

But don't start swapping out any caps yet. By all means check them on your ESR meter but keep it all original for now until we find out for certain what is causing the instability.
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Old 7th Jun 2015, 10:55 pm   #70
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Default Re: HP8568B calibrator noise / spurious signals

There are tantalum capacitors in the YIG coil driver circuits and these boards run hot. The capacitors cook and go high in value. Dig the driver board out, look for discoloured capacitors, and give them all a go with an ESR meter. you may also find some low value tant in series with a resistor used as a damper on the tuning coil. This is a highly possible part for erratic tuning.

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Old 7th Jun 2015, 11:49 pm   #71
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Default Re: HP8568B calibrator noise / spurious signals

I agree. You could check the health of C26 and Q5 on the A22 board. Also see if the jitter and noise behaviour can be seen to change as you cross from a span of just under 20MHz to one just over 20MHz. I'm not sure if that jitter can still be seen on such a large span though.

The YTO's FM tuning coil is used for sub 20MHz spans along with the above Q5 and C26 on A22. I'm guessing that Q6 switches in Q5 and C26 etc on spans below 20MHz so it's worth seeing if you can spot any change in the jitter either side of a 20MHz span.
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Old 8th Jun 2015, 12:11 am   #72
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Default Re: HP8568B calibrator noise / spurious signals

The other thing to note is that if you look back at my uploaded image in post #56 the A22 module is the long one with the thin perspex cover over it and you can see there are lots of labelled peepholes to testpoints in the perspex cover.

Every one of those testpoints corresponds to a 'black star' test point on the A22schematic and so there should be a similar label on the perspex cover and next to each black star logo on the A22 schematic. So it should be possible to monitor the YTO control lines via these test points and if there is a fault here it may be possible to spot some jitter on the control or reference voltages on a scope?
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Old 8th Jun 2015, 12:57 am   #73
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Default Re: HP8568B calibrator noise / spurious signals

Either C8 or C10 on the power supplies part of A22 board was one of the tants I mentioned before as having a higher ESR than its twin. One of them was about .93 ESR, the other zero.

There was another tant on that board that read a bit high, too. C24 I believe. I mentioned that as well, but didn't specifically ID it. Can I temporarily use electrolytics, maybe higher value if I cannot match exactly the capacitance?

I'll have another look when I can find the time to get at it again. I'll also find "David's" caps and check them out, I seem to recall either himself or someone else talked of these in another thread. Are they also on the A22 board? Finally I will also scope the control pins. Some great info and possible leads there, thanks gents!
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Old 8th Jun 2015, 1:36 am   #74
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Default Re: HP8568B calibrator noise / spurious signals

I think the filter caps C24 and C26 just need to be fairly stable with low (and consistent) leakage performance. I think the ESR of a replacement won't be as critical here. C26 is in series with a big resistor in the circuit for one thing...

But because the leakage needs to be low and consistent I'd stick with OEM equivalent tants here.

Did you also check C18?
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Old 8th Jun 2015, 6:57 am   #75
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Default Re: HP8568B calibrator noise / spurious signals

I have an 8566B waiting for me to get around to finding new tants for its coil driver board. In this case it's bad enough to give 'YTO unlock' warnings and it changes as the box warms up.

One of the capacitors was £80 at Farnell!

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Old 8th Jun 2015, 8:44 am   #76
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Default Re: HP8568B calibrator noise / spurious signals

I think I am right in saying the HP8568 does not have a separate YIG driver board, like the HP8566? And the driver stuff is on A22?

However there is a none boxed PCB down near the YIG oscillator whose function I cannot see as yet.


Jeremy, in post #74, do you mean I need to find the Kemet axial leaded tants exactly as per original, or can I use a modern tear drop shaped tants? Cheers.
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Old 8th Jun 2015, 12:14 pm   #77
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Default Re: HP8568B calibrator noise / spurious signals

On the A22 board I find 2 leak tants. C24, part of the "YTO main coil tune driver" circuit, shown in the schematic on P52, bottom middle of TRM-V2.pdf

It's a 1.5uF 20V device, and on my Peak ESR meter it shows no capacitance and an ESR of 1.14 Ohms.


The other leaky cap is another tant, same board.

C10, a 2.2uF 20V cap. It's in the power supplies -15V line, and is shown on page 50, bottom left of the same TRM-V2.pdf file.

The cap reads with an ESR of 0.68 Ohms.

I tested both with one leg pulled, so no circuit influences were added.

I have ordered some tear drop tants, do either look likely suspects?

As you suspected Jeremy, on scans just below and above 20MHz, the jitter is hard to see, but I *think* it's still there above a 20MHz span
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Old 8th Jun 2015, 4:03 pm   #78
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Default Re: HP8568B calibrator noise / spurious signals

I got home early, here are some `scope screen shots, probe at X10 on the A22 board TP£, "FM Sweep". When the noise is bad I see spikes on this test point. Significant or useful?

http://www.gatesgarth.com/SA/noise1.jpg

http://www.gatesgarth.com/SA/noise2.jpg

http://www.gatesgarth.com/SA/noise3.jpg
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Old 8th Jun 2015, 6:47 pm   #79
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Default Re: HP8568B calibrator noise / spurious signals

Could be significant... I don't think they are meant to be there.

Can you connect channel 2 of the scope to TP10 and see if it has the spikes too? You should see a ramp on TP10 "Attenuated Sweep"

Maybe experiment with different span settings whilst doing this too.

But does TP10 have corresponding glitches too?
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Old 8th Jun 2015, 7:09 pm   #80
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Default Re: HP8568B calibrator noise / spurious signals

Also, there is a test point TP4 on the A6 module. Can you transfer channel 2 of the scope to TP4 on A6? Does this have the glitches on it?

See image below for the location of the A6 module and TP4.
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