Video Teardown, Repair and Analysis of an HP 8562B 22GHz Spectrum Analyzer - Page 1
Forum section: electronics / testgear

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 14, 2015, 05:57:01 pm
content: THE PROBLEM HAS BEEN SOLVED! SEE MY LAST POST IN THIS THREADYou can watch the video here: [52 Minutes]http://youtu.be/hkedOOij8X4More videos at The Signal Path:http://www.TheSignalPath.com

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 14, 2015, 11:07:06 pm
content: For the problem you have could be that the lo frequency need to be filtered?Make sense or could be some ipedence matching between the 1st and 2nd mixer.Any way good video.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 14, 2015, 11:12:12 pm
content: Quote from: vaualbus on March 14, 2015, 11:07:06 pmI looked at the output of the Mini-Circuit mixer (as shown in the video also). It is very clean...

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 14, 2015, 11:53:33 pm
content: Ah not watch with enough attention!Also in the previous video you do a therdown of the modular spectrum analyzer, Do it use the same 1st converter module? It is made around the same time!Because the weired problem have to be caused by the mixer.You use a double side mixer but the instrument could use a single side one we don't know.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 15, 2015, 06:43:32 pm
content: Quote from: vaualbus on March 14, 2015, 11:53:33 pmI don't think the mixer was single sideband, furthermore it also shows the spurs even without an input signal present.

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 15, 2015, 09:09:40 pm
content: So it is a very strange problem! Or the other option is that two resistor(somehow).....A good think is to try to calibrate the intrument and see what happens.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 16, 2015, 02:09:08 am
content: Quote from: vaualbus on March 15, 2015, 09:09:40 pmYes, that is possible. It seems that the issue is due to the interaction between the first and second converter.

author: MarkL
author_rank: Unknown
timestamp: March 16, 2015, 06:19:36 pm
content: On the more detailed block diagrams (not the one you used in the video) the 8562A, in addition to a YTF preselector, has two filters FL2 and FL3 between the first and second converters, one for high band and one for low band. Perhaps those help?How about trying an isolator on the output of the mixer, or watching through a directional coupler to see if something is getting back into the mixer?Also, did you verify that the internal LO output is ok? I think I only saw you testing with the front panel output. I guess since the high bands don't have the spurious tones it's probably ok.Also, with the ZX05-U742MH+ mixer the low end is now 100kHz instead of 1kHz.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:38:28 pm
content: Quote from: MarkL on March 16, 2015, 06:19:36 pmIt would be interesting if I had a good band pass filter around 3.9107GHz. Do you happen to know one on top of your head?I can probably try the directional coupler to see the interactions between the mixers better.Thanks for the comments!

author: G0HZU
author_rank: Super Contributor
timestamp: March 16, 2015, 10:16:53 pm
content: Quote from: Hugoneus on March 15, 2015, 06:43:32 pmAt 50:22 in the video you can see a fixed spurious when the analyser is tuned to 1GHz on range 1.I think this is due to 3*LO1 - 4*LO2 = 310.7MHz = IF2eg (4903.567*3) - (3600*4) = 310.7MHz.This spurious is therefore an issue caused by poor isolation between the mixers or their LO feeds. There should be a 4.9GHz roofing filter between these mixers (FL2?) which would hopefully improve the conducted isolation path up at many GHz.Also, the original mixer in the first converter will be carefully designed to minimise the level of terms like this. So your crude Mini Circuits mixer will not perform well here in this respect and you also need to make sure you don't degrade the mixer isolation via LO cable leakage/layout.Note: A decent analyser will also be able to do some 'tricks' to dodge spurious terms like this and maybe the very basic HP8562 doesn't do this within its frequency plan. But even without the dodging tricks the analyser should be able to suppress this term to a much lower level.Has someone taken the FL2 filter out of your analyser and not replaced it? FL2 would help with conducted isolation at many GHz between the two mixers but there are also other ways the LOs can travel across to meet each other so FL2 might not give the wanted improvement on its own.If you look closely, this spur at 50:22 at about 992MHz looks like it has a narrower RBW response compared to the wanted tone at 50:26. This is because this term sweeps the RBW three times faster because the 3rd harmonic of LO1 sweeps at three times the MHz/millisecond that the fundamental of LO1 tunes. So it's spurious term looks 'skinny' compared to the wanted 1GHz tone at the centre of the screen.Also, you should ideally have some form of basic termination or diplexer at the mixer output port. eg a directional filter would be good here or even a small attenuator. eg a 4dB attenuator would be good here to preserve amplitude flatness across the full sweep of range1 and it would help with mixer IMD too. But a decent directional filter would perform a lot better.

author: MarkL
author_rank: Unknown
timestamp: March 17, 2015, 12:50:44 am
content: Quote from: Hugoneus on March 16, 2015, 06:38:28 pmGlad to participate! I really enjoy your videos - great job.This obviously isn't a permanent solution, but as a test if you have a YIG filter laying around from one of your teardowns you could try powering up the heaters, tuning it to 3.9107GHz, and then insert it after the mixer. That would be a pretty good and narrow bandpass filter, albeit with a fair amount of loss (depends on the filter, but ones I've worked with are around 6dB).If that works, you could try to score a used FL2.Quote from: G0HZU on March 16, 2015, 10:16:53 pmLooking at diagram 5-7 in the service manual, it's pointing to FL2 as something that looks like, to me anyway, a piece of plain hard-line coax. Is it possible this connector actually has the filter integral to it?If so, the re-vamped circuit has FL2 removed if I'm seeing it right on the video. Perhaps that's the issue.I thought I read somewhere in the manual that FL2 and FL3, along with the YTF, were specific to the "A" version. But I can't find that now, so I'm going to have to say FL2 and FL3 is part of both "A" and "B" models.

author: G0HZU
author_rank: Super Contributor
timestamp: March 17, 2015, 01:01:40 am
content: If FL3 isn't in the HP8562B then it would be a major omission. However, I would expect FL3 to be inside the second converter and it would be common to the A and B versions but that's just a guess.FL3 should be a decent BPF centred on 3.9GHz but it probably doesn't have very good stopband attenuation above 10GHz and will presumably have poor attenuation at approx 3 * 3.9GHz as there could be a re entrant mode here. So FL2 would be needed to provide decent stopband performance to many GHz. I'm not sure what FL2 looks like but it may well be in a tubular format. So it could be skinny enough to look a bit like coax.

author: MarkL
author_rank: Unknown
timestamp: March 17, 2015, 02:16:05 am
content: It would appear that both FL2 and FL3 are of the same type.Here's the wiring diagram 5-7 below, and also a snippet from parts list which includes what FL2 and FL3 connects "from" and "to". The coax pieces (W's on the diagram) are listed on a different parts page.On Keysight's Find-A-Part page both of these parts are described as "FILTER LP CABLE ASSY".Interesting.

author: G0HZU
author_rank: Super Contributor
timestamp: March 17, 2015, 02:33:06 am
content: By FL3 I was referring to the BPF that is inside the second converter A13. So it won't appear on that diagram. I'd expect FL3 to be a cavity filter that takes up a fair chunk of the aluminium section of the second converter A13.I would expect FL2 on that diagram to look like a rigid bit of coax but it will probably be as thick as the SMA connectors. So I would expect to see two bits of normal skinny coax with FL2 fitted in the middle.But the diagram just shows one long bit of coax. So this all looks a bit odd to me!

author: albert22
author_rank: Regular Contributor
timestamp: March 17, 2015, 11:55:24 am
content: It would be interesting to see what is inside the 2nd converter. I guess that it should have a filter on its input and possibly a diplexer to accurately match the impedance of the 1st converter and to suppress the reflected power from the filter.I see that you tested the output of the minicircuits mixer with a spectrum analyzer and there were no spurious. I think that this worked because it was correctly terminated to 50ohms in the entire band, by the instrument. And the LO level was ok.AFAIK this not happens with a cavity filter, the rejected band gets reflected back creating mismatch. That is why a diplexer or another component (isolator ?) is needed.Perhaps the diplexer is inside the 1st converter and thus you are bypassing it.+1 in inserting an attenuator to test.Thanks for another good video and looking forward for a 2nd part.Regards

author: MarkL
author_rank: Unknown
timestamp: March 17, 2015, 02:26:42 pm
content: I think there's little doubt the 3.9107GHz filter labeled inside the A13 second converter (connected to J1) is a cavity BPF.FL2 and FL3 are low pass filters, so they're not going to help with fundamental reflections, but they should stop any harmonics from the LO from getting into and being reflected from the 2nd converter. Perhaps reflected LO harmonics are causing the issue in the mixer.I don't see any isolators, and there's no diplexer shown on the diagram unless it's implicitly built in.Snippet of the mixer and 2nd converter below. The whole page is also below in PDF. Full manual these were extracted from here:http://literature.cdn.keysight.com/litweb/pdf/08562-90062.pdfEDIT: I'm also noticing a little coax symbol grouped inside FL2 and FL3, further hinting that these might be some kind of specially constructed coax filter.

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 17, 2015, 09:53:31 pm
content: So reading the manual seem that the two filter are indeed the cables.But We don't know the filter type/frequency.If Some one have a high frequency network analyzer and one of this SA so he can meseaure the filter charateristics.

author: G0HZU
author_rank: Super Contributor
timestamp: March 17, 2015, 10:44:12 pm
content: If a VNA is not available you could crudely test the response of the FL2 cable using the LO signal cable into a 10dB (SMA) attenuator and then into the FL2 cable and then into a power meter. I think Shahriar has such a power meter. But be very careful not to bend or stress the FL2 cable.i.e. set the analyser to a very slow sweep from 0 to 2.7GHz and watch to see if the power meter reading starts to drop sharply at around a 4 to 4.9GHz LO frequency.I didn't have a detailed block diagram until now so the 3.9GHz cavity BPF I was referring to as FL3 is the one arrowed by the blue arrow in the diagram I've added below. i.e. the one inside the second converter that is in series with FL2. This arrowed filter will be a cavity filter and it will presumably not have good stopband performance up at about 14GHz because it will start to show reentrant modes up here and the stopband rejection of this filter could actually be quite minimal up here at 14GHz. So this filter alone will not be able to provide wideband isolation between the mixers.Hence the need for FL2. If FL2 is fully absorbed into the coax cable then I'd be very careful about bending it. Usually, filters like this are inline tubular types that look like a fat section of rigid cable with SMA connectors at each end.

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 17, 2015, 11:03:49 pm
content: Wish that we will solve the problem.By the way is a shame that you can download the volume 1 of the service manual on the keysight site and what they claim to be the clip.At the end the clip don't have any schematics and the service manual volume 1 have no information.On the other hand it have the calibration procedure. (Somethingh that it should be done after the repair)I see that someone sell the volume 2 and 3 for more than 100$!, and the clip for other 180$! Insane.Not even artekmanuals have the schematics!Why keysight not upload the complete service manual and clip documents!

author: G0HZU
author_rank: Super Contributor
timestamp: March 17, 2015, 11:21:54 pm
content: I watched the end of the video again and the frequency of all of the spurious terms pointed to by arrows in the image below can be easily predicted with a few sums.I have an old DOS based program I wrote for this type of thing way back in the 1990s and it predicts the location of all of the arrowed spurious terms in the image below if I enter the frequency plan for range 1 of the HP8562B.It also finds two more that don't appear in the image below but maybe these have been cancelled to be below the noise level by lucky phasing effects.What is surprising is how high these terms are even without FL2 so there must be a significant leakage path between the two mixers.

author: MarkL
author_rank: Unknown
timestamp: March 18, 2015, 12:18:04 am
content: Quote from: G0HZU on March 17, 2015, 10:44:12 pmFL2 clearly needs to be back in there, but for the extra fun you could use the analyzer to see a scalar response of FL2.Using the Minicircuits mixer, you could inject 310.7MHz into the mixer IF port, feed the front panel LO into the mixer LO, and then take the mixer RF out and send it through FL2 and then into the input of the analyzer.Using Band 1 (2.75GHz to 6.46GHz) you should be able to directly see FL2's rolloff. Normalize if desired.

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 18, 2015, 09:02:33 am
content: Quote from: vaualbus on March 17, 2015, 09:53:31 pmi have the 8562a option 26 and a 8510c 26ghz network analyzer and even a cnc milling machine to mill a filter.but taking apart a working 3500 euro spectrum analyzer is not something i look forward to

author: vaualbus
author_rank: Frequent Contributor
timestamp: March 18, 2015, 11:43:33 am
content: Quote from: PE1RKI on March 18, 2015, 09:02:33 amBut even it will not help teardown. We need to measure the filter frequency. So given the fact that the filter is in the coax cable a teardown will not help.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 19, 2015, 04:41:16 am
content: Everyone,I want to thank you for all your comments and passionate investigation! This is exactly the kind of discussions I was hoping would arise from my videos.I went back to the unit and found the coax cable that was actually a filter! I had bent it and was using it for the LO signal!I should have paid more attention. Even though it was bent the response still looks good! (Attached you can see both the cable and the response). I used this coax filter line between the first and second converter and all the extra tones went away! Hoooora!Finally you can see the picture of the instrument now measuring a 1GHz tone. Clean and beautiful. The other little tone that you see is from the synthesizer itself.Thank you all again and I hope that you see the results. Let me know!

author: MarkL
author_rank: Unknown
timestamp: March 19, 2015, 11:32:30 am
content: Wow! That thing looks just like a piece of coax! There's not even a bulge in it. I don't think anyone would have identified that as a filter at first glance.Glad it's working now! That was definitely an out-of-the box repair with the external switch and mixer.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: March 19, 2015, 01:18:17 pm
content: Nice!How do they use this rigid coax as a filter? What is in it?

author: SeanB
author_rank: Super Contributor
timestamp: March 19, 2015, 07:46:42 pm
content: You use a resistive wire as the centre conductor, and wind it as a loose spiral inside to make a distributed LC low pass filter.

author: G0HZU
author_rank: Super Contributor
timestamp: March 19, 2015, 09:29:51 pm
content: That FL2 filter does look very skinnyI was expecting to see something like this one below:

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 19, 2015, 10:50:32 pm
content: Quote from: G0HZU on March 19, 2015, 09:29:51 pmYes, what is amazing is that I bent it in two places!

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 20, 2015, 05:01:56 pm
content: Quote from: Radio Tech on March 19, 2015, 01:18:17 pmThere are a few patents on this actually. Wire wrapping, changing the dialectic thickness across the cable, etc.