DSA-815 analyser schematic required - Page 1
Forum section: electronics / testgear

author: 58kk90
author_rank: Newbie
timestamp: January 21, 2025, 06:23:55 am
content: Does anyone have any schematics for the Rigol DSA-81 analyser? particularly the RF input section.Mine has gone 'deaf' across the entire bandwidth, I have taken it apart and there is no evidence whatsoever of any burned up components or other damage nor can I see any dry joints with the microscope, I could do with a schematic of the input section so I can inject a known level signal at say 10MHz and follow it through the various switches to see where it is attenuated.The tracking generator output level seems to be spot on when I feed it into a Motorola RF test set, certainly up to 500Mhz which is all I tested, but the input is 26dB down across the entire analyser bandwidth.I wonder if the 'CAL' signal is at the incorrect level and that is what would be causing it to calibrate incorrectly, if anyone knows the level of what it is supposed to be I could measure that to eliminate it as a possible cause, the last time I used it it worked perfectly, it seems to have gone deaf after being stored for a few months of not being used.Anyone helpRegardsTony

author: Kean
author_rank: Unknown
timestamp: January 21, 2025, 05:48:51 pm
content: I don't think a schematic is going to be readily available, but a few others have documented their repairs.I would suspect one of the RF input switching or mixing ICs. Maybe it was exposed to ESD or a high strength field while it was stored.Check outhttps://www.eevblog.com/forum/repair/rigol-dsa-815-repair/And the PDF attached to this posthttps://www.eevblog.com/forum/testgear/spectrum-analyzer-rigol-dsa815/msg1059060/#msg1059060This video may also be relevant

author: 58kk90
author_rank: Newbie
timestamp: January 22, 2025, 04:16:00 am
content: Thanks for those, I did already watch that video and it did help to show the signal flow from the N socket through the front end, but the thread with the pdf list of RF items was most helpful.I plan at the weekend to strip it down again and feed in a known level RF signal, say 5MHz and then follow the RF path looking at the amplitude of the 5Mhz signal on the oscilloscope to try and see where it is low in level.Hopefully I can then then identify the culprit, which I hope is either the PIN dioes on the input, or the first of the Rf switches, I suspect the mixer and second RF switch will probably be OK as it appears to pass calibration successfully.Tony

author: 58kk90
author_rank: Newbie
timestamp: January 22, 2025, 09:44:53 am
content: A quick update, I took the 815 apart, and fed a 1MHz sine wave signal into the RF input N connector at -10dBm, I then measured that with the N connected at the back of the N socket using a 300MHz scope and probe and followed it all the way through the RF input section right up to the input track to the PGA, there was almost no difference in signal amplitude, and there was no distortion on the sine wave whatsoever, so I am guessing the actual front end is fine.The analyser scfreen displayed this -10dBm signal at -36dBm.The interesting thing is though, whilst it was apart, I also measured the 45Mhz calibration signal by feeding it through a 1n capacitor into an Anritsu analyer, guess what this isn't -10dBm which I have read should be the calibration signal level, it is actually -35dBm, so 25db down.Interesting that the anaylser is about 25 - 26dB deaf, and the calibration signal is also low.Apart from that I have been over the front end with the microscope and I cannot find any signs of damage, RF or physical nor and dodgy solder joints.Without another identical analyser to confirm the measure calibration level, or knowledge of how it is generated, or adjusted, I may well remove the SMD cap C195 which the cal signal goes through to the RF switch, and feed in -10dBm @ 45MHz from my rubidum standard locked signal generator and then run a cal and see if it changes anything.If anyone has any other idea's I am all ears!Tony

author: Kean
author_rank: Unknown
timestamp: January 22, 2025, 10:53:54 am
content: I'm not too keen on opening my unit up to measure it, but the PDF linked earlier does mention the cal signal as being 45MHz -10dBm at U101.The only software thing I can think of is if you have a correction factor applied, but I don't think those are persistent between reboots.So I'd say you are on the right track.I recall there being a hidden mention for some calibration options, but I think it was primarily/only for the tracking generator - and pressing the wrong button would wipe the existing cal data.Edit: see attached PDFs and mentioned herehttps://www.eevblog.com/forum/testgear/sniffing-the-rigol_s-internal-i2c-bus/msg380395/#msg380395- but probably not helpful for your issue.

author: 58kk90
author_rank: Newbie
timestamp: January 23, 2025, 05:07:53 am
content: Thank you for those documents, I have scanned through but as you say mainly for adjusting the reference which seems to be pretty much spot on frequency, as is the level of the tracking generator.Everything seems to work perfectly other than the thing beng 26dB deaf, and the calibration signal level being low by what appears to be the same amount, which surely cannot be a coincidence?Tony

author: RFDx
author_rank: Regular Contributor
timestamp: January 23, 2025, 09:48:21 am
content: Quote from: 58kk90 on January 23, 2025, 05:07:53 amIt does not look like a coincidence at all. Can you inject your own 45MHz/-10dBm CAL signal to see if everything is back to normal after the SA goes through the CAL routine? Did you probe the 45 MHz CAL signal path to check where ~25dB of additional attenuation is introduced?

author: TurboTom
author_rank: Super Contributor
timestamp: January 23, 2025, 11:19:32 am
content: If the internal calibration signal is too low, the automatic CAL routine should adjust the SA rather too sensitive than "deaf", provided it doesn't characterize the measurd signal as "unreliable / unplausible" and possibly completely skip this step. Yet, then the "Uncal" indicator should get displayed on the screen.Strange that some component further down the signal path appears to have failed here. From my experience, if something breaks on these relatively young instruments, it's usually related to overloading the input and thus limited to suppressor diodes or the first RF switches, as others have already suggested. Interesting case...Fingers crossed that you'll get it fixed.

author: 58kk90
author_rank: Newbie
timestamp: January 24, 2025, 04:41:01 am
content: @RFDx, As yet I didn't pursue where the 25 or so dB loss is occuring, having no schematic will make that a difficult thing to do I thing, I will try and see if I can follow it but knowing where it is generated will make life easier.@TurboTom, I thought the same, if the CAL signal was low, then it should calibrate what it thinks is -10dBM but is actually -36dBm, then it would make it more sensitive not less.There is no indication on the screen of uncal, as I said it was fine the last time I used it several months ago, it then went into a storage cupboard, and immediatley I used it a few days ago I noticed straight away that it looked to be reading low.I wonder if any of you that have the same analyser could tell me what your noise floor level is with no preamp on? apologies for the non-technical description, but I notice on my faulty unit, with the center frequency at 750Mhz and a full sweep, (i.e. pressing Preset) the baseline noise floor is about half way up the screen with nothing connected to the input, it doesn't look any more noisy, just higher up the screen, i'm sure the last time I used it it was much lower down the screen than that.I am not at home where the analser is so cannot describe it better than that without seeing what is actually displayed and the level shown on the screen.Over the weekend, i'll inject a -10dBm signal at 45Mhz and see what it cal's like with that just to rule out that being the issue, or you never know it may resolve it!Tony

author: 58kk90
author_rank: Newbie
timestamp: January 28, 2025, 02:35:31 pm
content: So quick update to this, I removed C195 (I think it was labelled that) which is a 10n, then fed 45mhx in through a 10n cap from my RF test set.Did a full cal and it made no difference at all, the anaylser is still about 26dB deaf.So I then fed a 1MHz signal in and looked at the RF envelope on the scope all the way through the RF section, I had done this before but did it again just to confirm, there is almost no loss and no distortion from N socket to the output of the programmable attenuator U103, on C193.Then, I did the opposite, fed 1MHz through a 10n cap to various points along the RF path, such as the inputs and output of the RF switches, then the input and output of the RF attenuator U193, it always shows as 26db lower than what I am feeding in.So I am stumped now, I have put the 10n back (c105) and boxed it all back up until I get change to dig deeper, but without a schematic I think it's probably a lost cause.Tony