Is my specgtrum analyzer working properly? - Page 1
Forum section: electronics / testgear

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 26, 2020, 02:40:47 am
content: My ebay HP 8590d SA came in. Been spending time messing with it.I am slightly hampered by the 40 MHz limit of my Siglent 2042x. It is the highest bandwidth generator I own.It seems that the analyzer and the frequency generator don't completely agree. Also, the lower the frequency, the less they agree.After being on for a couple hours, and doing a self-cal, a 1 MHz 0 dBm output from the generator reads as a 715 KHz -4 dBm signal on the analyzer. The generator is set for 50 ohms sine wave. 3' SMA cable between the instruments.A 20 MHz 0 dBm signal reads as 19.92 MHz .53 dBm (power is HIGHER than what I thought I was sending!)A 40 MHz 0 dBm signal reads as 40.045 MHz, .2 dBmAll readings taken using 10 kHz RBW, 2 MHz span.As time passes, the analyzer seems to drift CLOSER to spec at all frequencies. This drift happens over the course of hours.I have always found my generator to compare properly with my other gear. I think it is reasonably in spec.As this is my first SA, I have questions. Is this normal? Especially that it is less accurate at low frequencies? Would this be considered badly out of cal, or just whats expected from an old instrument? It was sold as "working, but uncalibrated". I got it for $300, so at that price, is the cost of a proper cal worthwhile?I should mention I got the thing mostly for teaching myself the rf area of the hobby. I also am eyeballing frequency counters on eBay, so I probably don't need to have the SA be the thing telling me my exact freqs.Otherwise, the thing seems to work flawlessly, and I am very happy. Thoughts?EDIT: in the 15-20 minutes it took to write this, my 40 MHz signal went from reading 40.045 , to 40.01. Unit has probably been on for 3 hours.SECOND EDIT: Another 20 minutes. Now reads 40.005. Still drifting closer, rate of drift seems to be slowing. Curious if it settles down.

author: bob91343
author_rank: Super Contributor
timestamp: August 26, 2020, 03:15:25 am
content: What is the spectrum analyzer time base? Is it in an oven? If so, it needs hours to settle. Once settled, you should standardize it with WWV. If not in an oven, it's likely a TCXO which will drift somewhat with warmup depending on how good a unit it is.Read the specs on the SA time base.As for the amplitude discrepancy, expecting even 1 dB accuracy is a bit much. If you are within 3 dB it's probably okay, or about 30% voltage error into the proper impedance.In fact, many signal generators and spectrum analyzers develop errors at the ends of their frequency ranges. Again, read the specs and see if it's out or in.

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 03:17:01 am
content: This sounds like the internal frequency reference just may be a tad out when cold - which is somewhat expected.The frequency readout accuracy specification of the 8590D without option 013 seems to be low: +/-(5MHz +1% of span).I'm assuming is doesn't have option 013.Do you have a trustworthy external 10MHz reference - e.g. from an ovenized oscillator or GPSDO? A GPSDO is pretty cheap these days.I agree with bob re the amplitude accuracy, especially when neither unit is calibrated, plus unknown cable losses.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 26, 2020, 03:28:44 am
content: I actually have no idea if the reference it is a temp controlled XO or not. The thing is stock, no options. I do think it was a lower end analyzer in HP's line (if you can call an $8000 dollar unit in '93 low end!)I don't have a quality reference. Maybe I need to look into that.Sounds like it is working properly from what you guys say. Better than freq spec if Kean''s numbers are right!

author: fourfathom
author_rank: Super Contributor
timestamp: August 26, 2020, 03:40:49 am
content: Quote from: Dan Moos on August 26, 2020, 02:40:47 amThis amount of frequency error isn't due to a slightly-out-of spec or "tad out" internal reference oscillator. Something else is going on.

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 03:51:21 am
content: Quote from: fourfathom on August 26, 2020, 03:40:49 amI though so too until I looked at the specs of the base model 8590DSeehttps://www.testequipmentdepot.com/usedequipment/hewlettpackard/spectrumanalyzers/8590despecs.htmQuote

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 04:23:13 am
content: I'm not sure if it exists on the 8590D (can't find the manual), but there is a self-calibration procedure in the 8590E series user manual that uses the CAL OUT conenctor. This can self calibrate both amplitude and frequency without any additional equipment, just a low loss cable. It recommends a 30 minute warm up before attempting any measurements or running calibration. The self-calibration probably won't help with the poor frequency reference specification, but it is worth trying. Make sure you check the user guide to understand the procedure.Quote from: Dan Moos on August 26, 2020, 02:40:47 amReducing your span and RBW to home in on the signal may achieve better readings, at least on the higher frequencies and assuming the generator is any good.If you feel like getting more frequency precision, maybe get something like this as a permanent lab frequency reference (I can't vouch for these particular items)https://www.ebay.com/itm/262038924752orhttps://www.ebay.com/itm/402268690713You can get cheap (~$20+) second hand 10MHz OCXO's which will be better than the internal reference, but you will generally want a GPSDO to check and adjust them.

author: rf-loop
author_rank: Super Contributor
timestamp: August 26, 2020, 07:05:00 am
content: Quote from: Kean on August 26, 2020, 03:17:01 amOf course many freq reference are good, better, more better and best etc..But example this place, checking roughly this SA as OP do with low frequency like with SDG2040X generator, its frequency accuracy is well enough in practice to see that his SA is far off and need adjust/repair.SDGInitial accuracy -1 +1 ppm 25℃ / -2 +2 ppm 0~40℃1st-year aging -1 +1 ppm 25℃10-year aging -3.5 +3.5 ppm 25℃And these are nearly as worst case specs. In my experience these are usually better. Except I do not have any experience about its 10 year aging.Last time I have checked one using HP GPS DOCXO and after factory it was around 100ppb off. Of course individual SDGs can be what ever inside specs but typical is much better than specs limit values.Previously I have used same GPS added with 2 others for more trust for calibrate stand alone Rb oscilattors to inside these factory specs.And normally Siglent SDG2kX stay inside +/- 300ppb from gold to warm repeatedly. I do not have here more perfect details, im thousands of km from my workshop.1ppm is 40Hz when use 40MHz. This SA do not even have very narrow RBW.This SA displayed 1MHz error is - just terrible like totally out of order even if there is simplest uncompensated xtal osc. And even it do not explain why proportional offset is less with higher tune frequencies. What can next do is find cal/service manual for this SA without any "blind" tuning this and that.Also for check SA internal ref. Oh well... just listen it and you get value so do not need speculate if it is off and how terrible off. Ref offset do not explain nonlinear offset. If ref is 1% off all f are 1% off. If this is not case reason is other.Yes I have been one "time nut" in my previous life but after I understand bit more I will say, there is perfect accurate time only in case when we have just one clock. Yes time nutting may be fun but mostly only for time nutting itself. Can not anymore think what ever it matter if 10MHz is 10ppb or 0.01ppb off or not. So what. Of course if one do very extremely narrow BW things like ultra slow communication deep inside noise it may matter that freq is roughly ok... or if think clock have run 0.1s off after ten years or 1000 years. My internal clock run every day faster and faster, every day I surprise how this day feels shorter than yesterday. Practical meaning in electronics is very very close nonsense - mostly... even if it is fun.So, OP can quite sure trust his 10MHz from SDG is between 9999980 and 10000020 Hz. And mostly error is much less what is well enough accurate to tell that SA cal or something is out of order.

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 07:25:39 am
content: Quote from: rf-loop on August 26, 2020, 07:05:00 amAccording to docs those accuracy specifications only apply with option 013 "Frequency Accuracy Enhancements".Without that option the 8590D appears to have pretty poor frequency accuracy, per my post above - around +/- 5MHz.Maybe I'm reading the specs wrong. It seems pretty clear, but I've never owned one of these.

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 07:28:45 am
content: Oops, sorry you were quoting the SDG frequency accurancy specs.As mentioned above, these are the specs for the 8590D:Quote

author: TheDefpom
author_rank: Frequent Contributor
timestamp: August 26, 2020, 09:24:33 am
content: My 8590 is about 3MHz off after warming up... and drifts a lot for hours, I don't think it is great in that regard but it seems normal (unfortunately).If someone does know a way to get these things more accurate and stable it would be good to hear it, as apart from those two issues they are nice old units.

author: Kean
author_rank: Unknown
timestamp: August 26, 2020, 10:33:42 am
content: Quote from: TheDefpom on August 26, 2020, 09:24:33 amGood to have another data point that matches the specs.From looking at a few photos online, and an 8590D User Guide I found on xDevs, it appears that the EXT REF IN and REF OUT connectors are not standard. They are apparently only included with option 013, in which case they are normally connected together via reference connector 1250-1499, but you could feed in 10MHz from a GPSDO to potentially get even better accuracy.No joy without option 013 (or R13 the retrofit version) I'm afraid. You're unlikely to find one, so just keep it in mind as a limitation.

author: bob91343
author_rank: Super Contributor
timestamp: August 26, 2020, 06:49:58 pm
content: A suitable 10 MHz reference should be inexpensive, and you can calibrate it well enough from WWV. If there is a reference output you can measure it against WWV or any decent counter.Apparently the stock reference is not great so it might be a good idea to do something about that.

author: cgz2001
author_rank: Contributor
timestamp: September 02, 2020, 02:19:21 am
content: Dan Moos, I too have collected one of these units in the past week, though mine was saved from the trash and probably has an amplitude calibration issue with it (I am occasionally failing "RES BW AMPL FAIL").However, after allowing the unit to remain on for around an hour, I can say after calibration is performed successfully the frequency measurement is fairly spot on. I am applying a 30 MHz sine wave at 0dBm from a rigol signal generator and the HP 8590D sees exactly 30.00 MHz. In the past day I have seen it measure the correct frequency +- 0.4 MHz, though it may not have been calibrated correctly. Still, yesterday it did not seem to move over time (around an hour). Let me know if there are any comparison measurements you would like to make.

author: thinkfat
author_rank: Unknown
timestamp: September 02, 2020, 09:04:22 am
content: Quote from: TheDefpom on August 26, 2020, 09:24:33 amIf these have OCXO timebases, they're often affected by retrace behavior, i.e. the OCXO takes a long time to settle after a long turn-off period, and also settles at a different frequency.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 05, 2020, 10:14:43 pm
content: Ok, I have some questions now that I have had a week to play with the thing.First, I don't know what it has for a frequency reference. I don't believe its oven controlled though. Does anyone know specifically what an 8590D has stock for a reference (no options installed)? Would it be obvious to me if I opened the unit up and looked?Next question:Outside of finding the specific part, are there aftermarket references that are compatible with these things? Do plug and play options exist? I imagine if I did want an OCXO, I'd have to conjur a way to get the correct voltages to the oven.Third. As Kean mentioned, the displayed frequency accuracy of the thing according to the calibration manual is +/- 5 MHz + 1% of the span. That seems insanely bad to me to the point I feel I must be misinterpreting the spec. If that is indeed what it can do, mine is performing MUCH better tan spec. Its usually 10's of KHz off, and sometimes low hundreds of Khz. I just can't believe I'm reading that spec right! cgz2001's unit seems to imply that they perform much better than this.Quote from: cgz2001 on September 02, 2020, 02:19:21 amEDIT: just noticed cgz2001's post says it is +- .4 MHz , which is what mine is doing at it's worst. It's usually better. Maybe they just are that badNext question: Drift versus accuracy. I am willing to use my Siglent generator as a 10 MHz reference to tune the oscillator. I need no more accuracy than what the Siglent can do. But there is still the matter of drift. Since the 8590D has in external reference input, I'm not sure how to deal with this. Can one find the port inside the unit where the internal reference is taken, and simply cable it to my Siglent's 10 Mhz ref out?Would a professional calibration address drift, or is drift not a symptom of bad cal? (more a fault than a calibration issue)I am reluctant to open the thing up because it IS working well enough for my purposes.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 10:23:49 pm
content: Quote from: Dan Moos on September 05, 2020, 10:14:43 pmDoes the HP SA have a Ref in ?You can lock the SDG and SA together and it's a trick I use occasionally to null any differences between their clocks.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 05, 2020, 10:26:39 pm
content: Quote from: tautech on September 05, 2020, 10:23:49 pmNothing on the back panel. I'm sure I can find inside where the internal reference is taken, and swap in a feed from the SDG, but I'm scared of wrecking something.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 10:38:06 pm
content: OK, check the 8590D Calibration Guidehttps://doc.xdevs.com/doc/HP_Agilent_Keysight/HP%208590D%20Calibration.pdfSection 1 "Frequency Readout Accuracy" on page 1-10 it shows the min and max readings you should see at various frequencies.Then in section 2 for a unit with option 013 on page 1-12 they show the far higher accuracy at different spans.And near the end of the document it again lists frequency specifications. Seems pretty clear the standard model is not great.I'm pretty sure you don't get a ref input without opt 013. I couldn't find any info about the standard reference. I seem to remember looking at the block diagrams in the docs that there was no obvious place to connect an external reference without option 013. And I doubt a modification would be financially viable. But I am far from an expert.Some internal photos and repair attempts are on the web, so look at those before you open it up. It is pretty robust, so you are unlikely to upset it unless you start pulling out cables or other parts. And most things are labelled.e.g.http://faradaylab.blogspot.com/2014/10/hp-8590d-spectrum-analyzer-crt-repair.html

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 05, 2020, 10:52:06 pm
content: Quote from: Kean on September 05, 2020, 10:38:06 pmMy idea of a modification would be to unplug the internal ref and sneak a cable from my Siglent inside instead. I doubt it's that easy though, for reasons I'm hoping someone can tell me.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 11:13:07 pm
content: Quote from: Dan Moos on September 05, 2020, 10:52:06 pmExcept I don't think the internal 10MHz reference is at all easy to access.I did a little more digging in the 8590E Assembly level repair manual (I couldn't find one specifically for the 8590D).https://www.keysight.com/upload/cmc_upload/All/08590-90316.pdfPage 361 states this about the 10MHz Frequency ReferenceQuoteThere is a little more detail on page 367, or just search the doc for "A22", "OCXO", or "10 MHz" with the space.There is also info about the expected reference input & output levels (page 650), and the related connecting cables (page 513).On page 49 you can see where A22 (the optional OCXO) would be installed and adjusted.Search for mentions of "600 MHz" and it may fill in more blanks e.g. page 88, page 275, and page 360.Of course this is for the 8590E, and there may be some differences in the 8590D.I presume the older 8590D works much the same as the 8590E, and the option 013 adds a 10MHz OCXO and possibly new 600MHz oscillator locked to that (to simplify install).So there is very likely a 10MHz crystal oscillator embedded in your A9 module, and some way to hack it to get external output and input connectors. Maybe someone here knows more, or maybe you can dig a little more with Google and the above info.You can see the 8590D 600MHz frequency adjustment on A9 module in this photohttp://2.bp.blogspot.com/-h3RXiE-AcI0/VDqbCJIlNnI/AAAAAAAAAOU/_Zrw13EH-qE/s1600/photo%2B(.JPGAnd the 8590D 600MHz input at the left of this photo - presumably that cable goes to the A9 modulehttp://2.bp.blogspot.com/-dCv9ZnWLGWc/VDqZ7cIQ72I/AAAAAAAAAOA/WmhEHtBLL0E/s1600/photo%2B(6).JPG

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 05, 2020, 11:42:50 pm
content: Just opened the thing up. Don't think I'll attempt to go deeper. It's not obvious how to get at things beyond the initial cover removal.The aforementioned A9 unit does have adjustments for the 600 MHz and CAL signals. I have no counter that can read that high, so I'm not touching either. Definitely not going to attempt to remove and open the A9 unit. Can't even tell if its soldered or socketed ito the back plane. I know my limiationsI'm gonna leave well enough alone.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 11:50:28 pm
content: Could be a good project once you buy your next SA.

author: xmo
author_rank: Regular Contributor
timestamp: September 05, 2020, 11:52:03 pm
content: TheDefpom: "My 8590 is about 3MHz off after warming up... and drifts a lot for hours, I don't think it is great in that regard but it seems normal (unfortunately). If someone does know a way to get these things more accurate and stable it would be good to hear it"----------------------You didn't say what version 8590 you have. The behavior you described is normal for 8590A models because they have no internal time-base frequency reference.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 11:59:50 pm
content: Thinking about it further and I suspect option 013 also changes something else to give a higher precision frequency readout. The docs seem to imply the number of display digits is limited in the base model.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 06, 2020, 12:00:18 am
content: Quote from: Kean on September 05, 2020, 11:50:28 pmGod, if my wife read these threads...I'm totally plotting buying a newer Rigol or Siglent eventually.This thing really does seem to work great, and apparently is in spec. Far better spec than Defpom's.Next purchase is a proper frequency counter. An an RF signal generator. I accidentally seem to be tending towards used HP gear, and see no reason to quit!

author: Kean
author_rank: Unknown
timestamp: September 06, 2020, 12:09:24 am
content: I’ve got an HP 8657B 100kHz-2GHz generator I am repairing (mostly issues with output attenuation) but it keeps getting put off as I have since picked up a few other cheap RF signal generators. I do still want to fix it as it is quite a nice unit.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2020, 12:15:40 am
content: Quote from: Dan Moos on September 06, 2020, 12:00:18 amThe story of my life !QuoteSSA3021X Plus ....... if nothing else for its ability to be vastly improved.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: September 06, 2020, 12:23:55 am
content: Quote from: tautech on September 06, 2020, 12:15:40 amWhile I've got you, and you seem to be the resident Siglent guy, do you know if my SDG 2042x can still be "vastly improved" via reverting firmware and doing the "thing"?

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2020, 12:27:46 am
content: Quote from: Dan Moos on September 06, 2020, 12:23:55 amYep, to 120 MHz.Study the appropriate threads.