Tektronix 495P spectrum analyzer - Problem with center frequency / peak offset? - Page 1
Forum section: electronics / repair

author: MarosD
author_rank: Newbie
timestamp: May 18, 2025, 04:06:52 pm
content: Hello,at first sorry, if my question is pretty obvious, but I bought Tektronix 495P spectrum analyzer, because I sometimes need to measure HF signal (I am mostly in the audio range). I never had spectrum analyzer before, but I am quite experienced in the electronic field (LF). I bought it also with the service and user manual in a printed form + service cards + service RF cables. I made a deal with a seller that I can return analyzer, if I wont be satisfied with the functionality. He owned it a few years and changed 1st LO, because old one was not possible to repair. He used it very very rarely. Opticaly it looks very nice.I powered it on, status was changed to INT and I started self calibration process (shift+cal). Everything is OK! No errors.But - I have offset from center frequency (peak is not under center dot). To be more precise, is under the dot only in a small span setting (center_ok.jpg). Under bigger span or max. span its out of align (center_off.jpg).Is it "normal" to have this "display" error? Or I need to something set differently or calibrate something inside?PS: The big peak on the 0Hz is normal? I mean, I know its about heterodyn. mixing, but if its always big like this? Its over range indepedently to dB/div setting.I just need to be sure, and maybe here are users of it ( they are definitely here, I read very interesting topics about repairing) )Thanks a lot,Maros

author: KE5FX
author_rank: Super Contributor
timestamp: May 18, 2025, 04:50:18 pm
content: There's a reasonably-detailed procedure in the service manual for calibrating the dot marker position and other things. Looks like you may have a later-model unit going by the appearance of the CRT, so the 494A manuals would likely serve better than the earlier versions of the 495 manuals if that's the case for the printed copies you have. The vintage of the internal modules is more important in that respect than the model number on the front panel.One good thing about these units is that they are maintenance-friendly, but one bad thing is that they can also be maintenance-intensive. Years ago I used to "fix and flip" them on eBay, and I wrote upsome noteson common issues I encountered when working on them. I bring that up because if youdohave a newer version that has not yet been recapped, you need to treat that as a minor emergency and deal with it right away as described in the notes.The first order of business, even before that, is to familiarize yourself with the operator's manual so that you understand things like the 0-Hz spur. It originates downstream of the front-end attenuator, so its level will not change when the reference level is adjusted by changing the RF attenuation, only when the IF gain changes.Also, your UNCAL light is on, so you are running at the wrong sweep speed for the selected RBW filter, which can move the peak away from the dot by itself. There is almost never a reason to run with the sweep rate control set to anything but AUTO, so check that first.

author: MarosD
author_rank: Newbie
timestamp: May 19, 2025, 06:21:44 am
content: KE5FX - Thank you so much.1. I needed to be sure its only about dot marker calibration and its no problem to solve. It was suspicious for me the position changes, if I change the span. I thought the offset would be constant (independent of span setting) if its just out of align. If dot marker calibration helps, I am quite happy.2. Recap - of course, its planned. The notes are perfect.3. Yes. It confused me, because others SA has this 0 Hz spur much lower, inside of the screen view. Approach in 495P I like much more, because its good to distinguish it.4. Uncal - Thanks. Yes, I set time base from auto.. I played with it. Thank you for the warning.I do not want to look, like I need everything immedietaly and do not read manuals / long topics about SA. I have no problems to think about something for months, but I can return it in limited time. Thats why I started the topic.Thank you for the patience.

author: MarosD
author_rank: Newbie
timestamp: May 21, 2025, 05:06:25 pm
content: Dear KE5FX,today I tried to do the adjustment, a few question:1. During "Frequency loop cal" from <shift>+Pulse Stretcher menu everything was fine, until it wanted:"Connect DVM to PIN B on the controlled oscilator board, adjust C2011 for 11.3V reading".Maybe its because of almost night hours but I cannot see any board named like that. Even C2011, probably capacitor for the setting?2. I need also time mark generator, but can I use my Siglent 1062 generator? I know, manual wants 0.2us - no problem - but could you tell me correct pulse width? 50 ns? I cannot find any info. Also amplitude of the pulse?3. Just to be sure - Its OK to connect this generator (50 Ohm output) directly the RF input of the 495P? There is no DC on the generator (only the pulse for a very short time) ). Just asking if time mark generator was something special for it in the case of output impedance / level.Thanks a lot!Maros

author: KE5FX
author_rank: Super Contributor
timestamp: May 21, 2025, 05:14:26 pm
content: Quote from: MarosD on May 21, 2025, 05:06:25 pmThe pulse width doesn't matter, and in fact (as the document points out) you can even use a sine wave signal that's strong enough to show some harmonics. It will be obvious enough how to align the frequency-domain peaks of whatever signal you use with the markings on the graticule.But yes, as long as the level is low and there's no DC, the Siglent generator should be fine. About 1V peak (i.e., +10 dBm into 50 ohms) should be a good starting point.The "controlled oscillator" board is part of the multi-slot PLL assembly in the metal casting. There's not much you can do with it without a set of extenders. In general, you can stop the calibration process and call it good once you reach that step, unless there is a specific reason to recalibrate the PLL module. It rarely needs any attention (and it's a lot of work when it does.)

author: MarosD
author_rank: Newbie
timestamp: May 22, 2025, 11:06:07 am
content: Dear KE5FX,I made some progress, I send some pictures. Whole process was without any problems, except I did not adjust controller board of PLL. I also followed your notes. The center frequency is in the +20% tolerance of the SPAN resolution, but I am still quite suspicious about the center frequency. Here are some pictures, so please, take a look if its correct. When I compare it to the first post in this topic, its definitely better.Some points:1. At first, I calibrated digital storage as you suggest and also my manual. There was a significant error in horizontal align. I followed screen steps, but that storage / non storage blinking was very hard for my eyes to align. Much more I liked steps in the user manual. At the first picture is clear, that storare / non storage is calibrated well. Some errors in levels is due to make photo in non storage mode.2. I adjusted 0 VDC on TP10733. Then I started FREQUENCY LOOP routine. I aligned everything without problems and calibration was succeeded. One thing:3.1 I tried this process more times andevery timeI needed to set 0.000 VDC on TP1058 with center frequency knob on the panel. Without that there was appx. -0.030 VDC. Maybe its not a problem, and calibration routine set this offeset for some reason, but..4. R1065 was aligned to have peaks/marks on the grid, but only with some small trick, that I changed center frequency or horizontal position of the whole screen. Without that, all markers were out of grid, but by same amount. This trick was good for make it more confortable. After that, I move H position trimmer to the center. It was easy due to center dot.5. I applied 0.2 pulses, and align it like this. I use 50 ns pulse width and 16 ns rise edge.6. Also 50 us time marks. You can see the offset from the grid:7. Dot marker was set at the begining spur after shift+reset.The results:Wide SPAN. Markers are slightly of the grid. Probably its good because of +-20% of the SPAN resolution?***Narrow SPAN and I moved with STEP+- buttons for harmonics. 300 MHz is out of align, but 400 MHz is correct.***Marker at 100 MHz shows 101 MHz:***More detailed 100 MHz from the calibrator output. Its in the middle:100 MHz from the cal out and SA is set like for bandwidth measurement from the SM. Its again with some offset:***Phase sidebands from cal out. Its slightly worse in the contrast to SM, but its in the tolerance.Thank you so much,Maros

author: KE5FX
author_rank: Super Contributor
timestamp: May 23, 2025, 05:15:21 am
content: I'd say you're done. Due to limited sweep linearity, that's about as good as it gets. This is why later analyzers started drawing the graticule on the screen itself, with its own set of vectors.The only questionable part is the 300 MHz misalignment. That's not something you can chalk up to sweep linearity. The 1st LO calibration process really should have gotten it closer than that. I don't see any obvious problems with your procedure, although it's probably been 5-7 years since I went through it myself.

author: MarosD
author_rank: Newbie
timestamp: May 23, 2025, 08:56:16 am
content: Thanks.Yes, its weird. I spoke with a previous owner and he bought it from a different guy. That guy made pcb service extenders, but he couldnt fix it. So he sold it to the previous owner. He had to replace the YIG oscillator. This is original YIG he gave me:This is the replacement. I do not know the reputation of this company, but maybe a bigger phase noise is due to non-original part? Also tunning problems? Maybe its important to align also PLLs for proper operation like it was advised during adjustment on the screen?***I found this (connector). The red mark says "Mechanically touched - Tunnig fail". Its too rude for me, but I hope it has no affect on the behaviour, especialy on 3rd harmonics. The owner told me he must to clean it and re-tight it again.***Sorry, I bothering you with all this torture and stupid questions, but I highly appreciate it. Really. Its quite rare SA and information about that its hard to get + I am very new in RF field. I need it mainly for digital audio, so below, lets say 50 MHz. I also would like to try your fantastic PN program for measuring phase noise. I will get some GPIB converter. I just want to try it and see the figure from my oscillator for DAC. I got also a friend who can made a proper PN measurement, because he has PN analyzer for that. Unfortunately he has no time for SA repair work with me. He tries to catch the reflection from the moon and to get message from the received noise)Or diferently - I bought this SA for 450 Eur = 500 USD. I think, its worth it. (?)

author: KE5FX
author_rank: Super Contributor
timestamp: May 23, 2025, 10:08:49 pm
content: Yeah, hard to complain at $500! Looks like a nice clean unit.The part you show is not the YIG oscillator, but the 10 MHz reference oscillator. It's an ovenized crystal oscillator (OCXO). You don't have any problem with regard to phase noise, in any event.

author: MarosD
author_rank: Newbie
timestamp: June 03, 2025, 01:41:19 pm
content: Sorry for longer response time, I was overhauled with other projects.***I was also ashamed because my lack of knowledge with SA and RF part so I had to learn it deeply.I probably solved it.1. 3rd harmonics offset was suspicious and it also predicted a problem with SA. When I used narrow SPAN, under 500 kHz the SA gave me PHASELOCK error and 1st LO error. What was strange -only at 3rd harmonics from CAL OUT!So at 300 MHz something happend.2. I studied service manual and theory of it and I also tried to follow troubleshooting and so on. I was pretty angry for huge mistakes in SM. Tektronix is/was great company, but this was a hell - testpoint mentioned in the text, but not marked/drawed in schematic. Same with trimmers. If I would be a tech with 20 years of knowledge in servising this, OK, probably some intuition will work. But this is my first SA!3. During analyzing the problem I changed capacitors (center freq, PLL, PLL synth., YIG), I calibrated PLL for 1st LO. Service manual claimed 40mA/V * voltage on shunt for 1st LO = current through 1st LO coil. I got around 5V on the 45 Ohm shunt (I checked the shunt too), so it should be around 200 mA due to SM. But math is math - 5 / 45 = 111 mA. When I placed ammeter in series I got 111 mA. In the menu on the screen was DAC value also around 5V. From this point I ended to trust to the SM.4. I realized that problem with phaselock error popped up after I-U changed to INT. So I connected 10 MHz signal to REF IN from external generator. The label was changed to EXT, but problem was the same. Ref OSC was ok.5. I was thinking it the I-U / INT change is only "murphys coincidance" to temperature, so I started to blow cold air on the all RF part. Nothing. I tried it on center freq, span, 1st LO driver and nothing. When I tried it somewhere near aux. synth and counter -bingo! I saw the 300 MHz spur slowly showing from right side. After some time, it was placed right in the middle. No error.6. On AUX Synth. board were old caps and replaced 74ac74. It was badly soldered so I resolder it, changed capacitor and the phaselock error popped up immedietaly after power on. Crap. I thought that 7474 was hot after resolder. After changing caps/resolder - the error was there.7. I checked VCO on the board andbingo II- control voltage for the VCO was 6.5V instead of 5 V like SM suggested. I set it to 5 V and after that, I got no error on 300 MHz.I think that due to bad soldering / caps somebody tried to tune VCO and tune it badly. Maybe on that frequency was some resonation, or some offset, I do not know. My previous cooling helped to tune VCO.After this moment, I got every spur on the place.***I found in the user manual that DC spur should be -30 dBm, but I have it much over the screen. I tried to align load trimmer, but no cure for that. The manual suggest to set it on the 1st mixer, but there is also a warning label to not crack diode substrate. Question is:- If I will leave it like this, is there some problem with "overloading" of the mixer?- Its really - 30 dBm? (My trust to SM is very weak)Thanks a lot,Maros

author: KE5FX
author_rank: Super Contributor
timestamp: June 04, 2025, 05:24:27 pm
content: The DC spur doesn't really have a "dBm" level in the sense that it is equivalent to a 0 Hz signal at the mixer input. It occurs when the LO sweeps through the IF frequency at 2072 MHz, so that the LO-IF difference is near 0 Hz. What you see on the screen is not the result of mixing (multiplication) but simple leakage of the LO into the IF output. It is unavoidable and basically harmless. The 0 Hz feedthrough level shown in your screenshots is perfectly normal.Longer story: the spike's indicated signal level is determined solely by the amount of IF gain. It will change when the RF attenuator is adjusted, for example, but only because the IF gain is adjusted at the same time to maintain the same reference level. The reference level has to take both RF attenuation and the mixer's conversion loss into account, and the 0 Hz leakthrough signal is affected only by the mixer, not the attenuator. So that's why the manual gives you a specific RF attenuator setting to use (0 dB for the 496, not sure about the 495) when evaluating the 0 Hz feedthrough spike; they have to establish a known IF gain.These are single-balanced mixers that have to make a tradeoff between LO-IF and LO-RF isolation. Given that, it's usually better to design for the best possible LO-RF isolation, as nobody (especially important military customers) likes seeing LO emissions coming out of their spectrum analyzer's input jack. Meanwhile, at any frequency other than 0 Hz, the LO will still leak into the mixer's IF port, but it won't get very far since its frequency is not the same as the IF. If the mixer's conversion loss is around 14 dB and its IMD is within spec, you don't need to spend a lot of time worrying about the 0 Hz feedthrough.

author: MarosD
author_rank: Newbie
timestamp: June 04, 2025, 08:48:13 pm
content: I definitely understand and believe what you are saying, also thats completely normal. But imagine a situation, I would like to follow the SM. Even hypoteticaly.I was just thinking if its normal, why to mention it in the SM? Why the SM is trying to adjust it to some even "virtual level"?From SM:QuoteHmm well, its out of the screen.My imagination is that I will turn the screw and the spur will jump inside of the graticule and I will find minimum amplitude. I am asking because I really do not want to crack the diode substrate. I am looking for the courage in style "just be gentle, its not so easy to crack it". Because it will hurt a LOT if I "destroy" now fully functional machine by my own and with the adjustment which is not even needed - just because my curiosity.QuoteI tried to turn it without touching the 1st mixer screw, because I just want to see the behaviour. Only thing I saw was a slightly bigger side noise floor around 0 Hz spur. I set it in the middle position. Now I got feeling that even spur at 100 MHz is slightly higher as it was before this. Is here some other way how to properly set it? Or just leave it there and send LOG calibration again?***Thanks a lot. I am happy that it works right now without problems, so I really want to do my best and finally close it into chassis.

author: KE5FX
author_rank: Super Contributor
timestamp: June 04, 2025, 10:06:12 pm
content: On the usefulness and credibility scale, the Tektronix service manuals for the 49x analyzers are somewhere between the Protocols of the Elders of Zion and the Brothers Grimm. My recommendation: stop trying to fix stuff that isn't broken.