Spurious signals around LO feedthrough in HP 8591A spectrum analyzer? - Page 1
Forum section: electronics / repair

author: gabor.dani
author_rank: Contributor
timestamp: August 03, 2019, 08:56:56 pm
content: I am troubleshooting my HP 8591A spectrum analyzer. It is fully functioning, except for these spurious signals around the 0 Hz feedthrough. This noise is only within 15 Mhz of DC.Where should I look for the possible fault? I have checked every single signal path, test point, and everything is within the specs specified in the service manual.

author: MarkL
author_rank: Unknown
timestamp: August 03, 2019, 10:23:12 pm
content: Are the spurious signals continuous or do they jump around?I would address the "REF UNLOCK" condition before digging any further.According to the block diagram there is a jumper connecting the REF OUT to the REF IN. Check that this is in place. On the "E" models it's on the back panel. On the "A" models I'm not sure.You could also try applying a 10MHz external reference and see what happens.EDIT: Added further suggestion. And found a photo; the jumper is on the back.

author: gabor.dani
author_rank: Contributor
timestamp: August 04, 2019, 10:31:08 am
content: I connected the ref out to ref in, the message disappeared, but the signals stayed.Also, I measured the 600 Mhz TG output of the A9 third converter SAW, and found this: attaching the spectrum.Does it look normal for a SAW oscillator? Because I don't think so.

author: MarkL
author_rank: Unknown
timestamp: August 04, 2019, 05:31:49 pm
content: Do you mean you're measuring the output on J5 of A9? It should be clean.You might want to take a look at the tuning voltage input to see if it's stable (J1-11). If it's not, you could try supplying this voltage input externally to break the PLL feedback loop and then try to identify where the instability is occurring.Also, have you checked that all your power supply rails are in spec and without excessive ripple?Do you have the tracking generator option?Again, are the spurious signals continuous or do they jump around?

author: gabor.dani
author_rank: Contributor
timestamp: August 05, 2019, 06:29:09 pm
content: - "Do you mean you're measuring the output on J5 of A9" - Yes. It is not clean. The previous spectrum was taken from that.- "Do you have the tracking generator option?" - No.- "you could try supplying this voltage input externally" - Well I removed R60, the VCO input from the motherboard is Zero, no matter what I do. The R68 trimpot is now able to adjust the 600 Mhz to spot on. The spurs stayed and are exactly the same. By trimming R68, the spurs are a little bit different, but just a little. Amplitude changes, and some minor frequency drift, but generally it is the same.- The interference is very stable in the domain of both frequency and amplitude. It is not jumping around, it is continuous. This stability is present in both of the measurement methods: Looking at +-15Mhz of 0Hz DC on 8591, and looking at the 600 Mhz output with an external SA.- The PSU has no ripples, all 4 rails are good. Special attention to the 15V rails, which feed the SAW, they are good.- I am unable to tune the SAW to exactly 600 Mhz with R68, it is always below that.- The VCO input J2-11 looks normal. There is no ripple. If the R68 is adjusted to the left end, the value is -840 mV. If R68 is adjusted to the right, the value is exactly 0. (Correction: Only 0, when ext ref is not connected. When connected, it changes to -5.)- The frequency output of the SAW module (Collector of Q13) is hard to measure. If I attach a wire, the oscillation stops. If I touch the wire, the oscillation starts and the value is around 600 Mhz. Roughly the same as the J5.- I am unable to measure the spectrum of the SAW module. By attaching it to the other analyzer, the oscillation stops. Even if I am touching the wire at the same time.- There is a service diag menu, inside that there is a list of frequencies and errors measured. The third from top left is the "Error in Hz between the sampler IF and the actual counted frequency at the sampler IF" and it is jumping around zero.The way I see it, there aremultipleproblems:The fact that the VCO tune is always zero,and the spurs are always present.