R&S FSH3 spectrum analyzer repair - Page 1
Forum section: electronics / repair

author: EPTech
author_rank: Regular Contributor
timestamp: November 24, 2017, 01:31:15 pm
content: Hi all,I got a R&S broken FSH3 spectrum analyzer from a friend that I would like to fix. The power supply and the logic board work. I have no means of measuring frequencies above 500MHz but the frequencies coming from the dividers of the 2nd and 3rd IF seem to be OK. The frequency going to the ADC and logic board seems to be OK. I believe the issue to be in the first mixer. The frequency coming from the divider that feeds the phase detector is not in the range specified in the block schematic of the service manual. The tuning voltage going to the 3 VCO's should be 3 consecutive ramps but instead it consists of multiple ramps as if the phase detector attemps but cannot lock the divided oscillator frequency onto the reference sweep. However the supplies to the VCO's seem to be switched correctly. I can also see this when switching between bands by setting the start and stop frequencies. I highly suspect that the varicap diodes on the VCO's are damaged. The diodes are SOD-123 with marking 00A. As a test I replaced them with a random type and this drastically changed the tuning voltage but now the device seems completely out of tune.So here is my question: IS there anybody who knows the exact type of this varicap diode, marking 00A?Kind greetings,Pascal.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 24, 2017, 01:39:16 pm
content: Pascal and the group,Have you tried studying the behaviour in zero span?If you set the SPAN to 0Hz all the frequencies should be fixed.Regards,Jay_Diddy_B

author: EPTech
author_rank: Regular Contributor
timestamp: November 24, 2017, 03:19:28 pm
content: Hi there,Tried that and it showed that the phase detector cannot lock. But more importantly you advice pushed me to run over the bands in smaller steps and it got me to the point where I figured out that the PD cannot lock on to anything below 1GHz (band 1). In the second and third band the tuning voltage showed a short ramp, followed by a sawtooth-like signal coming from a limiter/smoothing circuit. When I put the scan at the beginnin of the second and third band, it becomes a short ramp, followed by a steady high.This tells me that the control circuit seems to work fine, but that I used the wrong varicap diodes. All of the VCO's seem to oscillate consistently, but to low. So I really need to know the type of this varicap diode.Kind greetings.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 24, 2017, 04:09:45 pm
content: Pascal,Have you seen this thread:https://www.eevblog.com/forum/testgear/teardown-repair-analysis-of-a-rohde-schwarz-fsh3-3-0ghz-spectrum-analyzer/msg723839/#msg723839If you know the characteristic of the Varicaps that you used, and you know how far off frequency you are you. You can make a guess at the capacitance you need. Or you could put a capacitor in and check the frequency.I started this thread:https://www.eevblog.com/forum/testgear/repair-rohde-shwarz-fsh3-portable-spectrum-analyzer/msg950989/#msg950989I was unsuccessful in my repair attempt until I got a second unit. This is described in this message:https://www.eevblog.com/forum/chat/what-did-you-buy-today-post-your-latest-purchase!/msg1314793/#msg1314793Regards,Jay_Diddy_B

author: EPTech
author_rank: Regular Contributor
timestamp: November 24, 2017, 04:15:26 pm
content: Hey thanks!I already found that thread and the pictures of the RF section definitely got me started. I am also subscribed to and a fan of Shahriar's channel.

author: EPTech
author_rank: Regular Contributor
timestamp: November 29, 2017, 08:34:07 am
content: Hi there,So I tried the varicap diodes BB857 from Infineon, which also have the marking OOa, but now the VCO's oscillate at too high a frequency. So I need to know the exact type of the varicap diodes or make some kind of calculated guess, based on the ones I tried. They are not extremely expensive so I have room to try some more. But if anyone could tell me the exact type, I would be grateful.Kind greetings.