Rohde & Schwarz FSEB-30 - Input level issue - Page 1
Forum section: electronics / repair

author: yngndrw
author_rank: Regular Contributor
timestamp: September 29, 2025, 01:46:12 pm
content: I purchased an FSEB-30 as a faulty item, but I'm wondering if I've bitten off more than I can chew. I'm not experienced with RF, but it's something I want to learn but perhaps a faulty spectrum analyser wasn't the best start. In my defence, it does sort of work - just it's lacking sensitivity.The seller thought that the attenuator was faulty and I checked that replacements are available before purchasing. The unit still has intact calibration labels so it likely hasn't been mucked around with since the fault occurred.I was hoping I could run through my initial diagnostics and reasoning, and hopefully receive some advice on how to proceed to narrow things down. The way its behaving are making me second-guess what I thought the issue was, but that might just be my inexperience.Observations:Self test fails with the error: RF-Module RF-Converter (As an aside, how do you close the self-test pop-up without restarting the Spectrum Analyser?)With signal from a FE-5650A frequency standard (10MHz, +7dBm minimum) - The output looks correct, but the peak is only -40dBmManually changing the attenuator with the signal connected - You can hear the relays within the attenuator and the noise floor moves but the peak does not changeWith the internal calibration input selected rather than RF input (120MHz, -40dBm) - You can hear the relay when switching from RF input to calibration input, but it mostly just shows noise - There's a tiny -90dBm 120MHz signal visible with averaging and a 100kHz span (This does not change as the attenuator is manually changed)The observation with the internal calibration signal really threw me off.Here's the attenuator: (X1 is the RF input, X2 is the cal signal, X3 is the attenuated output)A couple of possibilities stood out to me:1) An internal signal may be weak, affecting the CAL generation and internal mixing - But I wouldn't expect it to work at all if such a fundamental signal was so low that the CAL was unrecognisable.2) There's an issue within the attenuator between Z5 and Z4 - The weird thing for me is how both the input and cal signals don't change with the attenuator, but the noise does change. Is that possible with such a fault, or is there something else I'm missing?Also what type of connectors are used internally? It looks like they are SMB - I will need to buy some adapters. I have an RF power meter on the way (Working, yay) and also a signal generator (Also broken, but that should be much easier to fix) so my plan is to verify the strength of the CAL signal with the power meter and then the power of the output of the attenuator. If I can prove that it's the attenuator, I can then pick up a replacement for £250 as I don't think I'd be able to fix it. (Especially as I can hear the relays changing and can see it swapping between the RF input and the CAL signal.)

author: yngndrw
author_rank: Regular Contributor
timestamp: September 29, 2025, 10:56:29 pm
content: A little more investigation, bypassing the attenuator by feeding the CAL signal straight into the RF board.First thing I noticed was that even with the attenuator (Which includes the switch to the CAL signal) bypassed, the CAL signal only showed up once the unit was set into the "Input CAL" mode. I was expecting that signal to always be available.The second, it's still only reading as -80dBm when it should be -40dBm.The noise floor still changes when the attenuator setting is changed which suggests that something else internally tracks the attenuator.So on one hand, it looks like the unservicable attenuator isn't at fault. But on the other hand, it's no-longer just a part I can easily source and replace. I think I'll have to wait for my power meter to arrive in order to check some of the other signals.