CMU200 Internally Generated Signals in SA Mode - Page 1
Forum section: electronics / testgear

author: eb4eqa
author_rank: Regular Contributor
timestamp: October 31, 2016, 10:10:19 am
content: Hi,I've found I have a few internally-generated signals on the CMU200. Attached one example. Input used was RF4 and a 50 Ohm load was connected to it. It is not very strong, but is this expected? Does yours show the same, or similar, internal signals?Thank you,Roberto EB4EQA

author: larry42
author_rank: Regular Contributor
timestamp: October 31, 2016, 04:20:13 pm
content: Don't have one, but datasheet specifies spurs (20-2200MHz) as -50dBc, so not unlikely that this is with spec. ...

author: richnormand
author_rank: Unknown
timestamp: October 31, 2016, 10:01:46 pm
content: Tried to reproduce your exact parameters, unless I missed something.Looks like the noise floor is lower and "no birdie".On the other hand I did see a lot of these spurious signals from the multiple IF conversions and mixing while I was troubleshooting my TxRx module.Any leakage from bad contacts between the module cavity clamshells between stages, grounding, shielded cables and such. In particular the two rigid lines at the bottom of the TxRx module were leaking if the SMA connectors were loose or when I had an extension cable. Even the top shield plate is supposed to make a difference. See this links for photos.https://www.eevblog.com/forum/repair/rohde-and-schwartz-cmu200-rxtx-module-issues/They pretty much went away after putting back the system together.Is it possible you live near a 38MHz transmitter? Does it get bigger with the 50ohm out on RF4?I would be tempted to remove and reseat all modules and make sure all shielding is present before proceeding.best of luck with it.

author: eb4eqa
author_rank: Regular Contributor
timestamp: November 01, 2016, 10:55:45 am
content: Hi,Thank you for your repliesI have two other SAs and they don't show this, and it does not change with the input open, or with a short, or with a load. It is internal. Appart from the change in levels (noise floor too), there are no differences in that signal with the input selected to RF4, RF2 or RF1.Seeing your that clean spectrum in your CMU, now I want (need?) to get to the bottom of this. This CMU200 is complete and clean, I will try to reseat everything as you suggest.Thank you,Roberto EB4EQA

author: CJay
author_rank: Super Contributor
timestamp: November 01, 2016, 11:14:37 am
content: I will check mine this evening if I get chance but I don't remember seeing those spurs, most of what I use mine for is HF to low VHF at the moment so I would like to think I'd have noticed if they were present.My CMU200 arrived with a few faults, one of which was related to tarnished contacts between daughterboard and motherboard in one of the modules, it may be worth 'going in' if reseating cables and modules doesn't work.

author: oh2ftu
author_rank: Regular Contributor
timestamp: November 01, 2016, 02:08:59 pm
content: On my celeron-based I get a spur at 144.000548MHz. No idea where it comes from;RF1: -55dBmRF2: -94dBmRF4: -103dBm.But that's it.This is without the aux tx gens.

author: eb4eqa
author_rank: Regular Contributor
timestamp: November 04, 2016, 11:55:16 pm
content: Hi,I have re-connected and re-seated everything, I have tightened every screw and this hasn't changed at all.The instrument passes all self-tests but I've found a continuously flashing LED labeled ALERR at the back of the RF FRONTEND module (1100.3042.02). No idea what that means or if that is expected, but I don't like the -ERR at the end...Any ideas?Thank you,Roberto EB4EQA

author: eb4eqa
author_rank: Regular Contributor
timestamp: November 08, 2016, 10:49:32 pm
content: Let me reply to myself...I did some testing. While connecting the RF FRONTEND module RX1 output that normally goes to the RXTX MODULE to another SA I found something strange. If I injected a signal to RF1, RF2 or RF4, its level changed depending on the input used, as expected (i.e. RF1 has a 17dB input attenuator). But injecting a signal to the RXTX module RFRX1 input, and changing the input (which should compensate the level reading for the attenuation on each input), the level shown in the CMU screen did not change while changing inputs. What was going on? What I did was run the Version Manager application for boards being replaced, and guess what, it all started working as expected. Someone replaced the RF FRONTEND module and did not run the application so everything was a complete mess inside the instrument (there happens to be a good reason for that replacement, see below...)And that unexpected signal dissapeared (well, not actually but it moved down about 2MHz and the level changed to around -100dBm). And what is more interesting, its level now changed depending on the input used, by exactly the amount expected. For example, whe RF4IN is selected, it is 17dB lower than when I select RF1. And that is because the instrument is compensating for 17dB loss in the RF1 input to a signal that is generated AFTER the attenuator, so its level appears higher in the spectrum screen.I also found very low output on RF3, let me open another thread for that completely different problem...Thank you for your Help!Regards,Roberto EB4EQA