R&S CMU200 very low output - Page 1
Forum section: electronics / repair

author: Michael-VK2BEA
author_rank: Regular Contributor
timestamp: October 20, 2021, 04:42:43 pm
content: I'm trying to repair a Rohde & Schwarz CMU200 comms tester. It has very low output from the TXRX board (maximum of -80dBm).I don't know the history of the unit (purchased on eBay).If anyone has seen this particular issue and has a good idea what might have failed, please let me know.The service manual on line seems to have no schematics or component layouts which will make repair a challenge!There is a block diagram showing a switch & attenuator on the output of the TXRX module (TX path). These look physically OK (i.e. not burnt up) but that's probably not much of an indication.The other issue I see is that the spectrum analyzer is showing about -10dB lower than it should (although the wideband power meter is OK)

author: Yansi
author_rank: Super Contributor
timestamp: October 20, 2021, 05:04:03 pm
content: Which output port gives low output? In the service manual, there is a block diagram of the RXTX block. Check that TX signal goes into the TXRX block from the RF block.Then from what port do you see low output? Its been a while since I have operated a CMU200, but it has 4 ports: One is a dedicated input, one dedicated output and two RX/TX ports. So check also following the RXTX block diagram to figure out if any of the RF switches inside it may be blown.Also, I think the dedicated TX port has an RF amplifier in the RXTX block and that component is failure prone, just even by getting old. So if your issue is just that the dedicated RF out port gives low signal, the amplifier may be done. It is just a simple MMIC amplifier, I can upload some photos and the part number later, when I will get back to my PC.Simply put: If yo error is in the RXTX block, you have a pretty good chance repairing it. If yo failure is in the RF block, you better start looking for a replacement module.

author: Michael-VK2BEA
author_rank: Regular Contributor
timestamp: October 20, 2021, 05:23:59 pm
content: Yes, perhaps I wasn't clear, I was measuring from the RXTX board output (that has one RX and one TX port) not the 'RF Frontend board' that has the multiple input/outputs and switches.I am assuming that a -80dBm signal IS a low output for the RXTX unit... is that true? Maybe it's deliberately low so as not to interfere with the receiver. I see there is what looks like an amplifier/filter as the first item in the block diagram of the frontend.

author: Yansi
author_rank: Super Contributor
timestamp: October 20, 2021, 06:09:27 pm
content: -80dBm is definitely nothing coming out. In this case I have no other clues as where to look. You will need to open the RF module and do the guesswork. I do not remember what signal level is supposed to come out the RF module, but the RXTX block with the N connector does not have much amplification inside, so most of the power should come right out the RF module.Problem is, that without an extender, doing any measurement in the module while powered up will be close to impossible, as there is no good physical access to it.

author: Michael-VK2BEA
author_rank: Regular Contributor
timestamp: October 20, 2021, 06:50:07 pm
content: there is this thread ...https://www.eevblog.com/forum/repair/rohde-and-schwartz-cmu200-rxtx-module-issues/msg1020780/#msg1020780that is quite relevant and has some good information.Yes, without an extender (schematic or placement diagram) it will be tedious.