R&S CMU200 high crosstalk? - Page 1
Forum section: electronics / testgear

author: _Wim_
author_rank: Super Contributor
timestamp: September 28, 2015, 07:45:30 pm
content: Anoter question for CMU200 users here. I am still playing aroud with my "new" CMU200, and just discovered the following:when I set the signal generator to for example -27 dbm @2400Mhz, and sent this signal to a 50 ohm dummy load, the signal appears as a -50 dbm peak on the spectrum analyzer (altough no connection is made to the RF input). This seems very high to me, but I have no experience with this kind of RF equipment. It does not matter which of the two RF inputs or outputs I configure, I always get approxemately the same result. Is this normal or do I have a damaged unit?

author: AYuen
author_rank: Newbie
timestamp: September 29, 2015, 07:50:26 am
content: My CMU200:RF3 out -27dB 50ohm 2.4GHzRF4 in -89dB 50ohmRF3 out -27dB 50ohmRF1 in -48dB 50ohmRF3 out -27dB 50ohmRF2 in -74dB 50ohmRF2 out -27dB 50ohmRF4 in -71dB 50ohmRF2 out -27dB 50ohmRF1 in -45dB 50ohmRF1 out -27dB 50ohmRF4 in -80dB 50ohmRF1 out -27dB 50ohmRF2 in -42dB 50ohmAndrew

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: September 29, 2015, 09:23:42 am
content: Quote from: _Wim_ on September 28, 2015, 07:45:30 pmIt sounds like a defective RF matrix. Unless you setup input and output to the same port (RF 1 or RF 2) then you shouldn't see the transmitted signal on the SA if the output signal is that low and the output is indeed properly terminated.But I'll give it a try on mine tonight to be sure.Update:Did a quick check on a CMU200 and a CRTU with a -27dBm 2.4GHz CW signal going via a H&S RF cable into my standalone SA (R&S FSP7), with the following results:- Output RF 1, input RF 2 (50ohms terminated): -47dBm- Output RF 1, input RF 4 (50ohms terminated): -57dBm- Output RF 3, input RF 1 (50ohms terminated): -67dBmI probably didn't notice it before because I never used these devices in that way. It's a bit disappointing that the RF matrix doesn't offer a better channel separation, but I guess it's OK for the purpose these devices were designed for, and in practical terms it shouldn't matter when using either generator or SA, or when looking at a different frequency on the SA than the one set on the generator.Another alternative could be to disconnect the receiver or transmitter from the RF matrix and connect directly to the module.

author: Neganur
author_rank: Unknown
timestamp: September 29, 2015, 09:47:56 am
content: I have a similar experience with my CTRU, ports 1&2 and 3&4 seem to have something in common that has poor isolation.

author: _Wim_
author_rank: Super Contributor
timestamp: September 29, 2015, 06:23:42 pm
content: First of all, thanks for all the replies. I repeated my test now a bit more controlled (as done Andrew above), and his results are indeed very comparable to my unit:RF3 out -27dB 50ohm 2.4GHzRF4 in -95dB 50ohm (Andrew -89dB)RF3 out -27dB 50ohmRF1 in -51dB 50ohm (Andrew -48dB)RF3 out -27dB 50ohmRF2 in -79dB 50ohm (Andrew -74dB)RF2 out -27dB 50ohmRF4 in -72dB 50ohm (Andrew -71dB)RF2 out -27dB 50ohmRF1 in -47dB 50ohm (Andrew -45dB)RF1 out -27dB 50ohmRF4 in -74dB 50ohm (Andrew -80dB)RF1 out -27dB 50ohmRF2 in -49dB 50ohm (andrew -42dB)P.S: I measured with a 10MHz span centered around 2400MHz, on minimum RBW.So I guess if signal levels allow, I will be best of using RF3 and RF4...

author: AYuen
author_rank: Newbie
timestamp: September 30, 2015, 02:17:21 am
content: I measured with a 100KHz span , 1KHz RBW .

author: _Wim_
author_rank: Super Contributor
timestamp: September 30, 2015, 06:48:43 pm
content: Just retested, same results on your settings within a few dB

author: _Wim_
author_rank: Super Contributor
timestamp: October 04, 2015, 06:32:54 am
content: If I look at the block diagram of the front-end on page 3.2 of the service manual (see pic below), you see RF3 and RF4 are have the widest separation (assuming the actual layout is also structured like this), and this also shows in the measurements above. When using RF3 and RF4 you have >80dB, which I think is quite ok. You just need to be away to use these for sensitive measurements