Greetings, I am experiencing difficulties with unwanted transmitter harmonics in the FLEX 400 daughtercard. I am attempting to transmit a signal at 460 MHz and simultaneously listen for a responding signal at exactly twice that frequency, 920 MHz, with a FLEX 900. My USRP is configured to send and receive a 320 kSa/s complex signal both ways, with an interpolation of 400 and a decimation of 200.
GNU Radio appears to be working as expected, but the transmitter on the FLEX 400 is producing a harmonic on the frequency I'm trying to listen to. My setup is as follows: TX Signal USRP LPF 320 kSa/s --> usrp_sink_c --> Interp 400 --> FLEX 400 --> 700 MHz --> TX Antenna 10 kHz wide RX Signal USRP 320 kSa/s <-- usrp_source_c <-- Decim 200 <-- FLEX 900 <-- 50 Ohm Terminator 10 kHz wide I tested the USRP with a spectrum analyzer and a near-field probe. Attaching a low-pass filter, using a narrow-band antenna, and closing the USRP's case is sufficient to stop the harmonic from radiating into the environment, but that doesn't do anything about its presence inside the enclosure itself. I think the harmonic might be directly coupling with the receiver daughtercard, but I need to do more testing to be sure. The power level of the harmonic (as seen by the receiver) is roughly 25 dB above the noise floor. Strangely enough, the harmonic gets stronger if I lower the transmitter's gain (by applying a gain in software of 1000 instead of 10000). Since I'm using maximum interpolation, could this be an artifact of the interpolation process? Is there any way I can increase the electromagnetic compatibility of the USRP daughtercards—besides just putting them in separate USRPs? Is there any additional shielding or grounding that might help? Is it possible to build an extension cable so I can put the FLEX 400 in its own enclosure, or would this cause too many reflection/timing issues? This problem is not unique to the USRP—even the very expensive signal generators I've used seem to have this second harmonic. I'm going to experiment with this a bit more and see if I can find a solution. Thanks, Colin Stagner Mo S&T Electrical Engineering
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