Hi list, I am writing a C++ software which will scan through a given frequency range and perform an fft. The software seems to do what it should, however something weird happens when using LO offset tuning. With the following settings all looks correct:
Sample rate 6.4M - master clock 51.2M - 512 byte num_samples/binsize - offset 0. However I can clearly see the DC peak for every «jump» in frequency. With the same settings as above and a 3.2M offset there is a «ghost» signal 3.2MHz below the real signal, which mirrors the real signal with slightly lower amplitude. This happens everywhere in the spectrum where solid signals (approx 40-50 dBuV) are present. Using the command tune_request(freq, offset) and usrp->set_rx_freq(tune_request) to set frequencies. Have also tried with different gain settings from 0-40, no change. Have also tried with 10 dB attenuator between antenna and the radio, no change. The same happens on other sample rates and offsets as well, the ghost signal is always the same as the offset (+ or -). I am using UHD_3.14.1. Have also tried changing rx_bandwidth without any change. Is it possible to use LO offset and eliminate the «ghost» signals, or is this some radio/DSP limitation? I have pictures showing the problem clearly, I can send directly by email if necessary. Thanks, Jorn
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