Transmitting at low sampling rates causes signal to be sent at higher rate

System Development
1

I’m sending generated DE-QPSK frames in a loop and I’m relying on hardware to control the rate. I’ve set a TX sample rate of 108 kSps (108e3), yet with the LimeSDR-USB, it seems to transmit the signal at 10 Msps (10e6). I have tried different SDRs (B200, N200) and they do not have this issue (though signal quality is much lower compared to the LimeSDR – lots of aliasing on TX).

I am using SoapySDR with a straightforward setup. I have a fixed symbol rate and I do interpolation of my signal (8 times) to get the 108 kSps rate.

This is all verified through a spectrum analyzer (Agilent CSA Spectrum Analyzer N1996A).

2

It turns out it is controlling the rate properly. It’s just the noise floor around the signal is raised by a lot (for a 6 kHz wide channel, it’s raising it by >30 dB over 250 kHz). I don’t get the aliasing I see in the other SDRs, but given the smallest filter the LimeSDR takes is 5 MHz, I guess I’ll have to filter it further in DSP.

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