DC offset in low-IF configuration

Projects LimeSDR
#1

Hi,

I would like to use the LimeSDR in low-IF mode. I configured the LimeSDR with default settings in the LimeSuiteGui. Then, I have enabled the CMIX in RXTSP and the FIR filters for lowpass filtering. No matter which up or downconversion frequency I choose a DC offset is remaining, while the real DC spike shifts correctly. I checked the setup in the FFT Viewer with a CW generator and there is definitely no error in terms of frequency offset.

Has anybody seen the same problem? Thank you for your help!

Best regards,
Christian

#2

You have two options:

  • avoid center “bump” by mixing your I/Q TX signal with complex oscillator inside software app before send it to LimeSDR via USB cable:

Spectrum
Spectrum901×219 50.2 KB

(in the image center you can see TX local oscillator leakage, on the left and right two large area where you can shift your TX modulated signal; you can also perform I/Q correction).

  • use TX NCO above 30MHz! Set TX local oscillator to be higher than TX LPF and set TX NCO to downconerter mode. TX LPF will cut TX local oscillator leaving spurious free region for modulated signal. Downside is that you need higher sample rate (maximum TX NCO is Fsamplerate x 16).
#3

Probably you misunderstood my question. I want to receive data in low-if configuration. However, I found out that there is some rounding error of half a bit in the implemented CMIX causing the permanent DC spike.

Best regards,

Christian

#4

Same goes for RX as well. Maybe you should post image of your setup and spectrum image.

#5

Is software filtering after reception not an option?