LimeSDR: Rework calibration

[f4exb]

The calibration process is a little bit more complex than originally thought. See: https://wiki.myriadrf.org/LMS7002Mr3_Calibration_Using_MCU

There are actually 2 APIs for calibration:

• LMS_Calibrate uses channel bandwidth for calibration routines 1 and 2 i.e. DC offset and LO leakage (Tx), phase, gain imbalances
• LMS_SetLPFBW uses LP BW and is for calibration routines 3 and 4 i.e. analog filter bandwidth

The code does LMS_Calibrate after LMS_SetLPFBW and uses LP bandwidth. This seems to be quite wrong moreover since LMS_SetLPFBW invokes calibration routines all streams should be suspended while it runs.

[brendanthebig]

Edouard, I have tried version 3.8.2. Changing LP filter to anything above 30MHz no longer causes buffer under run problems BUT the value of the LP filter does not seem to be applied. No matter what I set it to, nothing changes as far as apparent gain goes in spectrum display.

[f4exb]

You have to have a very large host sample rate to be able to see any change in LP above 30 MHz i.e. you need a rate significantly above this figure in MS/s like 40 MS/s and I am not sure this is workable under all conditions.

If you try reasonable values like tuning the filter at a few MHz then with a 10 MS/s host sample rate without software decimation you will be able to see the difference (it does for me).

[brendanthebig]

I tried it again after restart of pc and it seems to be working now.

[f4exb]

OK fine.