LimeSDR mini (and maybe others) do not want IQ data to become a value below
-1 or above 1. This happens slightly when applying the IIR filter after
upsampling. To prevent this, we scale down the IQ level a little bit.
To test the problem:
- set the scale level to 1.0, so it does not take effect
- use LimeSDR mini
- use upsampling (sample rate differs SDR sample rate)
- run loopback mode 2 (-l 2)
- IIR filter must be initialized with 2 or more iterations to take effect
- check linear IQ graph; it will then plot dots inside the circle
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