SoftFM/NOTES.txt

This file contains random notitions
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Sample rates between 300001 Hz and 900000 Hz (inclusive) are not supported.
They cause an invalid configuration of the RTL chip.
  rsamp_ratio = 28.8 MHz * 2**22 / sample_rate
  If bit 27 and bit 28 of rsamp_ratio are different, the RTL chip malfunctions.

The RTL chip has a configurable 32-tap FIR filter.
RTL-SDR currently configures it for cutoff at 1.2 MHz (2.4 MS/s).

Casual test of ADC errors:
 * DC offset in order of 1 code step
 * I/Q gain mismatch in order of 4%
 * I/Q phase mismatch in order of 1% of sample interval

With tuner in auto-gain mode, device autonomously switches between gain
settings during a run. The LNA gain seems to switch between ~ 24 dB
and ~ 34 dB without intermediate steps.

With RTL in AGC mode, the level of the digital sample stream is normalized
to -6 dB FS.
Unknown whether this is an analog or digital gain stage.
Does this improve SNR or not?

At first I suspected that AGC mode may be a cooperation between the RTL and
the Elonics tuner. I thought that the RTL would monitor the level and send
digital control signals to the Elonics to dynamically change the tuner IF gain.
However that is in fact NOT what happens. (Manually changing IF gain in AGC
mode causes a brief level spike, while manually rewriting the same IF gain in
AGC mode does not have any effect).
It seems more likely that AGC is a digital gain in the downsampling filter.

Default settings in librtlsdr:
  Elonics LNA gain: when auto tuner gain: autonomous control with slow update
                    otherwise gain as configured via rtlsdr_set_tuner_gain
  Elonics mixer gain: autonomous control disabled,
                      gain depending on rtlsdr_set_tuner_gain
  Elonics IF linearity: optimize sensitivity (default), auto switch disabled
  Elonics IF gain: +6, +0, +0, +0, +9, +9 (non-standard mode)
  Elonics IF filters: matched to sample rate (note this may not be optimal)
  RTL AGC mode off


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radio3      96800000  (good)
radio4      94300000  (bad)
qmusic     100700000  (medium)
radio538   102100000  (medium)
radio10    103800000  (bad)
radio west  89300000  (medium)