This file contains random notitions ----------------------------------- Valid sample rates ------------------ 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. Behaviour of RTL and Elonics tuner ---------------------------------- The RTL chip has a configurable 32-tap FIR filter running at 28.8 MS/s. RTL-SDR currently configures it for cutoff at 1.2 MHz (2.4 MS/s). Casual test of ADC mismatch: * 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 Effect of settings on baseband SNR ---------------------------------- STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT radio3 1 MS/s 24 dB default off 150 kHz 0.19 -62.6 dB/Hz radio3 1.5 MS 24 dB default off 150 kHz 0.19 -62.7 dB/Hz radio3 2 MS/s 24 dB default off 150 kHz 0.18 -62.7 dB/Hz radio3 2 MS/s 34 dB default off 150 kHz 0.46 -64.0 dB/Hz radio3 2 MS/s 34 dB default off 80 kHz -64.0 dB/Hz radio3 2 MS/s 34 dB default off 150 kHz adccal -64.0 dB/Hz radio4 2 MS/s 24 dB default off 150 kHz 0.04 -41.1 dB/Hz radio4 1 MS/s 34 dB default off 150 kHz 0.06 -43.3 dB/Hz radio4 1 MS/s 34 dB default off 80 kHz -51.2 dB/Hz radio4 2 MS/s 34 dB default off 150 kHz 0.10 -42.4 dB/Hz radio4 2 MS/s 34 dB default off 80 kHz -48.2 dB/Hz radio4 2 MS/s 34 dB default off 150 kHz adccal -42.4 dB/Hz Note: all measurements 10 second duration Note: all measurements have LO frequency set to station + 250 kHz Conclusion: Sample rate (1 MS/s to 2 MS/s) has little effect on quality. Conclusion: LNA gain has little effect on quality. Conclusion: Narrow IF filter improves quality of weak station. Conclusion: ADC gain/offset calibration has no effect on quality. Local radio stations -------------------- radio2 92600000 (good) radio3 96800000 (good) radio4 94300000 (bad) qmusic 100700000 (medium) radio538 102100000 (medium) radio10 103800000 (bad) radio west 89300000 (medium) --