2014-01-05 16:33:05 +01:00
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This file contains random notitions
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-----------------------------------
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2014-01-11 23:19:20 +01:00
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Valid sample rates
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------------------
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2014-01-05 22:01:22 +01:00
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Sample rates between 300001 Hz and 900000 Hz (inclusive) are not supported.
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2014-01-05 16:33:05 +01:00
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They cause an invalid configuration of the RTL chip.
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2014-01-12 18:12:53 +01:00
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rsamp_ratio = 28.8 MHz * 2**22 / sample_rate
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If bit 27 and bit 28 of rsamp_ratio are different, the RTL chip malfunctions.
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2014-01-05 16:33:05 +01:00
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2014-01-11 23:19:20 +01:00
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Behaviour of RTL and Elonics tuner
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----------------------------------
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The RTL chip has a configurable 32-tap FIR filter running at 28.8 MS/s.
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2014-01-05 16:33:05 +01:00
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RTL-SDR currently configures it for cutoff at 1.2 MHz (2.4 MS/s).
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2014-01-11 23:19:20 +01:00
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Casual test of ADC mismatch:
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2014-01-05 16:33:05 +01:00
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* DC offset in order of 1 code step
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* I/Q gain mismatch in order of 4%
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* I/Q phase mismatch in order of 1% of sample interval
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2014-01-05 21:56:06 +01:00
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With tuner in auto-gain mode, device autonomously switches between gain
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2014-01-09 21:18:44 +01:00
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settings during a run. The LNA gain seems to switch between ~ 24 dB
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2014-01-05 21:56:06 +01:00
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and ~ 34 dB without intermediate steps.
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With RTL in AGC mode, the level of the digital sample stream is normalized
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2014-01-18 20:44:17 +01:00
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to -6 dB FS. Unknown whether this is an analog or digital gain stage.
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2014-01-05 21:56:06 +01:00
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2014-01-09 21:18:44 +01:00
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At first I suspected that AGC mode may be a cooperation between the RTL and
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the Elonics tuner. I thought that the RTL would monitor the level and send
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digital control signals to the Elonics to dynamically change the tuner IF gain.
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However that is in fact NOT what happens. (Manually changing IF gain in AGC
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mode causes a brief level spike, while manually rewriting the same IF gain in
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AGC mode does not have any effect).
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It seems more likely that AGC is a digital gain in the downsampling filter.
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2014-01-05 21:56:06 +01:00
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2014-01-11 23:19:20 +01:00
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Default settings in librtlsdr
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-----------------------------
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Elonics LNA gain: when auto tuner gain: autonomous control with slow update
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otherwise gain as configured via rtlsdr_set_tuner_gain
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Elonics mixer gain: autonomous control disabled,
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gain depending on rtlsdr_set_tuner_gain
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Elonics IF linearity: optimize sensitivity (default), auto switch disabled
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Elonics IF gain: +6, +0, +0, +0, +9, +9 (non-standard mode)
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Elonics IF filters: matched to sample rate (note this may not be optimal)
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RTL AGC mode off
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2014-01-12 18:12:53 +01:00
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Effect of IF signal filtering
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-----------------------------
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Carson bandwidth rule:
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IF_half_bandwidth = peak_freq_devation + modulating_freq
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In case of broadcast FM, this is
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75 kHz + 53 kHz = 128 kHz (worst case)
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19 kHz + 53 kHz = 72 kHz (typical case)
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Simulations of IF filtering show:
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* narrow IF filter reduces noise in the baseband
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* narrow IF filter causes gain roll-off for high modulating frequencies
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* narrow IF filter causes harmonic distortion at high modulating deviation
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IF filter with 100 kHz half-bandwidth:
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* baseband gain >= -1 dB up to 75 kHz
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* less than 0.1% distortion of modulating signal at 19 kHz peak deviation
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* ~ 2% distortion of modulating signal at 75 kHz peak devation
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IF filter with 75 kHz half-bandwidth:
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* baseband gain ~ -3 dB at 60 kHz, ~ -8 dB at 75 kHz
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* ~ 1% distortion of modulating signal at 19 kHz peak deviation
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Optimal IF bandwidth is probably somewhere between 75 and 100 kHz, with
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roll-off not too steep.
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Weak stations benefit from a narrow IF filter to reduce noise.
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Strong stations benefit from a wider IF filter to reduce harmonics.
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2014-01-11 23:19:20 +01:00
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Effect of settings on baseband SNR
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----------------------------------
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2014-01-12 22:08:22 +01:00
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Note: all measurements 10 second duration
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Note: all measurements have LO frequency set to station + 250 kHz
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2014-01-11 23:19:20 +01:00
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STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT
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radio3 1 MS/s 24 dB default off 150 kHz 0.19 -62.6 dB/Hz
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radio3 1.5 MS 24 dB default off 150 kHz 0.19 -62.7 dB/Hz
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radio3 2 MS/s 24 dB default off 150 kHz 0.18 -62.7 dB/Hz
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radio3 2 MS/s 34 dB default off 150 kHz 0.46 -64.0 dB/Hz
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radio3 2 MS/s 34 dB default off 80 kHz -64.0 dB/Hz
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radio3 2 MS/s 34 dB default off 150 kHz adccal -64.0 dB/Hz
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radio4 2 MS/s 24 dB default off 150 kHz 0.04 -41.1 dB/Hz
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radio4 1 MS/s 34 dB default off 150 kHz 0.06 -43.3 dB/Hz
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radio4 1 MS/s 34 dB default off 80 kHz -51.2 dB/Hz
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radio4 2 MS/s 34 dB default off 150 kHz 0.10 -42.4 dB/Hz
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radio4 2 MS/s 34 dB default off 80 kHz -48.2 dB/Hz
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radio4 2 MS/s 34 dB default off 150 kHz adccal -42.4 dB/Hz
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Conclusion: Sample rate (1 MS/s to 2 MS/s) has little effect on quality.
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Conclusion: LNA gain has little effect on quality.
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Conclusion: Narrow IF filter improves quality of weak station.
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Conclusion: ADC gain/offset calibration has no effect on quality.
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2014-01-12 22:08:22 +01:00
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STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT
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radio3 2 MS/s 34 dB 9,0,0,0,9,9 off 80 kHz 0.38 -63.2 dB/Hz
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radio3 2 MS/s 34 dB 0,0,0,0,3,3 off 80 kHz 0.03 -61.5 dB/Hz
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radio3 2 MS/s 34 dB 0,0,0,0,3,9 off 80 kHz 0.07 -61.5 dB/Hz
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radio3 2 MS/s 34 dB 0,0,0,0,3,15 off 80 kHz 0.11 -60.5 dB/Hz
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radio3 2 MS/s 34 dB 0,0,0,0,9,15 off 80 kHz 0.22 -61.0 dB/Hz
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radio3 2 MS/s 34 dB 0,0,0,0,15,15 off 80 kHz 0.36 -61.4 dB/Hz
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radio3 2 MS/s 34 dB 0,0,6,0,15,15 off 80 kHz 0.66 CLIP -63.5 dB/Hz
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radio3 2 MS/s 34 dB 0,6,0,0,3,3 off 80 kHz 0.07 -62.9 dB/Hz
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radio3 2 MS/s 34 dB 9,3,0,0,3,3 off 80 kHz 0.14 -63.9 dB/Hz
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radio3 2 MS/s 34 dB 9,9,0,0,3,3 off 80 kHz 0.26 -64.2 dB/Hz
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radio3 2 MS/s 34 dB 9,9,3,2,3,3 off 80 kHz 0.45 -64.1 dB/Hz
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radio3 2 MS/s 34 dB 9,9,6,0,3,3 off 80 kHz 0.49 -63.7 dB/Hz
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radio3 2 MS/s 34 dB 9,9,9,0,6,3 off 80 kHz 0.77 CLIP -61.2 dB/Hz
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radio4 2 MS/s 34 dB 9,0,0,0,9,9 off 80 kHz 0.09 -41.5 dB/Hz
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radio4 2 MS/s 34 dB 0,0,0,0,3,3 off 80 kHz 0.01 -36.7 dB/Hz
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radio4 2 MS/s 34 dB 0,0,0,0,3,9 off 80 kHz 0.02 -39.9 dB/Hz
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radio4 2 MS/s 34 dB 0,0,0,0,3,15 off 80 kHz 0.03 -40.4 dB/Hz
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radio4 2 MS/s 34 dB 0,0,0,0,9,15 off 80 kHz 0.06 -39.9 dB/Hz
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radio4 2 MS/s 34 dB 0,0,0,0,15,15 off 80 kHz 0.09 -40.9 dB/Hz
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radio4 2 MS/s 34 dB 0,0,6,0,15,15 off 80 kHz 0.17 -38.4 dB/Hz
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radio4 2 MS/s 34 dB 0,6,0,0,3,3 off 80 kHz 0.02 -36.9 dB/Hz
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radio4 2 MS/s 34 dB 9,3,0,0,3,3 off 80 kHz 0.03 -37.5 dB/Hz
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radio4 2 MS/s 34 dB 9,9,0,0,3,3 off 80 kHz 0.07 -39.4 dB/Hz
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radio4 2 MS/s 34 dB 9,9,3,2,3,3 off 80 kHz 0.11 -38.5 dB/Hz
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radio4 2 MS/s 34 dB 9,9,6,0,3,3 off 80 kHz 0.12 -38.0 dB/Hz
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radio4 2 MS/s 34 dB 9,9,9,0,6,3 off 80 kHz 0.22 -37.5 dB/Hz
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Conclusion: IF gain has little effect on quality, although very low gain
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has an adverse effect. Librtlsdr defaults look good.
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STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT
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radio3 2 MS/s 34 dB default off 80 kHz 0.36 -62.1 dB/Hz
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radio3 2 MS/s 34 dB default ON 80 kHz 0.36 -61.6 dB/Hz
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radio4 2 MS/s 34 dB default off 80 kHz 0.11 -38.5 dB/Hz
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radio4 2 MS/s 34 dB default ON 80 kHz 0.36 CLIP -38.0 dB/Hz
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Conclusion: AGC mode has little effect on quality.
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2014-01-19 20:00:17 +01:00
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Stereo pilot frequency
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----------------------
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2014-01-20 07:50:15 +01:00
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Measuring 19 kHz pilot frequency vs Internet NTP (ADSL):
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2014-01-22 23:10:23 +01:00
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radio2 19 kHz pilot = 18999.79 Hz +- 0.04 Hz (7 hours measurement)
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radio3 19 kHz pilot = 18999.8 Hz +- 0.05 Hz (5 hours measurement)
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radio538 19 kHz pilot = 18999.78 Hz +- 0.04 Hz (6 hours measurement)
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Conclusion: stereo pilot is not a reliable time source.
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2014-01-19 20:00:17 +01:00
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2014-01-26 20:31:38 +01:00
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Ferrites
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--------
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Claming a ferrite block on the USB cable, as close as possible to the DVB
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received, reduces disturbance peaks in the spectrum by ~ 6 dB.
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A second ferrite block at the PC side gives another small improvement.
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The ferrite causes a clearly audible improvement of weak stations.
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DIY antenna
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-----------
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Constructed antenna from a vertical telescopic rod antenna (85 cm)
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and two steel wire ground radials ~ 85 cm.
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Antenna placed indoors close to window.
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Antenna connected to DVB receiver via 1.5 meter 75 Ohm coax cable.
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The ground radials are floppy and look ridiculous but they are essential.
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No ground radials, no reception.
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The DIY antenna has ~ 20 dB more signal than the basic DVB antenna.
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The DIY antenna causes notable improvement of weak stations.
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Radio4 reception improves from very bad to almost good.
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However, strong stations (radio3) sound slightly worse with the DIY antenna
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than with the basic DVB antenna.
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2014-05-29 21:32:32 +02:00
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Theory: Distortion caused by clipping I/Q samples due to strong antenna signal.
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No, that's not it. Reducing LNA gain or IF gain does not help much;
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small DVB antenna still sounds better than DIY antenna.
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Difference only clear in stereo mode.
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Don't know what's going on here, maybe the DIY antenna is just not good.
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2014-01-26 20:31:38 +01:00
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2014-05-31 21:07:04 +02:00
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IF processing
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-------------
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Idea: Filter I/Q samples with 3rd order Butterworth filter
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instead of 10th order FIR filter.
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Implemented in branch "iirfilter".
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Speed is unchanged.
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Sound quality is not much changed for strong stations.
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Sound quality is a bit worse for weak stations (at 1 MS/s).
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Conclusion: not worthwhile.
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Idea: Downsample I/Q samples to ~ 250 kS/s BEFORE quadrature detection
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instead of immediately after detection. Would reduce amount of work in
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I/Q filtering for same or higher FIR filter order.
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Implemented in branch "filterif".
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CPU time reduced by ~ 25%.
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Sound quality very slightly worse.
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Conclusion: not worthwhile.
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2014-01-26 20:31:38 +01:00
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Local radio stations
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--------------------
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radio2 92600000 (good)
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radio3 96800000 (good)
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radio4 94300000 (bad)
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qmusic 100700000 (medium)
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radio538 102100000 (medium)
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radio10 103800000 (bad)
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radio west 89300000 (medium)
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2014-01-11 23:19:20 +01:00
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--
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