218 lines
8.6 KiB
Plaintext
218 lines
8.6 KiB
Plaintext
|
|
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.
|
|
|
|
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 IF signal filtering
|
|
-----------------------------
|
|
|
|
Carson bandwidth rule:
|
|
IF_half_bandwidth = peak_freq_devation + modulating_freq
|
|
|
|
In case of broadcast FM, this is
|
|
75 kHz + 53 kHz = 128 kHz (worst case)
|
|
19 kHz + 53 kHz = 72 kHz (typical case)
|
|
|
|
Simulations of IF filtering show:
|
|
* narrow IF filter reduces noise in the baseband
|
|
* narrow IF filter causes gain roll-off for high modulating frequencies
|
|
* narrow IF filter causes harmonic distortion at high modulating deviation
|
|
|
|
IF filter with 100 kHz half-bandwidth:
|
|
* baseband gain >= -1 dB up to 75 kHz
|
|
* less than 0.1% distortion of modulating signal at 19 kHz peak deviation
|
|
* ~ 2% distortion of modulating signal at 75 kHz peak devation
|
|
|
|
IF filter with 75 kHz half-bandwidth:
|
|
* baseband gain ~ -3 dB at 60 kHz, ~ -8 dB at 75 kHz
|
|
* ~ 1% distortion of modulating signal at 19 kHz peak deviation
|
|
|
|
Optimal IF bandwidth is probably somewhere between 75 and 100 kHz, with
|
|
roll-off not too steep.
|
|
Weak stations benefit from a narrow IF filter to reduce noise.
|
|
Strong stations benefit from a wider IF filter to reduce harmonics.
|
|
|
|
|
|
Effect of settings on baseband SNR
|
|
----------------------------------
|
|
|
|
Note: all measurements 10 second duration
|
|
Note: all measurements have LO frequency set to station + 250 kHz
|
|
|
|
|
|
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
|
|
|
|
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.
|
|
|
|
|
|
STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT
|
|
|
|
radio3 2 MS/s 34 dB 9,0,0,0,9,9 off 80 kHz 0.38 -63.2 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,0,0,3,3 off 80 kHz 0.03 -61.5 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,0,0,3,9 off 80 kHz 0.07 -61.5 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,0,0,3,15 off 80 kHz 0.11 -60.5 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,0,0,9,15 off 80 kHz 0.22 -61.0 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,0,0,15,15 off 80 kHz 0.36 -61.4 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,0,6,0,15,15 off 80 kHz 0.66 CLIP -63.5 dB/Hz
|
|
radio3 2 MS/s 34 dB 0,6,0,0,3,3 off 80 kHz 0.07 -62.9 dB/Hz
|
|
radio3 2 MS/s 34 dB 9,3,0,0,3,3 off 80 kHz 0.14 -63.9 dB/Hz
|
|
radio3 2 MS/s 34 dB 9,9,0,0,3,3 off 80 kHz 0.26 -64.2 dB/Hz
|
|
radio3 2 MS/s 34 dB 9,9,3,2,3,3 off 80 kHz 0.45 -64.1 dB/Hz
|
|
radio3 2 MS/s 34 dB 9,9,6,0,3,3 off 80 kHz 0.49 -63.7 dB/Hz
|
|
radio3 2 MS/s 34 dB 9,9,9,0,6,3 off 80 kHz 0.77 CLIP -61.2 dB/Hz
|
|
|
|
radio4 2 MS/s 34 dB 9,0,0,0,9,9 off 80 kHz 0.09 -41.5 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,0,0,3,3 off 80 kHz 0.01 -36.7 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,0,0,3,9 off 80 kHz 0.02 -39.9 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,0,0,3,15 off 80 kHz 0.03 -40.4 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,0,0,9,15 off 80 kHz 0.06 -39.9 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,0,0,15,15 off 80 kHz 0.09 -40.9 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,0,6,0,15,15 off 80 kHz 0.17 -38.4 dB/Hz
|
|
radio4 2 MS/s 34 dB 0,6,0,0,3,3 off 80 kHz 0.02 -36.9 dB/Hz
|
|
radio4 2 MS/s 34 dB 9,3,0,0,3,3 off 80 kHz 0.03 -37.5 dB/Hz
|
|
radio4 2 MS/s 34 dB 9,9,0,0,3,3 off 80 kHz 0.07 -39.4 dB/Hz
|
|
radio4 2 MS/s 34 dB 9,9,3,2,3,3 off 80 kHz 0.11 -38.5 dB/Hz
|
|
radio4 2 MS/s 34 dB 9,9,6,0,3,3 off 80 kHz 0.12 -38.0 dB/Hz
|
|
radio4 2 MS/s 34 dB 9,9,9,0,6,3 off 80 kHz 0.22 -37.5 dB/Hz
|
|
|
|
Conclusion: IF gain has little effect on quality, although very low gain
|
|
has an adverse effect. Librtlsdr defaults look good.
|
|
|
|
|
|
STATION SRATE LNA IF GAIN AGC SOFT BW IF LEVEL GUARD/PILOT
|
|
|
|
radio3 2 MS/s 34 dB default off 80 kHz 0.36 -62.1 dB/Hz
|
|
radio3 2 MS/s 34 dB default ON 80 kHz 0.36 -61.6 dB/Hz
|
|
|
|
radio4 2 MS/s 34 dB default off 80 kHz 0.11 -38.5 dB/Hz
|
|
radio4 2 MS/s 34 dB default ON 80 kHz 0.36 CLIP -38.0 dB/Hz
|
|
|
|
Conclusion: AGC mode has little effect on quality.
|
|
|
|
|
|
Stereo pilot frequency
|
|
----------------------
|
|
|
|
Measuring 19 kHz pilot frequency vs Internet NTP (ADSL):
|
|
|
|
radio2 19 kHz pilot = 18999.79 Hz +- 0.04 Hz (7 hours measurement)
|
|
radio3 19 kHz pilot = 18999.8 Hz +- 0.05 Hz (5 hours measurement)
|
|
radio538 19 kHz pilot = 18999.78 Hz +- 0.04 Hz (6 hours measurement)
|
|
|
|
Conclusion: stereo pilot is not a reliable time source.
|
|
|
|
|
|
Ferrites
|
|
--------
|
|
|
|
Claming a ferrite block on the USB cable, as close as possible to the DVB
|
|
received, reduces disturbance peaks in the spectrum by ~ 6 dB.
|
|
A second ferrite block at the PC side gives another small improvement.
|
|
|
|
The ferrite causes a clearly audible improvement of weak stations.
|
|
|
|
|
|
DIY antenna
|
|
-----------
|
|
|
|
Constructed antenna from a vertical telescopic rod antenna (85 cm)
|
|
and two steel wire ground radials ~ 85 cm.
|
|
Antenna placed indoors close to window.
|
|
Antenna connected to DVB receiver via 1.5 meter 75 Ohm coax cable.
|
|
|
|
The ground radials are floppy and look ridiculous but they are essential.
|
|
No ground radials, no reception.
|
|
|
|
The DIY antenna has ~ 20 dB more signal than the basic DVB antenna.
|
|
|
|
The DIY antenna causes notable improvement of weak stations.
|
|
Radio4 reception improves from very bad to almost good.
|
|
|
|
However, strong stations (radio3) sound slightly worse with the DIY antenna
|
|
than with the basic DVB antenna.
|
|
May be caused by clipping due to too strong signal from antenna.
|
|
|
|
|
|
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)
|
|
|
|
--
|