RTL2832U FIR filter
Michael Karcher
osmocom at mkarcher.dialup.fu-berlin.de
Sat Jul 14 17:46:48 UTC 2012
Hello people,
in some experiments with my RTL2832 stick, I managed to find out how the
FIR filter works. The FIR filter is running at the XTAL frequency
(typically 28.8MHz). In the SDR mode, the filter is a symmetric FIR
filter with 32 real-number-valued taps. Using the symmetry, only 16
coefficients need to be specified. The coefficients are stored in the 20
bytes written to the FIR register. The first coefficient describes the
outermost tap, the last one the tap 0.5 taps away from the symmetry
axis. The first 8 values are 8-bit numbers, the second 8 values are 12
bit numbers, resulting in 64 + 96 = 160 bits (20 bytes).
The first 8 bytes written to the chip are the first 8 coefficients. The
12-bit coefficients are stored like this pairwise in three bytes: In the
byte sequence "12 34 56", the first coefficient is 0x123 and the second
coefficient is 0x456. Both the 8-bit and the 12-bit numbers are to be
interpreted as two's complement signed numbers (thus the 8-bit
coefficients range from -128 to +127, while the 12-bit coefficients
range from -2048 to +2047). The resolution of all 16 coefficients is the
same.
I furthermore found out that this is not the only filtering going on in
the RTL2832 chip in SDR mode. There seems to be a second anti-aliasing
filter before the DAC, making signals disappear that are at frequencies
above 0.7 times the sample rate (in the baseband), even if the
programmable FIR filter lets them pass.
As an example, I provide the decoded default coefficients:
-54 -36 -41 -40 -32 -14 14 53 101 156 215 273 327 372 404 421
421 404 372 327 273 215 156 101 53 14 -14 -32 -40 -41 -36 -54
This filter has a quite flat passband in the interesting range for DAB
(-768kHz..+768kHz) but by far not enough attenuation at 1280kHz (which
will alias to 768kHz at a sampling rate of 2048kHz) to get satisfactory
adjacent channel rejection. The experimentally observed alias rejection
is good enough, which lead to the conclusion that there is another
low-pass filter, which might be part of the resampling unit.
BTW: the 900kHz low sample rate limit seems to be valid for a 36MHz
XTAL, not the common 28.8MHz XTAL.
Regards,
Michael Karcher
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