diff options
| author | Thomas Tsou <tom@tsou.cc> | 2013-09-17 20:12:26 -0400 |
|---|---|---|
| committer | Thomas Tsou <tom@tsou.cc> | 2013-10-18 13:10:17 -0400 |
| commit | 0e44ab360ef53b0d3d28228fe1e71aa99181ded7 (patch) | |
| tree | 2c45285ec3adfb460949b96cca55c7ec2ab6adb4 | |
| parent | 092f757ec259febf42ecd98ff07f7db1e938f55b (diff) | |
Transceiver52M: Narrow resampling filter bandwidth
This patch only applies to resampling use at 4 samples-per-symbol.
By extention that means only USRP2 / N2xx devices are affected.
At 4 samples-per-symbol we restrict output bandwidth to roughly
roughly 700 MHz, which combined with the 2 pulse Laurent
approximation yields < 0.5 degrees of RMS phase error at the
resampler output.
Signed-off-by: Thomas Tsou <tom@tsou.cc>
| -rw-r--r-- | Transceiver52M/radioInterfaceResamp.cpp | 19 |
1 files changed, 11 insertions, 8 deletions
diff --git a/Transceiver52M/radioInterfaceResamp.cpp b/Transceiver52M/radioInterfaceResamp.cpp index 2b59203..03cde30 100644 --- a/Transceiver52M/radioInterfaceResamp.cpp +++ b/Transceiver52M/radioInterfaceResamp.cpp @@ -39,7 +39,15 @@ extern "C" { /* Resampling parameters for 100 MHz clocking */ #define RESAMP_INRATE 52 #define RESAMP_OUTRATE 75 -#define RESAMP_FILT_LEN 16 + +/* + * Resampling filter bandwidth scaling factor + * This narrows the filter cutoff relative to the output bandwidth + * of the polyphase resampler. At 4 samples-per-symbol using the + * 2 pulse Laurent GMSK approximation gives us below 0.5 degrees + * RMS phase error at the resampler output. + */ +#define RESAMP_TX4_FILTER 0.45 #define INCHUNK (RESAMP_INRATE * 4) #define OUTCHUNK (RESAMP_OUTRATE * 4) @@ -92,13 +100,8 @@ bool RadioInterfaceResamp::init() close(); - /* - * With oversampling, restrict bandwidth to 150% of base rate. This also - * provides last ditch bandwith limiting if the pulse shaping filter is - * insufficient. - */ - if (sps > 1) - cutoff = 1.5 / sps; + if (mSPSTx == 4) + cutoff = RESAMP_TX4_FILTER; dnsampler = new Resampler(RESAMP_INRATE, RESAMP_OUTRATE); if (!dnsampler->init(cutoff)) { |