paulmataruso/osmo-trx
1
0
Fork 0
mirror of https://gitea.osmocom.org/cellular-infrastructure/osmo-trx.git synced 2025-11-03 05:33:16 +00:00
Code Issues Packages Projects Releases Wiki Activity

Transceiver52M: Narrow resampling filter bandwidth

Browse Source 
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>
This commit is contained in:
Thomas Tsou
2013-09-17 20:12:26 -04:00
parent 092f757ec2
commit 0e44ab360e
1 changed files with 11 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files

19
Transceiver52M/radioInterfaceResamp.cpp
View File

@@ -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)) {