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>
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 @@
/* 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 @@
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)) {