Added CIC compensation filter tap generator. Useful when working without the half-band filter in the USRP.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@8185 221aa14e-8319-0410-a670-987f0aec2ac5

2 files changed, 46 insertions, 0 deletions

diff --git a/gnuradio-core/src/utils/Makefile.am b/gnuradio-core/src/utils/Makefile.am
index a81e3ee98..78c80dab8 100644
--- a/gnuradio-core/src/utils/Makefile.am
+++ b/gnuradio-core/src/utils/Makefile.am
@@ -22,6 +22,7 @@
 include $(top_srcdir)/Makefile.common
 
 EXTRA_DIST = 			\
+	cic_comp_taps.m		\
 	db_width.m		\
 	filter_tools.m		\
 	plot_freq_response.m	\
diff --git a/gnuradio-core/src/utils/cic_comp_taps.m b/gnuradio-core/src/utils/cic_comp_taps.m
new file mode 100644
index 000000000..e0e0c2ebd
--- /dev/null
+++ b/gnuradio-core/src/utils/cic_comp_taps.m
@@ -0,0 +1,45 @@
+#
+# Copyright 2008 Free Software Foundation, Inc.
+# 
+# This file is part of GNU Radio
+# 
+# GNU Radio is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3, or (at your option)
+# any later version.
+# 
+# GNU Radio is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with GNU Radio; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+# 
+
+# See Altera Application Note AN455
+#
+# R  = decimation factor, 8-256
+# Fc = passband (one-sided) cutoff frequency
+# L  = number of taps
+
+function taps = cic_comp_taps(R, Fc, L)
+    M = 1;                                   %% Differential delay
+    N = 4;                                   %% Number of stages
+    B = 18;                                  %% Coeffi. Bit-width
+    Fs = 64e6;                               %% (High) Sampling freq in Hz before decimation
+    Fo = R*Fc/Fs;                            %% Normalized Cutoff freq; 0<Fo<=0.5/M;
+    p = 2e3;                                 %% Granularity
+    s = 0.25/p;                              %% Step size
+    fp = [0:s:Fo];                           %% Pass band frequency samples
+    fs = (Fo+s):s:0.5;                       %% Stop band frequency samples
+    f = [fp fs]*2;                           %% Normalized frequency samples; 0<=f<=1
+    Mp = ones(1,length(fp));                 %% Pass band response; Mp(1)=1
+    Mp(2:end) = abs( M*R*sin(pi*fp(2:end)/R)./sin(pi*M*fp(2:end))).^N;
+    Mf = [Mp zeros(1,length(fs))];
+    f(end) = 1;
+    h = fir2(L,f,Mf);                        %% Filter length L+1
+    taps = h/sum(h);                         %% Floating point coefficients
+endfunction