usrp/fft: one more analysis tool for antenna measurements

- usrp/fft.c: utility to read a set of samples obtained with
  usrp2_rx_cfile.py and to perform a Fourier transform on them
- usrp/Makefile: use LDLIBS for libraries, not LDFLAGS
- usrp/Makefile: added "fft"

usrp/Makefile

@@ -1,4 +1,8 @@
 CFLAGS=-Wall -O9
-LDFLAGS=-lm
+LDLIBS=-lm
+LDLIBS_fft=-lfftw3 -lm
 
-all:	peak avg
+all:	peak avg fft
+
+fft:	fft.c
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LDLIBS_fft)

usrp/fft.c

@@ -0,0 +1,162 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <math.h>
+#include <sys/types.h>
+
+#include <fftw3.h>
+
+
+#define  DEFAULT_THRESHOLD	100
+
+
+static void do_fft(int skip, int dump, int low, int high, double threshold)
+{
+	float c[2];
+	float *re = NULL, *im = NULL;
+	fftw_complex *in, *out;
+	fftw_plan plan;
+	int e = 0, n = 0;
+	int i;
+	double a;
+
+	while (1) {
+		size_t s;
+
+		s = fread(c, sizeof(c), 1, stdin);
+		if (!s) {
+			if (!ferror(stdin))
+				break;
+			if (s < 0) {
+				perror("read");
+				exit(1);
+			}
+		}
+		if (skip) {
+			skip--;
+			continue;
+		}
+		if (e <= n) {
+			e = e ? e*2 : 10000;
+			re = realloc(re, e*sizeof(float));
+			im = realloc(im, e*sizeof(float));
+			if (!re || !im) {
+				perror("realloc");
+				exit(1);
+			}
+		}
+		re[n] = c[0];
+		im[n] = c[1];
+		n++;
+	}
+
+	if (skip >= n) {
+		fprintf(stderr, "cannot skip %d of %d entries\n", skip, n);
+		exit(1);
+	}
+	re += skip;
+	im += skip;
+	n -= skip;
+
+	in = fftw_malloc(sizeof(fftw_complex)*n);
+	out = fftw_malloc(sizeof(fftw_complex)*n);
+
+	for (i = 0; i != n; i++) {
+		in[i][0] = re[i];
+		in[i][1] = im[i];
+	}
+
+	plan = fftw_plan_dft_1d(n, in, out, FFTW_FORWARD, FFTW_ESTIMATE);
+	fftw_execute(plan);
+
+	if (dump) {
+		for (i = 0; i < n; i += 1) {
+			a = hypot(out[i][0], out[i][1])/n;
+//			a = a*a;
+			printf("%d %g\n", i, 10*log(a)/log(10));
+		}
+	} else {
+		double s = 0;
+		double db;
+
+		if (high >= n+skip) {
+			fprintf(stderr, "end %d > number of samples %d\n",
+			    high, n+skip);
+			exit(1);
+		}
+		low = low*(double) n/(n+skip);
+		high = high*(double) n/(n+skip);
+		if (high < n)
+			high++;
+		if (low == high)
+			low--;
+		for (i = low; i != high; i++) {
+			a = hypot(out[i][0], out[i][1])/n;
+//			a = a*a;
+			db = 10*log(a)/log(10);
+			if (db >= threshold)
+				s += a;
+		}
+		printf("%f\n", 10*log(s)/log(10));
+	}
+}
+
+
+static void usage(const char *name)
+{
+	fprintf(stderr,
+"usage: %s [-s skip] low high [threshold]\n"
+"       %s [-s skip] -d\n\n"
+"  threshold   only use frequency bins with at least this power, in - dB.\n"
+"              E.g., a threshold value of 60 would be -60 dB. (default: %d\n"
+"              dB)\n"
+"  -d          dump frequency-domain \n"
+"  -s skip     skip this number of samples from the beginning (default: 0)\n"
+    , name, name, -DEFAULT_THRESHOLD);
+	exit(1);
+}
+
+
+int main(int argc, char **argv)
+{
+	int dump = 0, skip = 0;
+	int low, high;
+	double threshold = DEFAULT_THRESHOLD;
+	int c;
+
+	while ((c = getopt(argc, argv, "ds:")) != EOF)
+		switch (c) {
+		case 'd':
+			dump = 1;
+			break;
+		case 's':
+			skip = atoi(optarg);
+			break;
+		default:
+			usage(*argv);
+		}
+
+	switch (argc-optind) {
+	case 0:
+		if (!dump)
+			usage(*argv);
+		do_fft(skip, 1, 0, 0, 0);
+		break;
+	case 3:
+		threshold = -atof(argv[optind+2]);
+		/* fall through */
+	case 2:
+		if (dump)
+			usage(*argv);
+		low = atoi(argv[optind]);
+		high = atoi(argv[optind+1]);
+		if (low > high)
+			usage(*argv);
+		do_fft(skip, 0, low, high, threshold);
+		break;
+	default:
+		usage(*argv);
+	}
+
+	return 0;
+}