diff --git a/tests/gensin1.c b/tests/lpf1.c
similarity index 100%
rename from tests/gensin1.c
rename to tests/lpf1.c
diff --git a/tests/gensin3.c b/tests/lpf3.c
similarity index 100%
rename from tests/gensin3.c
rename to tests/lpf3.c
diff --git a/tests/gensin3a.png b/tests/lpf3a.png
similarity index 100%
rename from tests/gensin3a.png
rename to tests/lpf3a.png
diff --git a/tests/gensin3b.png b/tests/lpf3b.png
similarity index 100%
rename from tests/gensin3b.png
rename to tests/lpf3b.png
diff --git a/tests/lpf4.c b/tests/lpf4.c
new file mode 100644
index 0000000..f8d289e
--- /dev/null
+++ b/tests/lpf4.c
@@ -0,0 +1,156 @@
+/*
+ * Copyright 2022 Oleg Borodin  <borodin@unix7.org>
+ */
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <complex.h>
+
+typedef struct {
+    double x1;
+    double x2;
+    double x3;
+    double rc;
+} lpf_t;
+
+
+void lpf_init(lpf_t *lpf, double freq) {
+    lpf->x1 = 0.0;
+    lpf->x2 = 0.0;
+    lpf->x3 = 0.0;
+
+    double order = 3.0;
+    double c = 1.0 / sqrt(powf(2.0, 1.0 / order) - 1.0);
+    lpf->rc = 1.0 / (2.0 * c * M_PI * freq);
+}
+
+double lpf_apply(lpf_t *lpf, double x0, double dt) {
+
+    double k = dt / (lpf->rc + dt);
+
+    lpf->x1 = lpf->x1 + k * (x0 - lpf->x1);
+    lpf->x2 = lpf->x2 + k * (lpf->x1 - lpf->x2);
+    lpf->x3 = lpf->x3 + k * (lpf->x2 - lpf->x3);
+    return lpf->x3;
+}
+
+
+
+void dft(double* x1, double* y1, int m, int dir) {
+    long i, k;
+    double arg;
+    double cosarg, sinarg;
+
+    double x2[m];
+    double y2[m];
+
+    for (i = 0; i < m; i++) {
+        x2[i] = 0;
+        y2[i] = 0;
+        arg = -dir * 2.0 * M_PI * (double)i / (double)m;
+        for (k = 0; k < m; k++) {
+            cosarg = cos(k * arg);
+            sinarg = sin(k * arg);
+            x2[i] += (x1[k] * cosarg - y1[k] * sinarg);
+            y2[i] += (x1[k] * sinarg + y1[k] * cosarg);
+        }
+    }
+    if (dir == 1) {
+        for (i = 0; i < m; i++) {
+            x1[i] = x2[i] / (double)m;
+            y1[i] = y2[i] / (double)m;
+        }
+    } else {
+        for (i = 0; i < m; i++) {
+            x1[i] = x2[i];
+            y1[i] = y2[i];
+        }
+    }
+}
+
+
+int main(int argc, char** argv) {
+
+    double t = 0;
+    double s = 0;
+    double freq0 = 20;
+    double dt = 0.001;
+
+    lpf_t lpf;
+
+    lpf_init(&lpf, 20.0);
+
+    int c = 1024 * 2;
+    double xa[c];
+    double xb[c];
+    double y[c];
+
+    for (int i = 0; i < c; i++) {
+        t = i * dt;
+        for (int n = 0; n < 200; n += 20) {
+            double freq = freq0 + n;
+            s += cos(2 * M_PI * freq * t);
+        }
+
+        xa[i] = s;
+        xb[i] = lpf_apply(&lpf, s, dt);;
+    }
+
+    for (int i = 0; i < c; i++) {
+        y[i] = 0;
+    }
+
+    double rate = 1.0 / dt;
+
+
+    FILE* fd;
+
+    fd = popen("gnuplot -persistent", "w");
+
+    fprintf(fd, "set terminal png size 1600,1200\n");
+    fprintf(fd, "set output \"gensin4a.png\"\n");
+    fprintf(fd, "plot '-' with lines\n");
+
+    dft(xa, y, c, -1);
+    for (int i = 0; i < c; i++) {
+        double freq = rate * (double)i / (double)c;
+        double amp = sqrt(xa[i] * xa[i] + y[i] * y[i]);
+
+        if (freq < rate / 2) {
+            fprintf(fd, "%e %e\n", freq, amp);
+        }
+    }
+    fflush(fd);
+    fprintf(fd, "e");
+    fclose(fd);
+
+    for (int i = 0; i < c; i++) {
+        y[i] = 0;
+    }
+
+
+    fd = popen("gnuplot -persistent", "w");
+
+    fprintf(fd, "set terminal png size 1600,1200\n");
+    fprintf(fd, "set output \"gensin4b.png\"\n");
+    fprintf(fd, "plot '-' with lines\n");
+
+    dft(xb, y, c, -1);
+    for (int i = 0; i < c; i++) {
+        double freq = rate * (double)i / (double)c;
+        double amp = sqrt(xb[i] * xb[i] + y[i] * y[i]);
+
+        if (freq < rate / 2) {
+            fprintf(fd, "%e %e\n", freq, amp);
+        }
+    }
+    fflush(fd);
+    fprintf(fd, "e");
+    fclose(fd);
+
+    return 0;
+}
diff --git a/tests/lpf4a.png b/tests/lpf4a.png
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diff --git a/tests/lpf4b.png b/tests/lpf4b.png
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