diff --git a/PDH_error_signal/PDH_error_signal.m b/PDH_error_signal/PDH_error_signal.m
new file mode 100644
index 0000000..02b30f1
--- /dev/null
+++ b/PDH_error_signal/PDH_error_signal.m
@@ -0,0 +1,32 @@
+clear
+clc
+close all
+
+L = 10e-2
+c = 299792458
+R = 0.99991
+
+fsr = c/(2*L)
+
+fc = fsr*128760;
+df=20e6;
+
+f = 2*pi*linspace(fc-df, fc+df, 10000);
+
+fpt = ((1.-R).*exp(-2.*L.*i.*f./c))./(1.-R.*exp(-2.*L.*i.*f./c));
+fpr = sqrt(R).*(1.-exp(-2.*L.*i.*f./c))./(1.-R.*exp(-2.*L.*i.*f./c));
+
+figure
+plot(f,abs(fpr))
+
+
+fm = 23e6;
+
+fm1 = sqrt(R).*(1.-exp(-2.*L.*i.*(f.+fm)./c))./(1.-R.*exp(-2.*L.*i.*(f.+fm)./c));
+fm2 = sqrt(R).*(1.-exp(-2.*L.*i.*(f.-fm)./c))./(1.-R.*exp(-2.*L.*i.*(f.-fm)./c));
+
+ff=fpr.*conj(fm1)-conj(fpr).*fm2;
+iff = imag(ff);
+
+figure
+plot(f,iff)
diff --git a/PDH_error_signal/PDH_error_signal.py b/PDH_error_signal/PDH_error_signal.py
new file mode 100755
index 0000000..c093974
--- /dev/null
+++ b/PDH_error_signal/PDH_error_signal.py
@@ -0,0 +1,52 @@
+#!/usr/bin/env python
+
+import numpy
+import matplotlib.pyplot as plt
+
+L = 14e-2
+c = 299792458
+R = 0.99991
+
+fsr = c/(2*L)
+print("fsr = %f GHz"%(fsr/1e9))
+
+finesse = numpy.pi*R**0.25/(1-R**0.5)
+print("finesse = %i"%finesse)
+
+print("finesse/fsr = %f s"%(finesse/fsr))
+
+fc = fsr*130000
+df = 20e6
+#print(fc)
+
+f = 2*numpy.pi*numpy.linspace(fc-df, fc+df, 100000) # frequency array
+i = complex(0,1)
+
+fpt = ((1.-R)*numpy.exp(-2.*L*i*f/c))/(1.-R*numpy.exp(-2.*L*i*f/c))
+fpr = R**0.5*(1.-numpy.exp(-2.*L*i*f/c))/(1.-R*numpy.exp(-2.*L*i*f/c))
+
+plt.subplot(2, 1, 1)
+plt.plot(f, abs(fpr))
+plt.title('FP cavity')
+plt.ylabel('Reflective intensity')
+plt.grid()
+
+
+fm = 23e6
+ram = 0
+
+fm1 = R**0.5*(1.-numpy.exp(-2.*L*i*(f+fm)/c))/(1.-R*numpy.exp(-2.*L*i*(f+fm)/c)) + ram*i
+fm2 = R**0.5*(1.-numpy.exp(-2.*L*i*(f-fm)/c))/(1.-R*numpy.exp(-2.*L*i*(f-fm)/c)) + ram*i
+
+ff = fpr*fm1.conjugate()-fpr.conjugate()*fm2
+iff = ff.imag
+
+
+plt.subplot(2, 1, 2)
+plt.plot(f, iff)
+plt.xlabel('Frequency')
+plt.ylabel('PDH')
+plt.grid()
+
+
+plt.show()