0
$\begingroup$

I am trying to make a AM modem using Python Numpy and Matplotlib. I am successful in generating the AM Signal but I cannot demodulate it using a lowpass filter function that I made. The demodulation equation that I am using is;

$$A_{c}m(t)\cos(\omega_{c}t)\cdot 2\cos(\omega_{c}t)$$

My modulated output is this; enter image description here

The demodulated signal is however this; enter image description here

When it should have been this; enter image description here

The lowpass filter that I am using is; $$h(t) = \frac{\sin(2\pi Bt)}{\pi t}$$

Finally, this is my code;

import matplotlib.pyplot as plt
import numpy as np

t = np.arange(0, 100, 0.1)

message_freq = 50
message = np.sin(6.28*message_freq*t)
carrier_freq = 1000


class Filters():
    def LowPassFilter(self, cutoff):
        f = np.sin(6.28*cutoff*t)/(3.14*t)
        return f


class AmplitudeModem:
    def modulate(self, message, fc):
        c = np.cos(6.28*fc*t)

        mod = message*c
        return mod

    def demodulate(self, received_array, fc, fm):
        c = np.cos(6.28*fc*t)

        demod = 2*c*received_array
        baseband = demod #- self.modulate(Ac, received_array, 2*fc)
        return np.convolve(Filters().LowPassFilter(message_freq), baseband)

modulated = AmplitudeModem().modulate(message, carrier_freq)
demodulated = AmplitudeModem().demodulate(modulated, carrier_freq, message_freq)

Another problem that I am facing is that, I cannot plot the convolution (used in demodulate() function) with respect to time, due to their different dimensions.

$\endgroup$
11
  • 1
    $\begingroup$ (1) Make sure your sampling frequency is large enough (it looks like it is 10 Hz right now, but I could be reading your code wrong) and (2) you're missing a rectifier in the receiver. $\endgroup$
    – MBaz
    Mar 22, 2018 at 18:19
  • $\begingroup$ but do i need a rectifier at all in this demodulation. As far as I know, a lowpass filter is enough to remove high frequency content from the spectrum of DSB-SC. $\endgroup$ Mar 22, 2018 at 22:44
  • $\begingroup$ You do need it for DSB-LC: I misread your first equation and didn't realize you're doing DSB-SC with coherent detection. What about your sampling rate, is it high enough? Also, double-check your filter impulse response; I think you're missing a $B$ in the denominator. $\endgroup$
    – MBaz
    Mar 22, 2018 at 23:14
  • $\begingroup$ The problem is solved now. $\endgroup$ Mar 27, 2018 at 13:34
  • 1
    $\begingroup$ Himanshu, please consider adding an answer explaining what the problem was and how you solved it. You can even accept your own answer. $\endgroup$
    – MBaz
    Mar 27, 2018 at 13:57

1 Answer 1

2
$\begingroup$

enter image description hereThis is wrong Because if your carrier frequency is less than your message frequency, then you will have trouble in filtering .

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.