I am low pass filtering the PCM of an arbitrary Bell 103 (300 bps) signal. However, the output of the low pass filter shows random spikes in amplitude.
I have tried to increase the bandwidth, but no luck. Can someone explain why these spikes arise? My goal is to do some sort of envelope detection on this signal afterwards, to decode the 1's and 0's. For example by using a rectifier first.
The filter coefficients:
coeffs = [0.265842424474100 0.243276902464240 0.271096721997119 0.243276902464240 0.265842424474100]
The Matlab script used:
close all; clear all; % Generate a random bit sequence as input bitsIn = randi([0 1], 16, 1)'; % Generate the amplitude of the FSK signal amplitude = randi([7000 10000], 1, 1); % Generate a randomized length of randomized startup noise % x = randi([-20 20], 1, randi([0 3000], 1, 1)); x = ; % TODO % FSK module the input bit stream for i = 1:length(bitsIn) if (bitsIn(i) == 1) freq = 1270; else freq = 1070; end % Append the new PCM data for the new bit, with random phase phase = (2 * rand(1)) * pi; x = [x (amplitude * sin((2 * pi * freq * (0:26) / 8000) + phase))]; end % Lowpass filter the resulting FSK samples d = fdesign.lowpass('N,Fp,Fst,Ast',4,1070,1270,10,8000); Hd = design(d,'equiripple'); y = filter(Hd,x); figure(); plot(x); title('Input data'); figure(); plot(y); title('Low pass output'); ```