AM Detection example

am.m

# Generate test signal
t = linspace(0,120*pi, 1000)
baseband = cos(t/40)
carrier = cos(t)
modulated = carrier .* baseband

# Downshift and split into I and Q
fq_offset = 1.05 # Carrier and LO are off by 5%
i = cos(t*fq_offset) .* modulated
q = sin(t*fq_offset) .* modulated

lpf = fir1(51, 0.1)

# Filter signals
filtered_i = filter(lpf, 1, i)
filtered_q = filter(lpf, 1, q)

clf
subplot(4,1,1)
plot(t, baseband)
title("Input signal")

subplot(4,1,2)
plot(t, filtered_i)
title("I (unfiltered and filtered)")
ylabel("foo")
hold on
plot(t, i)
#plot(t, sign(filtered_i))

subplot(4,1,3)
plot(t, filtered_q)
hold on
plot(t, q)
#plot(t, sign(filtered_q))
title("Q (unfiltered and filtered)")


mag_sqr = (filtered_i.*filtered_i) + (filtered_q .* filtered_q)
mag = sqrt(mag_sqr)
filtered_mag = filter(lpf, 1, mag)
subplot(4,1,4)
plot(t, mag_sqr)
hold on
plot(t, mag)
# plot(t, sign(filtered_i) .* sign(filtered_q))
#plot(t, filtered_i+filtered_q)
plot(t, filtered_mag)
title("Mag and mag squared")
holf off