Re: AM demodulation-envelope detection method

On Jul 19, 10:06=A0pm, "BCLIM" <boonchun_...@xxxxxxxxx> wrote:
On Jul 16, 8:42=3DA0am, "BCLIM" <boonchun_...@xxxxxxxxx> wrote:
Hi All,
=3DA0 =3DA0 =3DA0 I have read out Richard Lyons's book about
approxima=
tion
enve=3D
lope
detection. In the book got mentione after taking magniture of
analytic
signal, which is square root[square(I)+ square(Q)] and the result
need
to
be low pass filter to smoothen the signal. From the maths, the
resultant =3D
of
the modulus of analytic signal will be only carrie(DC) plus message
signal(which is the envelope). In this case why we still need to
low
pass
filter to smoothen out the signal. I did a FFT plot of the signal
before
low pass filtering, notice there are harmoincs of signal present.
Does
an=3D
y
one know why the harmonic present? Thanks.

Aliasing? When you square the I and Q signals their bandwidths
double.
Do you have enough headroom between your signal content and the
sample
rate to avoid aliasing?

Clay

Hi Clay,

=A0 =A0 =A0 =A0 The I and Q signal are digitized signal, the square of
th=
ese
signals are done in the software. Therefore there shouldn't be any
aliasi=
ng
problem. But definetely sqaure of each the signal will have second
harmonis, in this case I thought the mathematical shown at the end i
will
only have the DC term(carrier) plus the message signal so I have no
idea
why the harmonics still present. Thanks.

Regards,
BC- Hide quoted text -

- Show quoted text -

Hello BC,

I think you missed the point. For example let's say you have a 1000 Hz
sinewave sampled at 2500 Hz. Here there is no aliasing. Now square the
1000 Hz sinewave. A simple trig theorem tells us that we now have a
mix of 0Hz and 2000 Hz. Thus the 2000 Hz component sampled at 2500Hz
becomes a 500 Hz component. The aliasing has folded the signal's
spectrum back on itself. It doesn't matter that this is done in
software after the original sampling process.

Now back to the main question. Is your sampling rate more than four
times the maximum frequency? (before any squaring)

Clay


Hi Clay,
Yup. The sampling rate is more than 4X. The signal is 7kHz and the
sampling rate 300kHz something.
BC
.

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