FM demodulation problem

Hi,

I've designed a complete FM demodulator under Simulink, but I've got a
problem.

My FM IF is 70 MHz, 250 kHz deviation, 250 kHz message, so my bandpass
signal is 1 MHz large (Carson BW =3D 2* (Delta F + Fmax message) =3D 1 MHz

I first undersample the IF at 13.75 MHz so my FM signal is translated
to 1.25 MHz, then I multiply with cosinus and sinus (1.25 MHz) to
obtain I and Q signal which have 500 kHz bandwith, then I need to low
pass filter at 500 kHz and it's here my problem...
If I low pass filter at 500 kHz I've got an ondulation on I=B2+Q=B2 then
if I filter at 2 MHz this ondulation is very low compared to a filter
at 500 kHz ?

So at what frequency do I need to filter after the downconversion ? 500
kHz or up ?

Thanks...

patrick.melet@dmradiocom.fr wrote:

> Hi,
>
> I've designed a complete FM demodulator under Simulink, but I've got a
> problem.

Hi Patrick,

 I have written 2 private messages to you. I hope you will remember me.
I have 4 FM demodulator designed in simulink, and related
documentation.

>
> My FM IF is 70 MHz, 250 kHz deviation, 250 kHz message, so my bandpass
> signal is 1 MHz large (Carson BW =3D 2* (Delta F + Fmax message) =3D 1 MHz
>
> I first undersample the IF at 13.75 MHz so my FM signal is translated
> to 1.25 MHz,.....
.=2E.........................Would you like explain how your FM signal is
translated to 1.25 Mhz?

 then I multiply with cosinus and sinus (1.25 MHz) to
> obtain I and Q signal which have 500 kHz bandwith, then I need to low
> pass filter at 500 kHz and it's here my problem...
> If I low pass filter at 500 kHz I've got an ondulation on I=B2+Q=B2 then
> if I filter at 2 MHz this ondulation is very low compared to a filter
> at 500 kHz ?
>
> So at what frequency do I need to filter after the downconversion ? 500
> kHz or up ?

As I mentioned in private messege, when I try to modify FM demodulator
designed for 10.7 MHz IF, I have same problem.

 I recommend you to record your undersampled data to Matlab workspace
by using <To workspace> block of Simulink. So you can look at its
spectrum by using FFT . The first biggest harmonic in symmetrical FM
signal gives you low pass filter cut-off frequency.
>=20
> Thanks...