Digital FM demodulation

Hi everybody,

I demodulate a FM signal with this : IdQ - QdI

But in a paper I see that after this, we have to add an Integrate and
Dump !! But for this operation, we need and integrate clock time over
the symbole to demodulate....

If I don't put this Integrate and Dump block I think that in presence
of noise the demodulator works bad !!!

Do you have suggestions ?

Thanks.

Patrick,

Please more info for the group.

How do you implement the dI and dQ?

Is the signal bandlimited to a range appropriate for dI and dQ
implementation?

Do you account for any delay between I and dI, Q and dQ?

Did you try to normalize by sqrt(I^2+Q^2)?


Dirk Bell
DSP Consultant



patrick.me...@dmradiocom.fr wrote:
> Hi everybody,
>
> I demodulate a FM signal with this : IdQ - QdI
>
> But in a paper I see that after this, we have to add an Integrate and
> Dump !! But for this operation, we need and integrate clock time over
> the symbole to demodulate....
>
> If I don't put this Integrate and Dump block I think that in presence
> of noise the demodulator works bad !!!
> 
> Do you have suggestions ?
> 
> Thanks.

I compute de dI =3D  I(n)-I(n-1), the same for dQ...

I first normalize I and Q by dividing I and Q by SQRT(I=B2+Q=B2) before
demoduling with the derivative.

So after the derivative, what's the better :

1 - Integrate and Dump then symbol decision

2 - Compare the symbol with a treshold beetween Max and Min

For 1 I need a decision clock, so a clock bit synchronizer

For 2 I don't need a bit clock, so if the transmission is asynchronous,
I just need to send the output of the comparator to an UART

thanks

patrick.melet@dmradiocom.fr wrote:
> I compute de dI =3D  I(n)-I(n-1), the same for dQ...
>
> I first normalize I and Q by dividing I and Q by SQRT(I=B2+Q=B2) before
> demoduling with the derivative.
>
> So after the derivative, what's the better :
>
> 1 - Integrate and Dump then symbol decision
>
> 2 - Compare the symbol with a treshold beetween Max and Min
>
> For 1 I need a decision clock, so a clock bit synchronizer
>
> For 2 I don't need a bit clock, so if the transmission is asynchronous,
> I just need to send the output of the comparator to an UART
>
> thanks

You should pass the discriminator through a one-bit long boxcar filter
in either case.

John

So your I,Q are 1/2 sample out of alignment with your dI,dQ, and your
differentiator only differentiates for that part of the spectrum that
is grossly oversampled.

Dirk Bell
DSP Consultant

patrick.melet@dmradiocom.fr wrote:
> I compute de dI =3D  I(n)-I(n-1), the same for dQ...
>
> I first normalize I and Q by dividing I and Q by SQRT(I=B2+Q=B2) before
> demoduling with the derivative.
>
> So after the derivative, what's the better :
>
> 1 - Integrate and Dump then symbol decision
>
> 2 - Compare the symbol with a treshold beetween Max and Min
>
> For 1 I need a decision clock, so a clock bit synchronizer
>
> For 2 I don't need a bit clock, so if the transmission is asynchronous,
> I just need to send the output of the comparator to an UART
>=20
> thanks

Yes I have a problem in my Matlab simulation...

After the demod which computes the demodulated signal, I've got an
amplitude offset and my signal is not normalized to 1

I have divided the output of the substractor by I=B2+Q=B2...

What do you mean by the delay beetwen I and dI and Q and dQ ? you talk
about a 1/2 sample ? The sampling frequency or the center frequency of
I and Q ?

My I and Q signals are center to 50 kHz with +/- 40 kHz deviation...
and I'm sampling at 2 MHz

hi

I have simulated the FM demodulation under matlab and I have a problem

At the output of the substractor, there's a DC offset

I made the differentiator with I[n]-I[n-1]

My I and Q signals are center to 50 kHz, with +/- 40 kHz deviation

What do you mean by I and dI (Q and dQ) alignment, we put a one sample
(frequency sampling at 2 MHz delay on the I and Q branch before
multpliying by dI and dQ

I don't understand why I have this DC offset

thanks

patrick.melet@dmradiocom.fr wrote:

> hi
> 
> I have simulated the FM demodulation under matlab and I have a problem
> 
> At the output of the substractor, there's a DC offset> 
> My I and Q signals are center to 50 kHz, with +/- 40 kHz deviation
> I don't understand why I have this DC offset

The DC offset is there because your signal are centered at 50 kHz.
Isn't it obvious?
You should center your signal at zero frequency.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

hi

I have simulated the FM demodulation under matlab and I have a problem

At the output of the substractor, there's a DC offset

I made the differentiator with I[n]-I[n-1]

My I and Q signals are center to 50 kHz, with +/- 40 kHz deviation

What do you mean by I and dI (Q and dQ) alignment, we put a one sample
(frequency sampling at 2 MHz delay on the I and Q branch before
multpliying by dI and dQ

I don't understand why I have this DC offset

thanks

hi

I have simulated the FM demodulation under matlab and I have a problem

At the output of the substractor, there's a DC offset

I made the differentiator with I[n]-I[n-1]

My I and Q signals are center to 50 kHz, with +/- 40 kHz deviation

What do you mean by I and dI (Q and dQ) alignment, we put a one sample
(frequency sampling at 2 MHz delay on the I and Q branch before
multpliying by dI and dQ

I don't understand why I have this DC offset

thanks