What is a one-bit long box car filter ?

Is it a FIR filter with all the coefficients at value one, so it's a
moving average filter ?

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
> 
> 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
> 

The DC offset is due to your signal not being centered at DC rather than 
at 50KHz as Vladmir mentioned.  It comes about because with the 50 KHz 
carrier, your phase difference from sample to sample is a constant due 
to the carrier frequency plus a delta due to the modulation.  The 
constant translates to a DC offset.  You need to either mix the signal 
down to complex baseband first, or do something with the output to 
remove the DC offset.  The former is the better solution because the DC 
offset can swamp your signal in real systems.

Patrick,

Please describe what you are doing. What you are trying to do is pretty
simple for DSP (not as simple as Algebra though!).

"Starting with a signal described as _____, creating I and Q signals by
_____ (detail here), ......"

Lots of detail and I bet you get lots of useful responses.

Dirk

Dirk Bell
DSP Consultant


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
>
> 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

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

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

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

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