Low Pass Filter for a PLL

Hi all.

I have done a PLL for tracking frecuency carrier in a MFSK demodulator. I
have used a moving filter average of length=64 as low pass filter. I cannot
rise lenght more than 64 because then PLL would not track frecuency
desviations of the transmitter. 

I dont know if there are other filters better than moving average filter
for this application, without surpass 64 coeeficients.

Can i use butterword filter low pass or similars??????

JAlbertoDJ wrote:

> Hi all.
> 
> I have done a PLL for tracking frecuency carrier in a MFSK demodulator. I
> have used a moving filter average of length=64 as low pass filter. I cannot
> rise lenght more than 64 because then PLL would not track frecuency
> desviations of the transmitter. 
> 
> I dont know if there are other filters better than moving average filter
> for this application, without surpass 64 coeeficients.
> 
> Can i use butterword filter low pass or similars??????
> 

On Sun, 20 Sep 2009 17:47:48 -0500, JAlbertoDJ wrote:

> Hi all.
> 
> I have done a PLL for tracking frecuency carrier in a MFSK demodulator.
> I have used a moving filter average of length=64 as low pass filter. I
> cannot rise lenght more than 64 because then PLL would not track
> frecuency desviations of the transmitter.
> 
> I dont know if there are other filters better than moving average filter
> for this application, without surpass 64 coeeficients.
> 
> Can i use butterword filter low pass or similars??????

Don't put FIR filters inside of control loops, unless you _really_ know 
what you're doing.

Are you really implementing a PLL, or are you just implementing a phase 
estimator and calling it a PLL?

Generally when you put a low pass filter inside a control loop the 
degradation to the loop response far exceeds the noise suppression of the 
low pass filter.  Furthermore, the filter poles will generally become 
less stable when you close the loop, so if you _do_ put a low pass filter 
in your loop you want to make it far more heavily damped than you would 
outside a loop.  

So you really only want a low pass filter in there if your measurement 
has _lots_ of high frequency noise, and if tracking is relatively 
unimportant.  This narrows things down: while there _are_ loops where low 
pass filters are indicated, you generally want a low-order (often just 
one pole) IIR filter, and you want to make sure you know what you're 
doing.

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