PSN-L Email List Message

Subject: Re: VRDT Experience
From: Brett Nordgren brett3nt@.............
Date: Thu, 18 Sep 2008 22:25:39 -0400

Hi Chris,

At 08:59 PM 9/18/2008 -0400, you wrote:

>Hi Brett,
>        You are limited by the inductance of the sensor coils and their 
> natural resonant frequencies. Since the harmonics are of higher frequency 
> than that of the carrier, where is the difficulty in removing them?

No fundamental difficulty.  Depending on the sensor design, the harmonics 
might in some cases be rather large and take considerable attenuation.  You 
just don't want to mess up your loop phase margin in the process.

>>If you move your carrier up to 20kHz,
>        You will get a load of unwanted and totally unneccessary problems.

I would certainly not consider a magnetic device for that frequency (which 
was intended to be my original point).  Capacitor maybe?

>>If you do want to go that route and use 5kHz, a 3rd order filter that cuts
>>off somewhere around 400Hz should be about optimum.  It would add about 5
>>degrees at 20Hz, not too bad, and it should nicely filter the harmonics.
>>Does anyone know of a nice 400Hz 3rd order, low noise, Bessel filter design
>>to use here?
>       There is no problem there.

I agree, no problem.  What sort of design would work well?



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