In a message dated 2008/09/19, brett3nt@............. writes:

> The biggest drawback that I can see is that they don't seem to do too well 
> with higher drive frequencies.  I usually see them run at 5-6kHz or so, 
> while some other sensor types are happy at much higher frequencies.  After 
> you demodulate the signal, it contains a large second harmonic component 
> along with the higher even harmonics, and you need to filter it to recover 
> the baseband seismic signal.  But in a feedback seismograph, that filter 
> can introduce phase shifts which tend to make the loop oscillate.

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?

> If you move your carrier up to 20kHz, 

       You will get a load of unwanted and totally unneccessary problems.

> 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.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/09/19, brett3nt@............. writes:



The biggest drawback that I can=
 see is that they don't seem to do too well 

with higher drive frequencies.  I usually see them run at 5-6kHz or so,=
 

while some other sensor types are happy at much higher frequencies.  Af=
ter 

you demodulate the signal, it contains a large second harmonic component 
along with the higher even harmonics, and you need to filter it to recover <=
BR>
the baseband seismic signal.  But in a feedback seismograph, that filte=
r 

can introduce phase shifts which tend to make the loop oscillate.



Hi Brett,



       You are limited by the inductance of th=
e sensor coils and their natural resonant frequencies. Since the harmonics a=
re of higher frequency than that of the carrier, where is the difficulty in=20=
removing them?



If you move your carrier up to=20=
20kHz, 




       You will get a load of unwanted and to=
tally unneccessary problems.



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=20=


to use here?




       There is no problem there.



       Regards,



       Chris Chapman