PSN-L Email List Message

Subject: Re: Volksmeter sensor and the AD774x CDC chip
From: ChrisAtUpw@.......
Date: Sun, 23 Apr 2006 22:36:00 EDT


In a message dated 23/04/2006, lcochrane@.............. writes:

>     Like the VM channel (LCTST) my LC8 channel is  pretty much the raw
>     data from the pickup  except there are two 60 second high-pass filters 
in the
>   signal path. My LC3 channel, the integrated output, should have a  
velocity >response from about 1 second, the period of the pendulum, to  about 50 
seconds. I
>     have not made any changes to my  SG sensor for many years so the
>     value of the  integrator parts should be the same.
> 
>     The  circuit at http://psn.quake.net/sg-schm.gif shows a 100 K Ohm 
> input  resistor, a 470 K Ohm feedback resistor and a 2 mu F parallel 
>  capacitor. This RC combination rolls off at a period of only 5.9 seconds  
> - a factor of 10 less than we want.
>     Do you  still use a 4.7 M Ohm feedback resistor, like in your 
> previous  circuit at http://psn.quake.net/sgproc.gif ?
>      Moreover, there does not seem to be a 1 sec low pass circuit to 
>  produce a velocity response?

I still use a 4.7M feedback resistor and a  2.2UF cap. There is a 60 second 
high-pass filter before the integrator to  limit the low frequencies and DC 
offset going into 
the  integrator.
    I appreciated that you had split the integrator  circuit over the two 
opamps. One way of improving the performance a bit is to  use a two pole high 
pass filter on the input, instead of the single pole. It  gives a much sharper 
cutoff.
    I also question whether we really do need a 60 sec  response for general 
purposes? I suspect that 30 to 40 sec would be quite  adequate?
    The other problem with the S/G circuit is that you  do need to make an 
effort to limit the system noise, since the low frequency  gains need to be 
quite high. This is a suitable application for CAZ  opamps.
    I note that to give a velocity signal you need to  differentiate the S/G 
output from 1 to 10 Hz. You would probably be able to  use four instead of six 
stages of 10 Hz LP filter if you did this? A  3.3 mu F capacitor in series 
with the 50 K Ohm variable input resistor to the HF  channel should do this 
quite well.

>     Note that it is usually possible to lengthen  the period using 
> digital processing - if there is sufficient signal  resolution, as seems 
> likely with the VM 24 bit ADC. However, doing  this to a displacement 
> signal is likely to give rather large  amplitudes at the low frequencies. 
> Maybe the VM displacement signal  could be converted into a velocity signal?

Remember that a pendulum  with a displacement pickup will act as an 
accelerometer below the period of  the pendulum, so both the SG and the new VM sensor 
are recording acceleration  for teleseismic events. The integrator, in the SG 
sensor electronics (my LC3  channel), should be producing a velocity signal. I 
should be able to do the  same thing in WinSDR by digitally integrating the 
acceleration signal from the  VM sensor.


Great! 
    Can you also differentiate the VM signals above the  1 Hz pendulum 
frequency to give a velocity signal from 1 to 10 Hz?  
 
    In the S/G Construction Notes the 'photos of my SG  sensors' and the 
first 'photos of my sensors here' both have broken  links.
    The link to Arie Verveer's S/G seismometers no  longer works. Could you 
put his photos on your website, please?
 
    Regards,
 
    Chris Chapman





In a message dated 23/04/2006, lcochrane@.............. writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20 size=3D2>>     Like the VM channel (LCTST) my LC8 channe= l is=20 pretty much the raw
>     data from the pickup=20 except there are two 60 second high-pass filters in the
> =20    signal path. My LC3 channel, the integrated output, should ha= ve a=20 velocity >response from about 1 second, the period of the pendulum= , to=20 about 50 seconds. I
>     have not made any changes t= o my=20 SG sensor for many years so the
>     value of the=20 integrator parts should be the same.
>
>     T= he=20 circuit at http://psn.quake.net/sg-schm.gif shows a 100 K Ohm
> inp= ut=20 resistor, a 470 K Ohm feedback resistor and a 2 mu F parallel
>=20 capacitor. This RC combination rolls off at a period of only 5.9 seconds=20
> - a factor of 10 less than we want.
>     Do= you=20 still use a 4.7 M Ohm feedback resistor, like in your
> previous=20 circuit at http://psn.quake.net/sgproc.gif ?
>    =20 Moreover, there does not seem to be a 1 sec low pass circuit to
>=20 produce a velocity response?

I still use a 4.7M feedback resistor a= nd a=20 2.2UF cap. There is a 60 second high-pass filter before the integrator to=20 limit the low frequencies and DC offset going into
the=20 integrator.
    I appreciated that you had split the integrator= =20 circuit over the two opamps. One way of improving the performance a bit is t= o=20 use a two pole high pass filter on the input, instead of the single pole. It= =20 gives a much sharper cutoff.
    I also question whether we really do need a 60=20= sec=20 response for general purposes? I suspect that 30 to 40 sec would be quite=20 adequate?
    The other problem with the S/G circuit is that=20= you=20 do need to make an effort to limit the system noise, since the low frequency= =20 gains need to be quite high. This is a suitable application for CAZ=20 opamps.
    I note that to give a velocity signal you need=20= to=20 differentiate the S/G output from 1 to 10 Hz. You would probably be able to=20 use four instead of six stages of 10 Hz LP filter if you did this?= A=20 3.3 mu F capacitor in series with the 50 K Ohm variable input resistor to th= e HF=20 channel should do this quite well.
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20 size=3D2>>     Note that it is usually possible to lengt= hen=20 the period using
> digital processing - if there is sufficient sign= al=20 resolution, as seems
> likely with the VM 24 bit ADC. However, doin= g=20 this to a displacement
> signal is likely to give rather large=20 amplitudes at the low frequencies.
> Maybe the VM displacement sign= al=20 could be converted into a velocity signal?

Remember that a pendulum= =20 with a displacement pickup will act as an accelerometer below the period o= f=20 the pendulum, so both the SG and the new VM sensor are recording accelerat= ion=20 for teleseismic events. The integrator, in the SG sensor electronics (my L= C3=20 channel), should be producing a velocity signal. I should be able to do th= e=20 same thing in WinSDR by digitally integrating the acceleration signal from= the=20 VM sensor.
    Great!
    Can you also differentiate the VM signals above= the=20 1 Hz pendulum frequency to give a velocity signal from 1 to 10 Hz?  
 
    In the S/G Construction Notes the 'photos of my= SG=20 sensors' and the first 'photos of my sensors here' both have broken=20 links.
    The link to Arie Verveer's S/G seismometers no=20 longer works. Could you put his photos on your website, please?
 
    Regards,
 
    Chris Chapman

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