PSN-L Email List Message

Subject: Re: Volksmeter sensor and the AD774x CDC chip
From: Larry Cochrane lcochrane@..............
Date: Sun, 23 Apr 2006 02:01:24 -0700

Hi Chris,

ChrisAtUpw@....... wrote:

> Hi Larry,
>     The original S/G circuit used a 62.5 sec AC coupled integrator with 
> a gain of 10, a 1 sec low pass filter and critical damping. This was to 
> give it an output linear with velocity.
>      >     I note that in your SG EMail reference
>     (, you claim a flat response
>      > from 50 sec to 3 Hz, but remarked that you had to substantially
>     increase
>      > the damping to get this. I note that in your latest circuit
>     diagram, you
>      > seem to have greatly reduced the itegration time and wondered if the
>      > circuit values were correct?
>     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 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. This is why I asked if the published 
> circuit values are correct??
>     Do you still use a 4.7 M Ohm feedback resistor, like in your 
> previous circuit at ?
>     Moreover, there does not seem to be a 1 sec low pass circuit to 
> produce a velocity response?
>     It is possible to ''squash'' this sort of humped response by heavy 
> overdamping and a considerable increase in the amplifier gain, but you 
> tend to run into serious noise problems.
>     The original circuit used an input capacitor to the integrator, 
> which should effectively limit the VLF 1/f noise. A 100 mu F non polar 
> capacitor?

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.

>     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.

Larry Cochrane
Redwood City, PSN


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