In a message dated 20/04/2006, lcochrane@.............. writes:

> Hi  Larry,
>     What period compensation are you claiming  for this sensor?
>     Do the plots show a compensated  velocity / compensated displacement 
> / uncompensated signal, or  what?
>     How are you performing the velocity feedback  / damping necessary to 
> stabilise the response?

There is no  period compensation in either the SG sensor or the Volksmeter 
(VM). Both use  simple damping. The SG sensor has a feedback loop but it's just 
there to damp  the pendulum. The VM sensor uses eddy-current damping and has 
no feedback  system. 
Randall will be sending a response to this shorty.

The  plots are from the raw data right out of the sensor, but with a 60 
second 2  pole 
high-pass and 2 Hz 2 pole low-pass filter applied to the data before  making 
the GIF 
image. The event files are the raw data from the AD7746  chip.
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 
(_http://psn.quake.net/freqtest.html_ (http://psn.quake.net/freqtest.html) ),  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 _http://psn.quake.net/sg-schm.gif_ 
(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. 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 _http://psn.quake.net/sgproc.gif_ (http://psn.quake.net/sgproc.gif)  ?
    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?
 
    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?
 
    Regards,
 
    Chris Chapman





In a message dated 20/04/2006, lcochrane@.............. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>> Hi=20
  Larry,
>     What period compensation are you claimin=
g=20
  for this sensor?
>     Do the plots show a compensate=
d=20
  velocity / compensated displacement 
> / uncompensated signal, or=20
  what?
>     How are you performing the velocity feedb=
ack=20
  / damping necessary to 
> stabilise the response?

There is no=
=20
  period compensation in either the SG sensor or the Volksmeter (VM). Both u=
se=20
  simple damping. The SG sensor has a feedback loop but it's just there to d=
amp=20
  the pendulum. The VM sensor uses eddy-current damping and has no feedback=20
  system. 
Randall will be sending a response to this shorty.

The=20
  plots are from the raw data right out of the sensor, but with a 60 second=20=
2=20
  pole 
high-pass and 2 Hz 2 pole low-pass filter applied to the data bef=
ore=20
  making the GIF 
image. The event files are the raw data from the AD7746=
=20
  chip.
Hi Larry,
 
    The original S/G circuit used a 62.5 sec AC cou=
pled=20
integrator with a gain of 10, a 1 sec low pass filter and critical damp=
ing.=20
This was to give it an output linear with velocity. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>     I note that in your SG EMail reference (<=
A=20
  href=3D"http://psn.quake.net/freqtest.html">http://psn.quake.net/freqtest.=
html),=20
  you claim a flat response 
> from 50 sec to 3 Hz, but remarked that=20=
you=20
  had to substantially increase 
> the damping to get this. I note tha=
t in=20
  your latest circuit diagram, you 
> seem to have greatly reduced the=
=20
  itegration time and wondered if the 
> circuit values were=20
  correct?

Like the VM channel (LCTST) my LC8 channel is pretty much=20=
the=20
  raw data from the 
pickup except there are two 60 second high-pass filt=
ers=20
  in the signal path. My LC3 
channel, the integrated output, should have=
 a=20
  velocity response from about 1 second, the period of the pendulum, to abou=
t 50=20
  seconds. I have not made any changes to my SG sensor for many years so the=
=20
  value of the integrator parts should be the same.


    The circuit at http://psn.quake.net/sg-schm.gif shows=20
a 100 K Ohm input resistor, a 470 K Ohm feedback resistor and a 2 mu F=20
parallel capacitor. This RC combination rolls off at a period of only 5.9=
=20
seconds - a factor of 10 less than we want. This is why I asked if the=20
published circuit values are correct?? 
    Do you still use a 4.7 M Ohm feedback resistor,=
=20
like in your previous circuit at http://psn.quake.net/sgproc.gif=
 ?
    Moreover, there does not seem to be a 1 sec low=
=20
pass circuit to produce a velocity response? 
 
    It is possible to ''squash'' this sort of humpe=
d=20
response by heavy overdamping and a considerable increase in the amplif=
ier=20
gain, but you tend to run into serious noise problems. 
    The original circuit used an input capacitor to=
 the=20
integrator, which should effectively limit the VLF 1/f noise. A 100 mu=20=
F=20
non polar capacitor?
 
    Note that it is usually possible to length=
en=20
the period using digital processing - if there is sufficient signal=20
resolution, as seems likely with the VM 24 bit ADC. However, doing this=
 to=20
a displacement signal is likely to give rather large amplitudes at the low=20
frequencies. Maybe the VM displacement signal could be converted into a velo=
city=20
signal?
 
    Regards,
 
    Chris Chapman