Disregard last message Sent by mistake before it was finished
From: chrisatupw@..........
Sent: Sunday, December 30, 2012 6:11 PM
To: psnlist@.................
Subject: Re: Instrumentation Question
Hi Dave,=20
****A couple of comments that I forgot to include !
****If you are getting serious problems with 1/f long period noise when =
trying to look at Earth=20
Eigemodes, why don't you try building a chopper amplifier ? They are =
IMMUNE to 1/f noise !=20
****The only factor which limits Lehmans like the SEPUK on period is the =
daily ground tilt drift. If I=20
eliminate this using very long period optically sensed force feedback, I =
can set mine up fairly easily=20
to run at 60 seconds period. If you added a capacitative position sensor =
to drive the force feedback=20
and detect the signals, you should be able to sense stably out to 1,000 =
seconds. The commercial=20
HS3 used this technique.=20
The basic concepts for the Lippman circuit and the Robertson circuit =
are described in detail in the "Applications Manual for Computing =
Amplifiers" by Philbrick Research Laboratories in 1965. This is FAR from =
new science. The French School of Seismometers may have extended a 4.5 =
Hz geophone to 20 seconds but we don't see them in the field today =
because it was, and still is, impractical because of high noise. A =
practical, operationally useful extended period geophone has limits.
****I suggest that you have a look at some of the French School web =
sites sometime. There are about 50 of them 'in the field today' ! The =
seismometer response graph is on their web site. You can go to =
particular schools and display the current traces on line. I was a bit =
sceptical too, so I looked, but they don't seem to suffer from excessive =
long period noise. But I would agree that the French do seem to have =
pushed the technique as far as it can go. =20
I don't have access to the Philbrick manual, but in 1965 we didn't =
have Integrated Circuit opamps. Do they describe chopper amplifiers ? We =
were using them in nuclear applications then. I think that I may still =
have one tucked away in the loft !=20
Sure there are practical limits, but it can be fun to see just how =
far you can push them !
The absolute maximum in damping is not necessary or desired. My goal was =
0.5 Hz not because that is the best that can be done but because that is =
all that is required by local/regional earthquake location and shake map =
generation. If you can extend the range to 20 seconds still have a =
practical instrument - - go for it.
The practical, not absolute, limit seems to be about 10 to 1 in =
frequency.
****Do check out www.lennartz-electronic.de/index.php They show a graph =
of a 20:1 extension and the 2 Hz to 20 seconds seismometer is 40:1. =
There may be operational reasons why a seemingly 'old fashioned' system =
may be the preferred choice. Reliability, guaranteed life and repair =
policies are not always quoted. I haven't noticed Nanometrics boasting =
about any of these.=20
Incidentally, you can make a reasonably good 'school grade' =
seismometer using the 44mm OD piezo sounder disks.=20
Have you got two circuit boards for me please ?=20
Regards,=20
Chris Chapman=20
Disregard last message Sent by =
mistake before=20
it was finished
From: chrisatupw@.......
Sent: Sunday, December 30, 2012 6:11 PM
Subject: Re: Instrumentation Question
Hi Dave,
****A couple of comments =
that I forgot=20
to include !
****If you are=20
getting serious problems with 1/f =
long period=20
noise when trying to look at Earth=20
Eigemodes, why don't you try building a =
chopper=20
amplifier ? They are IMMUNE to 1/f noise !
****The only=20
factor which limits Lehmans like the SEPUK on period is the daily ground =
tilt=20
drift. If I
eliminate this using very long period =
optically=20
sensed force feedback, I can set mine up fairly=20
easily
to run at 60 seconds period. If you added a capacitative =
position=20
sensor to drive the force feedback
and detect the =
signals, you=20
should be able to sense stably out to 1,000 seconds. The commercial =
HS3 used=20
this technique.=20
The basic concepts for =
the Lippman=20
circuit and the Robertson circuit are described in detail in the =
"Applications=20
Manual for Computing Amplifiers" by Philbrick Research Laboratories in =
1965.=20
This is FAR from new science. The French School of Seismometers may have =
extended a 4.5 Hz geophone to 20 seconds but we don't see them in =
the field=20
today because it was, and still is, impractical because of high =
noise. A=20
practical, operationally useful extended period geophone has=20
limits.
****I suggest that you have a =
look at=20
some of the French School web sites sometime. There are =
about 50 of=20
them 'in the field today' !=20
The seismometer response graph is on their web =
site.=20
You can go to particular schools and display the current traces on line. I was a bit sceptical too, so I=20
looked, but they don't seem to suffer from excessive long period noise.=20
But I would agree that the =
French do=20
seem to have pushed the technique as far as it can go.
I don't have access to the =
Philbrick=20
manual, but in 1965 we didn't have =
Integrated=20
Circuit opamps. Do they describe chopper amplifiers ? We were using them =
in=20
nuclear applications then. I think that I may still have =
one tucked=20
away in the loft !
Sure there are practical limits, but it can =
be fun to=20
see just how far you can push them !
The absolute maximum in damping is not =
necessary or=20
desired. My goal was 0.5 Hz not because that is the best that can be =
done but=20
because that is all that is required by local/regional earthquake =
location and=20
shake map generation. If you can extend the range to 20 seconds =
still have=20
a practical instrument - - go for it.
The practical, not absolute, limit =
seems to be=20
about 10 to 1 in frequency.
****Do check out=20
=
Incidentally,=20
you can make a reasonably good 'school grade' seismometer using the=20
44mm OD piezo sounder disks.
=
Have you=20
got two circuit boards for me please ?
=
=20
Regards,
=
Chris Chapman =