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 try=
ing 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 da=
ily ground tilt drift. If I=20
eliminate this using very long period optically sensed force feedback, I ca=
n 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 sec=
onds. The commercial=20
HS3 used this technique.=20


The basic concepts  for the Lippman circuit and the Robertson circuit are d=
escribed 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 se=
conds 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 ext=
ended period geophone has limits.

****I suggest that you have a look at some of the French School web sites s=
ometime. 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 d=
isplay 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 ca=
n 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 u=
sing them in nuclear applications then. I think that I may still have one t=
ucked 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  gener=
ation. If you can extend the range to 20 seconds still have a practical ins=
trument - -  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 pr=
eferred choice. Reliability, guaranteed life and repair policies are not al=
ways 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

=20
=20
=20
=20




















































Hi Dave, 











****A couple of comments that I forgot to include !











****If you are getting serious problems with 1/f long peri=
od noise when trying to look at Ea=
rth 







Eigemodes, why don't you try building a chop=
per amplifier ? They are IMMUNE to 1/f noise ! 















****The only factor which limits Lehmans like the SEPUK on=
 period is the daily ground tilt drift. If I 







eliminate this using very long period optically sensed for=
ce feedback, I can set mine up fai=
rly easily 







to run at 60 seconds period. If you added a capacitative position sensor to drive the force feedback 



and detect the signals, you should be able to sense stably out to 1,000 sec=
onds. The commercial 

HS3 used this technique. 












The basic concepts  for=
 the Lippman=20
circuit and the Robertson circuit are described in detail in the "Applicati=
ons=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=20
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 limits.=











****I suggest that you have a look at=
 some of the French School web sites sometime. There are a=
bout 50 of them 'in the field today' ! The seismometer response grap=
h 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 techni=
que as far as it can go.  



    I do=
n't have access to the Philbrick manu=
al, but in 1965 we didn't have Integrated Circuit opamps. Do they describe =
chopper amplifiers ? We were using them in nuclear applications then. I thi=
nk that I may still have one tucked away in the loft ! 



    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=20
desired. My goal was 0.5 Hz not because that is the best that can be done b=
ut=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 see=
ms to be=20
about 10 to 1 in frequency.










****Do check out www.lennartz-electronic.de/index.php They s=
how a graph of a 20:1 extension and the 2 Hz to 20 seconds seismometer is 4=
0:1. There may be operational reasons why a seemingly 'old fashioned' syste=
m may be the preferred choice. Reliability, guaranteed life and repair poli=
cies are not always quoted. I haven't noticed Nanometrics boasting about an=
y of these. 



    Incidentally, you =
can make a reasonably good 'school grade' seismometer usin=
g the 44mm OD piezo sounder disks. 



    Have you got two circuit boards for me =
please ? 



    Regards, 



    Chris Chapman