Hi Chris ,

I believe the Lennartz 20 second instrument is based on a 1 second =
geophone. It has a self noise level  at least 20 db above the low noise =
model at 20 seconds. My standard  of comparison for  an expensive =
vertical seismometer is below the low noise model at 20 seconds . Even =
my own homemade verticals are below the low noise model to 35 seconds . =
I just don't buy it but I do understand why some people use them. =20

 Yes the 1965 Philbrick manual does discuss chopper stabilized =
amplifiers even though some of their modules were still using vacuum =
tubes (valves). The major application was analog computing which is =
where the term Operational Amplifier was first used. Only guys our age =
would know about that.

I always try chopper op amps in all my systems to see if they help with =
long period noise -- usually the AD 706 is best. I have tried many op =
amps and made noise tests using an HP 35665A.  The chopper amps are good =
but I have not found them  right for my applications. It seems =
counterintuitive but so far I have not found them necessary or =
desirable. However ,I always keep them in mind and in stock. =20

The broadband force balance sensors with the period extension to 500 or =
1000 seconds produce a signal which is entirely consistent with the =
measured  self noise. So far no excess noise has been observed.

Regarding circuit boards, I assume you are referring to the forced =
balance instruments.  I have not released any boards  unless I know =
exactly how they will be used.

Regards , Dave=20











 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





 




 
Hi Chris ,
 
I believe the Lennartz 20 second =
instrument is=20
based on a 1 second geophone. It has a self noise level  at =
least 20=20
db above the low noise model at 20 seconds. My standard  of=20
comparison for  an expensive vertical seismometer is below the =
low=20
noise model at 20 seconds . Even my own homemade verticals are below the =
low=20
noise model to 35 seconds . I just don't buy it but I do understand why =
some=20
people use them.  
 
 Yes the 1965 Philbrick manual =
does discuss=20
chopper stabilized amplifiers even though some of their modules were =
still using=20
vacuum tubes (valves). The major application was analog computing which =
is where=20
the term Operational Amplifier was first used. Only guys our age would=20
know about that.
 
I always try chopper op amps in all my =
systems to=20
see if they help with long period noise -- usually the AD 706 is best. I =

have tried many op amps and made noise tests using an HP=20
35665A.  The chopper amps are good but I have not found them  =
right=20
for my applications. It seems counterintuitive but so far I have not =
found them=20
necessary or desirable. However ,I always keep them in mind and in =
stock. =20

 
The broadband force balance sensors =
with the period=20
extension to 500 or 1000 seconds produce a signal which is entirely =
consistent=20
with the measured  self noise. So far no excess noise has been =

observed.
 
Regarding circuit boards, I assume you =
are=20
referring to the forced balance instruments.  I have not =
released any=20
boards  unless I know exactly how they will be used.
 
Regards , Dave 
 
 
 
 
 
 
 
 
 
 
 
 From: chrisatupw@....... 


Sent: Sunday, December 30, 2012 6:11 PM
To: psnlist@.............. 
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
www.lennartz-electronic.de/index.php=20
They show a graph of a 20:1 extension and the 2 Hz to 20 seconds =
seismometer is=20
40:1. There may be operational reasons why a seemingly 'old fashioned' =
system=20
may be the preferred choice. Reliability, guaranteed life and repair =
policies=20
are not always quoted. I haven't noticed Nanometrics boasting about any =
of=20
these. 

    =
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 =