****Hi Chris,
My goal is get AS1 equivalent performance out of a 4.5 Hz geophone,so I nee=
d to=20
go to 1/100 of the resonant frequency, all the way downto 0.045 Hz in the r=
ange=20
of surface wave frequencies.  I am not sure that the Roberts circuit is up =
to that.
..=20
 Hi Dave,=20
=20
    The origonal AS1 had resonant period of ~1.5 seconds. The response rang=
e=20
was extended from about 1.5 / 2 Hz down to 4.5 seconds. With an extracted t=
race,=20
the range could be extended out to about 20 seconds using the digital Filte=
r built=20
into AmaSeis - just so long as the digital signal is big enough. This only =
requires=20
an extra gain of x20 at 20 seconds.=20
    I have checked back on my notes on the Roberts' circuit and the best ex=
tension=20
that I achieved was x20, which requires an amplifier gain of x400. Any more=
 than=20
this ran me into noise problems. With a very low noise amplifier and sharp =
cut off=20
high pass filters, I could probably achieve x30 but this seems to be ~ the =
limit.=20
I did not try using a chopper amplifier.=20
    An alternative method is to use the geophone as a current generator. Fo=
r this,=20
you need an amplifier with a -ve input impedance to compensate for the resi=
stance=20
of the geophone. One problem with this method is that the signal is very sm=
all=20
indeed, so you need an extremely low noise amplifier, probably with discree=
t long=20
tailed pair of input transistors before the first opamp. The output charact=
eristic is a=20
signal rising in amplitude with frequency, so it needs to be frequency filt=
ered to give=20
a velocity output.=20
    Lennartz use both compensation types commercially on their seismometers=
,=20
which are described below, together with graphs.=20

http://www.lennartz-electronic.de/index.php?option=3Dcom_phocadownload&view=
=3Dcategory&id=3D1&Itemid=3D57
    The sensor patent is in German 'DE 3307575 C2' by Erich Lippmann=20
dated 1983/4, so it is now 'out of date'. Anyone can use it.
    But the LE-3D/20 second period seismometer uses 2 Hz geophones.=20
    The sensor period extension graph shows only ~ x30 extension, which is =
roughly=20
right. Only the French school seismometer system 'claims' to get a 20 secon=
d=20
performance from a 4.5 Hz geophone.
.. =20
****I have a very simple circuit that approaches that performance.
ttp://www.infiltec.com/seismo/qm45.htm
    If you just put a zero input impedance amplifier across a geophone - a =
current=20
to voltage converter - the range is extended but you still get a seriously =
'dog-legged'=20
amplitude response. =20
=20



































































=
****Hi Chris,

My goal is get AS1 equivalent performance out of a 4.5 Hz geophone,so I nee=
d to 


=
go to 1/100 of the res=
onant frequency, all the way downto 0.045 Hz in the range 


=
of surface wave freque=
ncies.  I am not sure that the Roberts circuit is up to that.

.. 


=
 Hi Dave, <=
/FONT>



=
 



=
  &nbs=
p; The origonal AS1 had resonant period of ~1.5 seconds. The response range 


=
was extended fro=
m about 1.5 / 2 Hz down to 4.5 seconds. With an extracted trace, 


=
the range could =
be extended out to about 20 seconds using the digital Filter built 


=
into AmaSeis - j=
ust so long as the digital sign=
al is big enough. This only requires 


=
an extra ga=
in of x20 at 20 seconds. 


=
    I h=
ave checked back on my notes on the Roberts' circuit and the best exte=
nsion 


=
that I achieved was x2=
0, which requires an amplifier gain of x400. Any more than 


=
this ran me into =
noise problems. With a very low noise amplifier and sharp cut off 


=
high pass filters, I could probably achieve x30=
 but this seems to be ~ the limit. 


=
I did not try using a chopper amplifier. 



=
    An =
alternative method is to use the geophone as a current generator. For this,=
 


=
you need an amplifier =
with a -ve input impedance to compensate for the resistance 


=
of the geophone. One p=
roblem with this method is that the signal is very small 


=
indeed, so you need an extremely low noise amplifier, probably with discreet lo=
ng 


=
tailed pair of <=
/FONT>inp=
ut transistors before the =
first opamp. The output characteristic is a 



=
signal rising in=
 a=
mplitude with frequency, so it needs to be frequency =
filtered to give 


=
a velocity outpu=
t. 


=
  =
  Lennartz use both compen=
sation types commercially on their seismometers, 


=
which are described below, together with =
graphs. 




http://www.lennartz-electronic.de/index.php?opti=
on=3Dcom_phocadownload&view=3Dcategory&id=3D1&Itemid=3D57
    The sensor patent is in Germ=
an 'DE 3307575 C2' by Erich Lippmann 
dated 1983/4, so it is now 'out of date'. Anyone can use it.<=
/FONT>
    But the LE-3D/20 sec=
ond period seismometer uses 2 Hz geophones. 
    The sensor period extension graph shows only ~ x30=
 extension, which is roughly 
right. Only the French school seismometer system 'claims' to get a 20=
 second 
performance from a 4.5 Hz geophone.
.  

*=
***I have a very simple circuit that approaches that performance.
=
http://www.infiltec.co=
m/seismo/qm45.htm

    If you just put a zero input impedance amplifier=
 across a geophone - a current 

to voltage converter=
 - the range is extended but you still get a seriously 'dog-legged'