Hi Chris ,

Yes I agree. I have been working extensively on period extension =
circuits for 4.5Hz geophones extending the period to 0.5 Hz.=20

The so called Lippman circuit is just a well known negative impedance =
converter circuit  ( NIC) applied to geophones.  With the original =
Lippman circuit the output of the NIC is proportional to acceleration =
and subsequent circuits shape the spectrum to give the desired velocity =
response with the 2 slope roll off below the long period corner.  I have =
taken it a step further and modified the NIC to provide a velocity =
response at the output of the NIC. The result  is that there is no point =
in the active signal path where the signal is proportional  to =
acceleration.  The advantage is a significant improvement in clipping =
margin for a strong local event and better DC stability. =20

Brett has created  a spice model which has been very helpful in =
optimizing the selection of the negative impedance load on the geophone. =
I am confident the same circuit could be used to extend the period of a =
1 second geophone to ~ 20 seconds. I was able to use the circuit equally =
well for 4.5 Hz geophones  with both 380  and 4000 ohm coils. For the 1 =
second geophone a high resistance coil would be necessary to avoid =
impractical component values.

 The DC gain of the NIC will become very high ( potentially unstable)  =
if you attempt to match the coil resistance with a negative resistance =
resulting in a near zero net resistance.  There is an optimum negative =
resistance  which provides good DC stability and the desired frequency =
response.

The major disadvantage of all period extension methods is long period =
noise . A look at the Lennartz noise curves illustrates that very well. =
It is a unavoidable consequence of the technique ,however, for some =
applications like volcano monitoring it may not be important.  Low nose =
op amps are essential as you point out  The force feedback technique is =
dramatically lower noise so is the preferred method for periods longer =
than a few seconds.

  I have been doing this work for a friend for a commercial application =
but the circuits are available. We can put them on Brett's website since =
I do not have a website.=20

I am writing a PSN posting on my period extension experience -- just =
need to find the time.=20

Regards,=20

Dave Nelson
Rolling Hills Estates, California=20

I add my location to avoid confusion with my namesake from "downunder"=20






From: chrisatupw@..........
Sent: Thursday, December 27, 2012 2:57 PM
To: psnlist@.................
Subject: Re: Instrumentation Question


From: Dave Nelson 

To: psnlist 
Sent: Mon, 24 Dec 2012 18:23
Subject: Re: Instrumentation Question


DC drift is very common --almost universal -- in seismometers. Even the=20
best broadband instruments have a drift proportional to the rate of=20
temperature change. The frequency content of the drift are usually well=20
below the seismic information so is of no consequence.

A 24 hour periodic drift is of absolutely no consequence for a geophone. =
For=20
a 1 second instrument, even with period extension, the longest practical =

seismic signal which can be observed is probably around 20 seconds. The=20
noise level at 20 seconds, after period extension, will be so high that =
only=20
very large events will be observable above the instrument/system noise. =
A=20
high pass filter in Winsdr at .05 Hz 1 pole or AC coupling with a time=20
constant of about 3 seconds (20 seconds / 2pi) would be about right.

Cheers,
Dave Nelson
Rolling Hills Estates,=20

Hi Dave,
****The noise that you see from a period extension circuit depends on =
the opamps used and on the=20
circuit chosen.
Period Extensions of up to x10 can be achieved using the Roberts' =
Circuit. The internal gain is x100.=20
For Period Extensions of up to x50, you need the Lippmann circuit and a =
VERY low noise amplifier.=20
The output short circuit current is very low.  =20
As far as I remember, Geoff is trying to use a Lippmann type circuit, so =
he should get reasonably low=20
noise signals at 20 seconds.=20

****Brett Nordgren wrote ;
Even professional instruments show substantial DC drift. Broadband=20
verticals have an output proportional to the rate of temperature=20
change and over a 24-hour period can easily vary by a volt or more=20
depending on their thermal insulation. We routinely put in a=20
one-pole digital high-pass filter at the lowest frequency available=20
(0.002 Hz in WinSDR) which makes all that go away.

****An offset of over a volt could exceed the input range of some ADCs =
!!=20
Such poor quality electronics would be totally unacceptable in my =
opinion.=20

 >> Subject: Instrumentation Question
>>
>> Yet I can not rid a small DC drift which is either related to
>> ambient AC noise level or DC drift of resistance or
>> thermocouple voltage related to the soldering of junctions.
>> This DC drift is related to the 24 hour cycle.
>> The dc drift is on the order of micro volts
>> which seem to be originating on the input.
>> The overall DC gain is 80Dbv or X10000.
>> I should be able to achieve a DC free drift at this gain.
>> I am using a op177G op amp ??
>> The sensor is HS10-1 Geophone.
>> The 40 foot of cable is designed for burial
>> it has silicon grease (I think) impregnated and
>> has a heavy copper jacket.
>> It is soldered at the geophone and also at the sensor
>> has soldered connections.
>
>      If you use a CAZ type opamp like the LTC1150 you will get zero=20
> temperature drift.
>
> The problem does not seem to be the amplifier itself.
> But rather, the variables dealing with the input.
> Cable/Geophone/Common mode stuff/
> Yet I have found a wide variation within the
> op amp offset voltages.
> I am currently using a Chopper amp (or so I think)
> In the front end. LTC1050 ???
> Yes, I have troubles finding decent opamps
> at civilian prices, the best are all seem to be MilSpec
> rip offs. Outrageous prices. The milspec parts are by far
> the best and all others should simply be scrapped.
>
> Regards,
> geoff
> LTC1050 - Precision Zero-Drift Operational Amplifier with Internal=20
> Capacitors

***So is the LTC1150 ! Check it out ? !=20

Regards,=20

Chris Chapman









Hi Chris ,
 
Yes I agree. I have been working =
extensively on=20
period extension circuits for 4.5Hz geophones extending the period to =
0.5 Hz.=20

 
The so called Lippman circuit is just a =
well known=20
negative impedance converter circuit  ( NIC) applied to geophones.=20
 With the original Lippman =
circuit the=20
output of the NIC is proportional to acceleration and subsequent =
circuits shape=20
the spectrum to give the desired velocity response with the 2 slope roll =

off below the long period corner.  I have taken it a step further and modified the NIC to =
provide a=20
velocity response at the output of the NIC. The result  is =
that there=20
is no point in the active signal path where the signal is =
proportional  to=20
acceleration.  The advantage is a significant improvement in =
clipping=20
margin for a strong local event and better DC stability.  =

 
Brett has created  a =
spice model=20
which has been very helpful in optimizing the selection of the negative=20
impedance load on the geophone. I am confident the same circuit could be =
used to=20
extend the period of a 1 second geophone to ~ 20 seconds. I was able to =
use the=20
circuit equally well for 4.5 Hz geophones  with both 380  =
and=20
4000 ohm coils. For the 1 second geophone a high resistance coil would=20
be necessary to avoid impractical component values.
 
 The DC gain of the NIC will =
become very high=20
( potentially unstable)  if you attempt to match the coil =
resistance with a=20
negative resistance resulting in a near zero net resistance.  =
There is=20
an optimum negative resistance  which provides good DC =
stability and=20
the desired frequency response.
 
The major disadvantage of all period =
extension=20
methods is long period noise . A look at the Lennartz noise curves =
illustrates=20
that very well. It is a unavoidable consequence of the technique =
,however, for=20
some applications like volcano monitoring it may not be important.  =
Low=20
nose op amps are essential as you point out  The force feedback =
technique=20
is dramatically lower noise so is the preferred method for periods =
longer than a=20
few seconds.
 
  I have been doing this work for =
a friend for=20
a commercial application but the =
circuits are=20
available. We can put them on Brett's website since I do not have a =

website. 
 
I am writing a PSN posting on my period =
extension=20
experience -- just need to find the time. 
 
Regards, 
 
Dave Nelson
Rolling Hills Estates, California =

 
I add my location to avoid confusion =
with my=20
namesake from "downunder" 
 
 
 
 




From: chrisatupw@....... 
Sent: Thursday, December 27, 2012 2:57 PM
To: psnlist@.............. 
Subject: Re: Instrumentation Question







From: =
Dave Nelson <davefnelson@.......>

To: psnlist <psnlist@..............>
=
Sent:=20
Mon, 24 Dec 2012 18:23
Subject: Re: Instrumentation=20
Question


DC drift is =
very common --almost universal -- in seismometers. Even the=20
best broadband instruments have a drift proportional to the rate of=20
temperature change. The frequency content of the drift are usually well=20
below the seismic information so is of no consequence.

A 24 hour periodic drift is of absolutely no consequence for a geophone. =
For=20
a 1 second instrument, =
even with period extension, the longest practical=20
seismic signal which can be observed is probably around 20 seconds. The=20
noise level at 20 seconds, after period extension, will be so high that =
only=20
very large events will =
be observable above the instrument/system noise. A=20
high pass filter in Winsdr at .05 Hz 1 pole or AC coupling with a time=20
constant of about 3 seconds (20 seconds / 2pi) would be about right.

Cheers,
Dave Nelson
Rolling Hills Estates,=20

Hi Dave,
****The noise that you =
see from a period extension circuit depends on the opamps =
used and on the 

circuit chosen.

Period Extensions of up to =
x10 can be achieved using the Roberts' Circuit. The internal gain is =
x100. 

For Period Extensions of up to x50, you need the Lippmann =
circuit and a VERY low noise amplifier. 

The output short circuit current is very low.  =
 =


As far as I remember, Geoff =
is trying to use a Lippmann type circuit, so he should get reasonably =
low 

noise signals at 20 seconds. 

****Brett Nordgren wrote ;
Even professional instruments show substantial DC drift. =
Broadband=20
verticals have an output proportional to the rate of temperature=20
change and over a 24-hour period can easily vary by a volt or more=20
depending on their thermal insulation. We routinely put in a=20
one-pole digital high-pass filter at the lowest frequency available=20
(0.002 Hz in WinSDR) which makes all that go away.


****An offset of =
over a volt could exceed the input range of some ADCs !! 

Such poor quality =
electronics would be totally unacceptable in my opinion. 


 >> Subject: Instrumentation Question
>>
>> Yet I can not rid a small DC drift which is either related to
>> ambient AC noise level or DC drift of resistance or
>> thermocouple voltage related to the soldering of junctions.
>> This DC drift is related to the 24 hour cycle.
>> The dc drift is on the order of micro volts
>> which seem to be originating on the input.
>> The overall DC gain is 80Dbv or X10000.
>> I should be able to achieve a DC free drift at this gain.
>> I am using a op177G op amp ??
>> The sensor is HS10-1 Geophone.
>> The 40 foot of cable is designed for burial
>> it has silicon grease (I think) impregnated and
>> has a heavy copper jacket.
>> It is soldered at the geophone and also at the sensor
>> has soldered connections.
>
>      If you use a CAZ type opamp like the LTC1150 you will get zero =

> temperature drift.
>
> The problem does not seem to be the amplifier itself.
> But rather, the variables dealing with the input.
> Cable/Geophone/Common mode stuff/
> Yet I have found a wide variation within the
> op amp offset voltages.
> I am currently using a Chopper amp (or so I think)
> In the front end. LTC1050 ???
> Yes, I have troubles finding decent opamps
> at civilian prices, the best are all seem to be MilSpec
> rip offs. Outrageous prices. The milspec parts are by far
> the best and all others should simply be scrapped.
>
> Regards,
> geoff
> LTC1050 - Precision Zero-Drift Operational Amplifier with Internal=20
> Capacitors



***So is the LTC1150 ! Check it out ? =
! 



Regards, 


Chris Chapman