Hi Bob,

I have tried the real-time period extending filter in WinSdr with very =
good results . I am using it to extend the period of broadband force =
balance seismometers from 50 or 120 seconds to 1000 seconds   I know =
that was not the original intent but it really works well.  I have =
compared its performance with an analog "inverse filter" with =
essentially the same transfer function .The resulting waveforms are =
essentially identical. =20

I have not yet tried the WinQuake version but I will do so

My objective is to provide a system with real-time capability to observe =
earth normal modes following a large event and analyze the spectrum with =
WinQuake.  The increase in noise at  very long periods is clearly =
evident but that is expected and OK. The noise source  is mostly the =
instrument self noise enhanced by the period extension filter since the =
instrument noise is significantly above the low noise model beyond 100 =
seconds. The very long period waveforms from side by side broadband =
instruments are incoherent but they will be coherent  following a large =
teleseismic event upon excitation of the Normal modes. The period =
extension process could be either in the data acquisition as I am now =
doing , or in post processing using  WinQuake. The pros do it in post =
processing but they don't get so excited watching waveforms in real time =
like the amateurs.=20

Just one comment on the relative merits of the inverse  filtering =
techniques vs. the negative impedance loading for a geophone. When =
filtering a critically damped geophone the geophone dynamics are =
unchanged and the clipping level is unchanged. When the geophone is =
loaded with a negative impedance the dynamics of the seismic mass are =
changed dramatically. The mass is heavily over-damped and  moves much =
less in response to large ground displacements. One of the problems in =
seismic instrumentation for public safety is clipping. The over-damped =
geophone has a much larger dynamic range as well as period extension.

Thanks for the heads up on WinQuake and sorry for the overly long =
response. This is really interesting stuff and it gets me going :-}=20

Best Regards,

Dave =20


From: Bob McClure=20
Sent: Thursday, December 27, 2012 7:22 PM
To: psnlist@.................
Subject: Re: Instrumentation Question


Hi Dave, =20


 The latest version of WinQuake incorporates my period extending filter. =
Download WinQuake 3.2.6 from=20
http://www.seismicnet.com/software.html#WinQuake. The filter can easily =
provide a 10X improvement in bandwidth. I suggest you at least try it =
out.


Bob


On Thu, Dec 27, 2012 at 12:34 PM, Dave Nelson  =
wrote:

  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" =







  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 Bob,
 
I have tried the real-time period =
extending=20
filter in WinSdr with very good results . I am using it to extend =
the=20
period of broadband force balance seismometers from 50 or 120 seconds to =
1000=20
seconds   I know that was not the original intent but it =
really works=20
well.  I have compared its performance with an analog "inverse =
filter" with=20
essentially the same transfer function .The resulting waveforms are =
essentially=20
identical.  
 
I have not yet tried the WinQuake =
version but I will do so
 
My objective is to provide a =
system with=20
real-time capability to observe earth normal modes following a large =
event and=20
analyze the spectrum with WinQuake.  The increase in noise at  =
very=20
long periods is clearly evident but that is expected and OK. The noise =
source=20
 is mostly the instrument self noise enhanced by the period =
extension=20
filter since the instrument noise is significantly above the low noise =
model=20
beyond 100 seconds. The very long period waveforms from side by side =
broadband=20
instruments are incoherent but they will be coherent =
 following a=20
large teleseismic event upon excitation of the Normal modes. The period=20
extension process could be either in the data acquisition as I am now =
doing , or=20
in post processing using  WinQuake. The pros do it in post =
processing=20
but they don't get so excited watching waveforms in real time like the=20
amateurs. 
 
Just one comment on the relative merits =
of the=20
inverse  filtering techniques vs. the negative impedance loading =
for a=20
geophone. When filtering a critically damped geophone the geophone =
dynamics are=20
unchanged and the clipping level is unchanged. When the geophone is =
loaded with=20
a negative impedance the dynamics of the seismic mass are changed =
dramatically.=20
The mass is heavily over-damped and  moves much less in response to =
large=20
ground displacements. One of the problems in seismic instrumentation for =
public=20
safety is clipping. The over-damped geophone has a much larger =
dynamic=20
range as well as period extension.
 
Thanks for the heads up on WinQuake and =
sorry for=20
the overly long response. This is really interesting stuff and it gets =
me going=20
:-} 
 
Best Regards,
 
Dave  
 
 


From: Bob McClure 
Sent: Thursday, December 27, 2012 7:22 PM
To: psnlist@.............. 
Subject: Re: Instrumentation Question

Hi=20
Dave, =20


 The =
latest=20
version of WinQuake incorporates my period extending filter.=20
Download WinQuake=20
3.2.6 from 

http://www.seis=
micnet.com/software.html#WinQuake.=20
The filter can easily provide a 10X improvement in bandwidth. I suggest =
you at=20
least try it out.


Bob


On Thu, Dec 27, 2012 at 12:34 PM, Dave Nelson =
<davefnelson@.......> wrote:


  

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

  
  
 
  
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 =
output of=20
  the NIC is proportional to acceleration and subsequent circuits shape =
the=20
  spectrum to give the desired velocity response with the 2 slope roll=20
  off below the long period corner.  I=20
  have taken it a step further and modified the NIC to provide a =
velocity=20
  response at the output of the NIC. The result  is that there =
is no=20
  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 which=20
  has been very helpful in optimizing the selection of the negative =
impedance=20
  load on the geophone. I am confident the same circuit could be used to =
extend=20
  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 =

  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=20
  is an optimum negative resistance  which provides good DC =
stability=20
  and the desired frequency response.
  
 
  
The major disadvantage of all period extension =
methods=20
  is long period noise . A look at the Lennartz noise curves illustrates =
that=20
  very well. It is a unavoidable consequence of the technique ,however, =
for some=20
  applications like volcano monitoring it may not be important.  =
Low nose=20
  op amps are essential as you point out  The force feedback =
technique is=20
  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 a=20
  commercial application but the circuits are =
available.=20
  We can put them on Brett's website since I do not have a=20
  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 =
namesake=20
  from "downunder" 
  
 
  
 
  
 
  
 
  

  


  

  
From: chrisatupw@....... 
  
Sent: Thursday, December 27, 2012 2:57 PM
  
To: psnlist@..............=20
  
  
Subject: Re: Instrumentation Question
  


  

  

  

  

  

  
From: Dave=20
  Nelson <davefnelson@.......>

  
To:=20
  psnlist <psnlist@..............>
Sent: Mon, 24 Dec =
2012=20
  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=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