PSN-L Message - Subject: Re: Instrumentation Question

Sent: Thu, 27 Dec 2012 17:35 Subject: Re: Instrumentation Question I have been working extensively on period extension circuits for 4.5Hz = geophones extending the period to 0.5 Hz.=20 Hi Dave,=20 ****The French School Seismometers use 4.5 Hz geophones extended to 20 = seconds period. One of the commercial Lennartz seismometers uses 2 Hz geophones extended = to 20 seconds period. I used the Roberts' circuit to extend the period of my 380 Ohm 4.5 Hz = geophones to 0.5 Hz - about five~six years ago !=20 =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 ****The Lippmann circuit is a current to voltage converter with a = negative impedance input. The output voltage is proportional to f, so it = is not too difficult to add 1/f compensation. 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.=20 ****It can do, but it is easy to be a "bit more clever". The resistance = of the copper coil is highly temperature sensitive, but fairly linear, = so you make the -ve input impedance ~equally sensitive using Pt = resistance foil sensors stuck onto the geophone. Not difficult to do or = too expensive ! The lower the resultant total resistance the higher the = damping and the less the coil movement.=20 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 noise = 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. ****You may need to be careful which Lennartz curves you reference. = Lennartz use both the Roberts and the Lippmann circuits for various = sensors. For a period extension of x10, the Roberts circuit requires an = ADDITIONAL gain of x100 at the low frequency corner.=20 For a period extension of x10, the Lippmann circuit ONLY requires an = ADDITIONAL gain of x10 ! But, since the current output is so small, the = overall amplifier gain needs to be much larger than for a geophone and a = voltage input.=20 The problem with using the Roberts circuit is that to get a gain of x100 = at the low frequency corner, the DC gain needs to be about x500. This = can generate a lot of very low frequency noise. You can't use a two pole = bass boost filter for period extension, since you then get a null at the = roll over frequency. You have to use two single pole filters in series. = To control the VLF noise adequately, you may need to use either a two or = a four pole high pass filter. 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 Regards,=20 Chris Chapman

Hi Chris,

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 limits ---=20 whether or not the Negative Impedance = Converter (Lippman) ,some=20 kind of inverse filter (Robertson) , a Digital Filter( = McClure) or a=20 combination is used.

My work is clearly and = unashamedly based on a=20 long history of work in this arena with a clear goal of simplicity and=20 practicality. I have recognized some shortcomings in the art and = made=20 minor but significant changes to address 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.

I know exactly how Lennartz does = temperature=20 compensation ,it is simple and practical at the expense of the absolute = optimum=20 damping. I avoid the issue by making the whole system = less=20 sensitive to coil resistance again at the expense of the absolute = maximum=20 damping. But then I don't need the absolute maximum in = damping to=20 reach my goal.

I personally question the = practicality of=20 Lennartz instruments and wonder how they sell them at the prices they = get for=20 them. The Nanometrics Trillium = Compact=20 Exploration ( 20 second to 100 Hz. 3 axis force balance = instrument )=20 has a noise level at 10 seconds 40 to 50 Db below a geophone = ,in any=20 configuration ,at about he same price as a Lennartz.

I do not recommend pushing = geophones to long=20 periods by any method -- EXCEPT to make a very=20 inexpensive, very simple seismometer for the amateur or a very low = cost=20 instrument for a focused monitoring system for public=20 safety. Definitely not for science. The basic = geophone=20 has a place in science for VERY local

studies (My = opinion).

Everyone in this business, whether = amateur or=20 professional, has personal opinions about technical issues and the best = way to=20 accomplish a goal. I like to listen to all ideas and always hope someone = will=20 come up with a better idea.

Chris keep it up ! I may = not always=20 agree but you have my most humble respect.

Best Regards,

Dave Nelson

From: chrisatupw@.......

Sent: Sunday, December 30, 2012 2:49 AM

To: psnlist@..............

Subject: Re: Instrumentation Question

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

To: psnlist <psnlist@..............>
= Sent:=20 Thu, 27 Dec 2012 17:35
Subject: Re: Instrumentation=20 Question

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

Hi Dave,=20

****The French School Seismometers use 4.5 Hz = geophones=20 extended to 20 seconds period.
One of the commercial = Lennartz=20 seismometers uses 2 Hz geophones extended to = 20=20 seconds period.
I used the Roberts' circuit to extend=20 the period of my 380 Ohm = 4.5 Hz=20 geophones to 0.5 Hz - about five~six years ago=20 !
=20

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

****The=20 Lippmann circuit is a current to voltage converter with a = negative=20 impedance input. The output voltage is proportional to f, = so it is=20 not too difficult to add 1/f=20 compensation.

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 an=20 optimum negative resistance which provides good DC stability and = the=20 desired frequency response.

****It can do, but=20 it is easy to be a "bit more clever". The = resistance of the=20 copper coil is highly temperature sensitive, but fairly linear, so you = make the=20 -ve input impedance ~equally = sensitive using Pt=20 resistance foil sensors stuck onto the geophone. Not difficult to do or = too=20 expensive ! The lower the resultant total resistance the = higher the=20 damping and the less the coil movement.=20

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 = noise op=20 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.

****You=20 may need to be careful which Lennartz curves you = reference.=20 Lennartz use both the Roberts and the Lippmann circuits for various sensors.
For a period = extension of x10,=20 the Roberts circuit requires an ADDITIONAL gain of x100 = at the low=20 frequency corner.

For a period extension of x10, the = Lippmann circuit ONLY requires an ADDITIONAL gain of x10 ! But, = since the=20 current output is so small, the overall amplifier gain needs to be much = larger=20 than for a geophone and a voltage input.
The problem = with using=20 the Roberts circuit is that to get a gain of x100 at the = low=20 frequency corner, the DC gain needs to be=20 about x500. This = can=20 generate a lot of very low frequency noise. You can't use a two pole = bass boost=20 filter for period extension, since you then get a null at = the roll=20 over frequency. You have to use two single pole filters = in series.=20 To control the VLF noise adequately, you may need to use either a two or = a four=20 pole high pass = filter.

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

Regards,

Chris=20 Chapman

PSN-L Email List Message