The so called Lippman circuit is just =
a well known=20
negative impedance converter circuit (NIC) applied to geophones.=20
With the original Lippman circ=
uit the=20
output of the NIC is proportional to acceleration and subsequent circuits s=
hape=20
the spectrum to give the desired velocity response with the 2 slope roll=20
off below the long period corner. I have taken it a step further and modified the NIC to provid=
e a=20
velocity response at the output of the NIC. The resul=
t 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.
****The Lippmann circuit is a current to voltage converter with a negative impedance input. Th=
e output voltage is proportional to f, so it is not too di=
fficult to add 1/f compensation.
Brett has created a sp=
ice 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 us=
ed 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 an=
d=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 beco=
me 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.
****It can do, but it is easy to be a "bit more clever". The resistance of the copper coil i=
s highly temperature sensitive, but fairly linear, so you make the -ve input impedance ~equally sensitive usi=
ng Pt resistance foil sensors stuck onto the geophone. Not difficult to do =
or too expensive ! The lower the resultant total resistanc=
e the higher the damping and the less the coil movement.
The major disadvantage of all period e=
xtension=20
methods is long period noise. A look at the Lennartz noise curves illustrat=
es=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
noise 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 t=
han a=20
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 require=
s an ADDITIONAL gain of x100 at the low frequency corner. =
For a period extension of x10, the Lip=
pmann circuit ONLY requires an ADDITIONAL gain of x=
10 ! 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.
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 fre=
quency noise. You can't use a two pole bass boost filter for period extensi=
on, since you then get a null at the roll over frequency. =
You have to use two single pole filters in series. To cont=
rol the VLF noise adequately, you may need to use either a two or a four po=
le high pass filter.
I have been doing this work for a friend for=20
a commercial application but the circuits a=
re=20
available. We can put them on Brett's website since I do not have a=20
website.
Regards,
Chris Chapman