Hi to all, in different discussion (closed) it was explained how extend lower frequency band of geophone using low-pass active filter; in this case I have to take care of different parameters (such corner frequency, damp factor, coil mass, coil resistor, and so on).
Most of theese parameters are nominal, so little tolerances (also in low-pass components) make extended frequency response not flat.
Generic RLC high-pass filter as simple geophone model.
From Encyclopedia of Petroleum Science & Engineering V3
Geophone equivalent circuit values:
Since K and B are usually not provided on geophone spec. sheets we must calculate them from other parameters which are available.
Geophone PS-4.5B datasheet:
Natural frequency [Hz] = 4.5 ±10%
Typical open-circuit damping = 0.6
Coil resistance [Ω] = 375 ±5%
Moving mass [g] = 11.3
Sensitivity [V/cm/s] = 28.8
Now, let’s take a look to SM-24 geophone:
Natural frequency [Hz] = 10.0 ±2.5%
Typical open-circuit damping = 0.25
Coil resistance [Ω] = 375 ±2.5%
Moving mass [g] = 11
Sensitivity [V/cm/s] = 28.8 ±2.5%
Typical damping (@Rs = 1339Ω) = 0.60
Typical damping (@Rs = 1000Ω) = 0.69
In open-circuit we got following parameters:
Transfer function becames:
(obviously)
Ok, geophone response changes upon shunt resistor so let’s take care of shunt resistor (in this case we don’t take care of coil’s inductance):
this resistor is in parallel to
That’s why we get different damping value for different shunt resistors.
Since shunt resistor acts as resistive partitor, it changes also sensitivity:
Then we have new equivalent system:
Ok, all these theorical shunt resistor values are agree to SM-24 geophone datasheet graphs, so geophone equivalent model correctly works.
My problem is this: how can a NIC (negative impedence converter) lower geophone high-pass frequency keeping flatness???
