Guralp Systems CMG-3TB - Manual (Page 71)

Operator's guide
In a feedback seismometer with a displacement transducer, it is
essential to monitor the acceleration output. This provides the position
of the displacement transducer and therefore also the mass position, as
the displacement transducer is attached to the sensor inertial mass.
The sensor should always be operated with the displacement
transducer centred or nulled, so that the response to input acceleration
is linear.
There are two types of feedback system which can be used in a 3TB
instrument, known as hybrid and conventional-response feedback.
Hybrid feedback
The hybrid feedback circuit contains a single capacitor in parallel with
a resistor, resulting in a single dominant pole at 0.033 Hz (30 s). Below
this frequency, the response of the seismometer is flat to ground
acceleration; above it, the response is flat to velocity. (Other values for
the acceleration-velocity corner can be provided upon request.)
Hybrid-feedback systems provide a stable response, particularly for
portable systems, with a high saturation level at high frequencies and a
high dynamic range at long periods.
An active low-pass filter provides a high-frequency cutoff point at a
frequency you specify. Without the filter, the velocity response is flat
up to 100 Hz. Outside the feedback loop there is an active high-pass
filter with a corner frequency of 0.01 Hz (100 s) or 0.005 Hz (200 s),
which serves to remove any DC offsets.
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