GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 18, Page 2
18.1 USE OF THE ATTENUATOR
The attenuator function in the GDP-32
II
is useful for limiting the input signal for low frequency
surveys using fractional N-spacings or other configurations where the input voltage may exceed
3.5 volts.
The attenuator should not normally be used for high frequency measurements, such as TEM,
CSAMT, or AMT, where phase accuracy is necessary and the input load resistance varies. The
reason for this restriction is that the attenuator circuit works as a low-pass filter when engaged,
due to its high impedance and associated parasitic capacitance. Some typical phase shifts with
the attenuator IN and a 10 KΩ input load are:
16 Hz 1 mr
64 4
256 12
1024 52
4096 210
8192 375
With its low-pass filter characteristics, the attenuator can be used with resistivity and IP surveys
to filter out strong radio signals from sources such as broadcast stations and air navigation
stations, without affecting the IP response.
If the input load resistance is constant, the phase shift values can be calibrated out for high
frequency CSAMT and AMT measurements, but not for TEM. The added filtering will affect
the early times up to about 1 ms.
18.2 RECALIBRATE WHEN REPLACING ANALOG CARDS
With the ease in changing the configuration of the GDP-32
II
analog cards, clients have been
moving cards between channel positions in the internal card rack when they suspect some kind
of electrical problem. With the new software, the calibrates in the calibrate cache do not have to
be in the same order as the analog cards. However, if the GDP does not find a corresponding
serial number for each card that is active, an error message will come up on the screen, and the
operator can decide whether to go on or not.
When analog cards are replaced, the operator must calibrate all channels before gathering data
with the new configuration, so that a valid calibrate is available in the calibrate cache.
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