Agilent Technologies 1141A, Руководство пользователя и сервисное руководство

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Service 4
Theory of Operation
1141A and 1142A User’s Guide 63
Offset Functions
There are two offset functions developed in the control module:
variable offset and dc reject. A variable offset voltage with coarse
and fine adjustments can be selected by the front panel controls.
The offset level is buffered by U8 and selected by multiplexer U3 as
the input to offset amp U7. The output of the offset amp is
summed with the low- frequency signal and feedback which gives dc
coupling in the probe. Front panel screwdriver adjustment Offset
Null zeros the dc output from the probe when the dc input and
offset are zero.
For dc Reject, an output from the low- frequency amplifier in the
probe (LFSIG) is used to develop a voltage used to null the dc
component of the input signal. LFSIG is an input to U6, an
inverting amplifier and low- pass filter. Multiplexer U3 selects one
of three capacitors to set a roll- off frequency of 0.05, 0.5, or 5 Hz.
The output of U6 is selected, again by U3, as the input to the offset
amp. When the output of the offset amp is summed into the low
frequency path, the result is cancellation of the dc component of
the input signal. Front panel screwdriver adjustment DC Reject
Gain adjusts the gain of the dc reject circuit.
Local and Remote Control
The front panel switch controls the dc reject and offset functions. It
also selects remote operation, which allows control through the rear
panel remote input connector.
Power Supply
The supply provides ±6V and ±15V for the probe and analog control
circuitry as well as +5V for the digital control circuitry.
Attenuator Adapters
The 10x and 100x Attenuator Adapter are similar. A ceramic
substrate carries two attenuators, one for each input polarity. A
variable resistor adjusts the low- frequency balance (
LF CMRR )
between the two attenuators. The high- frequency adjustments are
the same for each attenuator. Each attenuator is adjusted
differently. One attenuator is adjusted for optimum pulse response
and the other for best high- frequency CMRR.