Keysight N5171B EXG Service Manual page 29

Troubleshooting
Overall Block Description
Basic A7 Micro Deck Operation
The A7 Micro Deck provides output power level and ALC control using the
following circuitry.
— ALC board
— Low-band detector
— High-band detector
— Low-band modulator
— High-band modulator attenuator
ALC On Operation
In ALC On mode (closed loop operation), the RF output level is monitored and
compared to a reference voltage. A difference between the reference and
detected voltages results in a change to the ALC modulator drive, which
causes a change in the output power. When the reference and detected
voltages agree, the ALC modulator drive level is maintained, resulting in a
constant RF output power level.
To achieve leveled output power, the CPU sets the ALC reference voltage
(calibrated) based on the set power level and applies this voltage as one input
to the ALC integrator. The detector voltage (low and high band detectors) is
the second input to the integrator and provides the feedback necessary for the
ALC assembly to maintain leveled output power. The ALC integrator circuit
provides the drive to the ALC modulator circuit, which controls (increases or
decreases) the amplitude of the RF signal passing through ALC modulator.
When both of the input voltages (reference and detector) are at the same DC
voltage level, the integrator output is constant. When the voltages to the
integrator's inputs are not the same, the integrator output increases or
decreases the modulator drive, resulting in a change in the output power.
The A7 Micro Deck contains a low band coupler/detector and a high band
coupler/detector. The low band coupler/detector operates from 0 to 2 GHz.
The high band coupler/detector operates from > 2 GHz to 20 GHz (Option 520)
or from > 2 GHz to 44 GHz (Option 532/540).
ALC Off Operation
In ALC off mode (open loop operation), only the reference voltage is used to set
the modulator drive level. Since there is no detector voltage (feedback)
indicating the RF output level, the power level may drift over time and with
temperature changes.
Power Search Mode Operation
Power search mode used with modulation is similar to ALC Off mode. Using
pulse modulation as an example, the difference between ALC Off mode and
power search mode is that when power search is first executed, pulse
modulation is turned off, and the ALC loop is closed and leveled. The ALC loop
22 Keysight X-Series Signal Generators Service Guide