Keysight Agile Signal Generator Service Guide 17
Troubleshooting
Output Power Level/Automatic Leveling Control
The output power control circuitry, commonly referred to as the ALC loop,
comprises the following:
— Two detectors: the Lowband Coupler/Detector (frequencies ≤ 3 GHz) in the
A18 Lowband Output Module and the Highband Coupler/Highband
Detector (frequencies > 3 GHz) in the A20 Output Leveling Module. The
couplers couple off a small portion of the RF signal and route it to the
detectors. The detectors convert the RF signal to a DC voltage proportional
to the RF power level. This DC voltage is routed to the A15 RF Deck
Interface. Depending either low or high band the voltage is digitized by the
low or high band ADC then routed to the A12 DAC board and used to adjust
the output power.
— There are two limiting amplifiers in the A17 Agile Modulator Filter to be
used for output power level control. The mid band limiting amplifier is used
to control the power level for frequency between 3 to 6 GHz and the high
band limiting amplifier is used to control the power level for frequency
> 6 GHz.
— The output power in low band 10 MHz to 3 GHz is controlled via the output
level of the A12 DAC board.
In ALC off (open loop operation (default)), the DAC modulator in A12 board is
used to control the limiting amplifiers in the A17. The DAC modulator applies
stored calibration data to correct for any losses that occur after the
coupler/detector which results of the power level set on the front panel. The
stored calibration data is correction information stored in the instrument
during power level calibration (no feedback is provided).
In ALC on (closed loop operation), the output level is detected from the
coupler/detector then the voltage is digitized and fed back to the DAC board
for correcting power if any variation occurred. For frequency between 3 to
6 GHz, the midband limiting amplifier is used to control the power level. For
frequency greater than 6 GHz, the highband limiting amplifier is used to control
the power level.
In low band, 40 MHz to 3 GHz, the output is detected via the low band detector
and goes through the A15 ADC and then fed back to A12 DAC board to control
the power level. For frequency lesser than 40 MHz, the output goes directly
from the lowband coupler (< 40 MHz path) to the ADC in the A15 board and
output to the A12 DAC board for power leveling control.
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