194 Keysight CXG, EXG, and MXG X-Series Signal Generators Service Guide
RF Assembly
A3 RF Assembly Overview
In addition to three switchable attenuator sections, Atten 1 contains an input
selection switch as well as a bypass switch. It has the ability to switch in 5 dB
to 55 dB of attenuation.
Atten 2 contains only one switchable attenuator section, 20 dB.
In addition to three switchable attenuator sections, Atten 3 contains a bypass
switch and the reverse power sensing and protection circuitry. It has the ability
to switch in 5 dB to 55 dB of attenuation.
Automatic Level Control Section
The Automatic Leveling Control (ALC) loop provides leveled output power. The
ALC loop is a feedback control system that monitors RF power and maintains
power at the user-selected level.
There are two different Automatic Level Control sections, one for the RF
instruments and one for the microwave instruments. For the RF instruments
this section contains the reference DAC, temperature compensation DAC,
amplitude modulation amplifiers, output detector preamp, log amplifier, loop
integrator, unlevel detector, ALC amplifiers and filters. For the microwave
instruments this section contains only amplitude modulation amplifiers, output
detector preamp, ALC amplifiers and filters. For the microwave instruments
most of the automatic level control circuitry resides on the A7A1 Microwave
ALC board on the A7 Micro-Deck assembly.
There are several adjustments related to the Automatic Level Control section
found in the performance verification and adjustment software for these
instruments, which will be covered later in this chapter.
Closed Loop
In closed loop mode (ALC On) leveled output power is obtained by comparing a
detected voltage with a reference voltage. The detected voltage is generated
by coupling off a portion of the RF output signal and converting it to a DC
voltage using detector diodes. The reference voltage is generated using
calibrated DACs. When the reference and detected levels are not the same, the
ALC integrator output ramps up or down to increase or decrease the detected
level. If the integrator cannot achieve a match between the detected and
reference voltages, an unleveled indicator is displayed.
Open Loop
In open loop mode (ALC Off) leveled output power is obtained with a reference
voltage. The reference voltage is used to control the modulation drive current.
The reference voltage is determined by setting the desired power. It includes
the stored calibration data used to compensate for any losses that occur after
the detector.
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