Anritsu MG3702xA Series, Руководство по обслуживанию

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Chapter 2 — Functional Description 2-4 ALC/Pulse Modulation
MG3702xA MM PN: 10370-10372 Rev. B 2-9
RF Outputs 10 MHz to 20 GHz
Refer to the block diagrams shown in Figure 2-3 on page 2-13 and Figure 2-4 on page 2-14 for the
following descriptions. The MG3702xA uses VCOs and multipliers capable of generating RF signals in
the frequency range of 2.0 GHz to 20 GHz. All other frequencies output by the instrument are derived
from the fundamental frequencies generated by the VCO and multipliers.
0.01 to 2.2 GHz (Option 004)
RF output frequencies of 0.01 to 2.2 GHz are developed by down converting the fundamental frequencies
of 2 GHz to 4.4 GHz. This is achieved by using a series of dividers and 16 bandpass filters. Precise
control of the 0.01GHz to 2.2 GHz frequencies to 0.01 Hz resolution is achieved through phase-lock
control of the fundamental frequencies prior to division.
Step and List Sweep Mode
Step (digital) frequency sweeps of the VCO oscillator and multiplier RF output consist of a series of
discrete, synthesized steps between a start and stop frequency. Each frequency step is made by changing
the fine and coarse loops, then phase-locking the RF output. Every frequency step in the sweep range is
phase-locked.
2-4 ALC/Pulse Modulation
The MG3702xA ALC and pulse modulation subsystems provide automatic level control (ALC) and pulse
modulation of the RF output signal. The ALC loop consists of circuits located on the A6 ALC PCB. These
circuits interface with the A10 switched filter assembly, the digital down converter assembly and the
directional coupler/level detector (all located on the RF deck). Pulse modulation of the RF output signal is
provided by circuits on the A6 ALC PCB. These circuits interface directly with the switched filter assembly
located on the RF deck via coaxial cables. (In instruments with Option 4, these circuits interface directly with
the digital down converter and are looped through the digital down converter to the switched filter assembly.)
The ALC subsystem is shown in
Figure 2-4 . The following paragraphs describe the operation of the subsystem
components.
ALC Loop Operation
In the MG3702xA, a portion of the RF output is detected and coupled out of the directional coupler/level
detector as the feedback input to the ALC loop. The feedback signal from the detector is routed to the A6 ALC
PCB where it is compared with a reference voltage that represents the desired RF power output level. If the
two voltages do not match, an error correction signal is fed to the modulator shaper amplifier circuits located
on the A6 PCB. The resulting ALC control voltage output causes the level control circuits, located on the A10
switched filter assembly, to adjust the RF output level. Thus, the feedback signal voltage from the level
detector will be set equal to the reference voltage.
The level reference DAC, under the control of the CPU, provides the RF level reference voltage. By setting the
output of this DAC to the appropriate voltage, the CPU adjusts the RF output power to the level selected by the
user.
Note
The instrument uses two internal level detection circuits. For frequencies < 2 GHz, the level detector
is part of the digital down converter. The signal from this detector is routed to the A6 ALC PCB as the
Detector 0 input. For frequencies

2 GHz, the level detector is part of the main directional coupler.
The signal from this detector is routed to the A6 ALC PCB as the Detector 1 input.