FUNCTIONAL

ALC/AM/PULSE

DESCRIPTION

MODULATION

 

Step Sweep

Step (digital) frequency sweeps of the YIG-tuned os-

 

Mode

cillator RF output consist of a series of discrete, syn-

 

 

thesized steps between a start and stop frequency.

 

 

Each frequency step is generated by applying the

 

 

tuning signal (from the A12 Analog Instruction

 

 

PCB) to the YIG-tuned oscillator’s main tuning coil,

 

 

then phase-locking the RF output. Every frequency

 

 

step in the sweep range is phase-locked.

2-4ALC/AM/PULSE

The ALC, AM, and pulse modulation subsystem provides automatic

MODULATION

level control (ALC), amplitude modulation (AM), and pulse modulation

 

of the signal generator’s RF output signal. The ALC loop consists of

 

circuits located on the A10 ALC PCB, the A9 PIN Control PCB, and

 

the A14 SDM, SQM Driver PCB. These circuits interface with the

 

Switched Filter assembly, the Down Converter assembly, the Source

 

Quadrupler Module (SQM), and the Directional Coupler/Level Detec-

 

tor (all located on the RF deck). AM modulation circuits (located on

 

the A10 ALC PCB) are included in this loop.

 

Pulse modulation of the RF output signal is provided by circuits lo-

 

cated on the A6 Pulse Generator PCB. These circuits interface directly

 

with the Switched Filter assembly located on the RF deck. (In 682X5B/

 

683X5B models, these circuits interface directly to the Digital Down

 

Converter assembly and from there to the Switched Filter assembly.)

 

The overall block diagram of the ALC and modulation subsystem is

 

shown in Figure 2-3, page 2-17. The following paragraphs describe the

 

operation of the subsystem components.

 

ALC Loop

In the 682XXB/683XXB, a portion of the RF output

 

Operation

is detected and coupled out of the Directional Cou-

 

 

pler/Level Detector as the feedback input to the

 

 

ALC loop. The feedback signal from the detector is

 

 

routed to the A10 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 from the A10 ALC PCB to the modulator shaper amplifier circuits located on the A9 PIN Control PCB and the A14 SDM, SQM Driver PCB, if installed. The resulting modulator control voltage output causes the modu- lators, located in the Switched Filter assembly, the SQM, and the Digital Down Converter assembly (682X5B/683X5B models only), to adjust the RF out- put level. Thus, the feedback signal from the detec- tor will be set equal to the reference voltage.

682XXB/683XXB MM

2-15

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Anritsu 683XXB, 682XXB manual Functional ALC/AM/PULSE Description Modulation, 4ALC/AM/PULSE
Page 42 Page 44

682XXB, 683XXB specifications

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