80 Chapter 2
Block Diagrams
A19 Synthesizer Assembly
A19 Synthesizer Assembly
The A19 synthesizer assembly supplies the first local oscillator
frequency and power to the A17 RF assembly. This LO is fed to the first
mixer for the first down conversion to a 321.4 MHz first IF.
The synthesizer assembly performs the following main functions:
• detects unlock problems
• provides VCO phase lock loop
• provides three selectable bandpass filters
• provides the first doubler
• provides the second doubler
Lock Detector
If the 500 to 1000 MHz VCO phase lock loop is not able to lock to the
10 MHz internal reference, the lock detector output logic will be
interpreted by the A22 CPU and display an LO unlock error message in
the error message block on the instrument display.
VCO Phase Lock Loop
The VCO range is 500 MHz to 1000 MHz. The frequency output of the
VCO depends on the center frequency of the instrument. This output
frequency is very stable because it is phase locked to the 10 MHz
reference signal from the A18 reference assembly.
Bandpass Filters
There are three firmware selectable bandpass filters. The frequency
ranges of the filters are 250 to 396 MHz, 396 to 628 MHz, and 628 to
1000 MHz. The center frequency of the instrument will determine the
VCO frequency, and which one of these bandpass filters will be
switched in.
First Doubler
The center frequency of the instrument determines the signal path
through the first doubler. The fundamental VCO frequency may bypass
this block of circuitry.
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