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Theory of Operation
3-28
2715 Spectrum Analyzer Service Manual
FSTROBE
FM
MAIN
Main
DAC
SWP
DAC
DIG
1st LO Beat to
Counter
Phase Gate
Harm GenVCO Module
Summing
and Fixed
Scaling
VOFS
1st LO
1st LO to 1st Converter
PLL
DAC
VREF
DIG
Microprocessor
Serial Out
Serial In
FM Coil
Driver
FM
DAC
DIG
Summing
and Fixed
Scaling
Main Coil
Driver
Summing
and Fixed
Scaling
DIG
1st LO
VOFS
DIG
Decade
Atten
Analog Sweep In
Figure 3- 12: Phase Lock Center Fr equency Control Configuration for Wide Spans (Unlocked, Sweep Main Coil)
The VCO is not used except intermittently for frequency corrections, and it is
tuned by a fixed bias summed with VCFCU arising from DAC U950B .
Moderate Span Operation (50 kHz/div through 5 MHz/div Span). The Main coil is
driven by a fixed bias plus a DAC output (U950A).
The FM coil is driven with VCFC FM arising from DAC U850A (routed
through U757A) summed with the sweep signal (routed through U830B). Total
current is routed through Q910 and Q911 to FM coil. See Figure 3--13.