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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-29
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- 13: Phase Lock Center Fr equency Control Configuration for Moderate Spans ( Unlocked, Sweep FM Coil)
The VCO is not used except intermittently for frequency corrections and is tuned
by a fixed bias summed with VCFCU arising from DAC U950B.
Narrow Span Operation (20 kHz/div and Smaller Spans). The instrument is phase
locked.
The Main coil is driven with a fixed bias summed with DAC U950A output. See
Figure 3--14.