Theory of Operation
3-28 2715 Spectrum Analyzer Service Manual
FSTROBE
FM
MAIN
Main
DAC
SWP
DAC
DIG
1stLO Beat to
Counter
PhaseG ate
HarmGenVCO Module
Summing
andF ixed
Scaling
VOFS
1stLO
1stLO to 1st Converter
PLL
DAC
VREF
DIG
Microprocessor
SerialO ut
SerialIn
FMCoil
Driver
FM
DAC
DIG Summing
andF ixed
Scaling
MainCoil
Driver
Summing
andF ixed
Scaling
DIG
1stLO
VOFS
DIG Decade
Atten
AnalogSweep In
Figure 3- 12: Phase Lock Center Frequency Control Configuration for Wide Spans (Unlocked, Sweep Main Coil)
The VCO is not used except intermittently forfrequency 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 (routedthrough U830B). Total
current is routed through Q910 andQ911 to FM coil. See Figure 3--13.