Tektronix 2715 2715 Spectrum Analyzer Service Manual 3-29

Theory of Operation
2715 Spectrum Analyzer Service Manual 3-29
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- 13: Phase Lock Center Frequency Control Configuration for Moderate Spans (Unlocked, Sweep FM Coil)
The VCO is not used except intermittently forfrequency 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 fixedbias summed with DAC U950A output. S ee
Figure 3--14.