Theory of Operation
2715 Spectrum Analyzer Service Manual 3-15
When the FM coil is used to sweep the oscillator,the relay on the Interface board
closes and couples a largecapacitor (two capacitors in parallel) across themain
coil. The capacitors lower the noise bandwidth of the main coil driving circuit
while the FM coil is in operation. The heater provides temperature stability.
The rest of the circuitryon the Interface board provides operating voltages for
the two amplifiers in the 1st LO assembly.
The output of the 1st LO drives the 1st LO Buffer Amplifier.
The 1st LO Buffer Amplifier consists of the following:
HAutomatic Level Controlled (ALC) Amplifier
HStrobe Driver
HSampling Gate
ALC Amplifier.The ALC Amplifier is composed of a wide band amplifier with
impedance matching transmissionlines at the input and output, a directional
coupler, a detector,a level comparator, and active bias. See Figure 3--7.
1stLO In
1stLO Out
Detector
-- V R E F
BeatNote to Phase
LockCF C Module
Sampling
Gate
STROBE from Center
Frequency ControlBoard
(25.7325MHz to 26.05 MHz)
Reservedfor Options
ALCAmplifier
StrobeDriver
Terminator
replacedby
semi-rigidcable
whenO ption15
isinstalled
Figure 3-7: 1st LO Buffer Am plifierDiagram
1st LO Buffer Amplifier