Raytheon Pathfinder 1200 Instruction Manual page 79

6.2.3 Receiver
The general layout for the receiver is shown in Figure 6-5. Receiver consists of Gunn
Oscillator, Attenuator, Mixer, and Receiver Printed Circuit Board.
1) Gunn Oscillator
By means of both mechanical and electrical controls, A30l is tuned to give a
continuous output frequency 38 MHz higher than that of the magnetron. Mechanical
tuning is achieved by the adjustment of a screw on A30l; electrical tuning is achieved
by the adjustment of the operator's TUNE control on the display unit.
-,
2) Attenuator
Normally a Gunn Oscillator provides a higher output than required for
eff'~cient
mixing. An attenuator is set to optimize the signal-to-noise ratio and this condition
is obtained when the mixer current is 0.5 rnA at either test point TPI or TP2.
3) Mixer
The mixer is the balanced type and uses diode types lN23E and lN23ER. The
balanced type mixer presents a good signal-to-noise ratio to the receiver system.
The mixer output is resonant at 38 MHz.
4) I.F. Amplifier
The I. F. Amplifier consists of two amplifying stages. The first stage is transistor
amplifier which is designed to provide a good noise figure. The noise figure of this
circuit is determined by the collector current of the transistor, approximately 3.5 rnA.
The second stage is the integrated circuit
(K')
amplifier. This stage is controlled
by the GAIN-STC circuit in the display unit. The GAIN-STC control voltage is fed
to the bias terminal
(3)
oflC30 1.
GAIN-STC control voltage on TB10l-GS (at STC control min) for maximum
gain is approximately 5 V and for minimum gain approximately 9 V.
5) Second Detector
The second detector is an IC detector which operates as a sensitive detector ampli-
fier. The negative going video signal appears across R13, the I.F. component is re-
moved, and the video signal is fed to the video circuit.
6) Video Circuit
The video circuit consists of an emitter follower/video integrator. The emitter
follower operates as an impedance transformer to drive the coaxial cable which feeds
the video signal to the display unit.
The video integrator circuit is provided for test purposes when the Gunn Oscil-
lator is being tuned.
The circuit is a de restorer circuit. When the negative going video pulses are fed
on CD8, the charging current flows via CD8 and C24. As a result, C24 is charged
negatively to approximately -1 V de which appears on TB 101-32. Proper tuning of
Gunn Oscillator is achieved when the tuning voltage on TB 10 1-32 is at a peak value.
6-13