TM 32-6625-022-24P
FUNCTIONING OF EQUIPMENT
3-1 GENERAL. The following paragraphs provide a
3-5 UHF OSCILLATOR. The UHF oscillator is a tuned
functional description of the electronic functions of the
line, varactor swept oscillator. The oscillator is tunable
major elements of Sweep Signal Generator SG-677/U.
over a frequency range of 300 MHz to 1200 MHz. The
Refer to the Functional Block Diagram shown in figure
sweep drive voltage for the oscillator is coupled to varactor
3-1.
CR 502, which changes the electrical length of the tuned
line, thereby changing the oscillator's output frequency.
3-2 SWEEP RATE. The sweep rate stage provides
When the sweep generator is used in the UHF range, this
triangular and square wave outputs as required for opera-
output is fed directly to the monitor stage. When the
tion by the vertical blanking, horizontal, and sweep drive
VHF range is desired, a 700 MHz to 1000 MHz signal
stages. This maintains synchronization of the sweep gener-
from the oscillator is fed to the VHF circuitry before
ator. There are three modes of operation, LINE, MANUAL,
being coupled to the monitor stage.
and VARIABLE. They are achieved by changing the
operating characteristics of a Schmidt trigger and Miller
3-6 VHF CIRCUITRY. The VHF frequency range is
integrator by means of the SWEEP RATE switch. In the
generated by sweeping the UHF oscillator from 700 MHz
LINE mode a line frequency signal from the primary
to 1000 MHz and heterodyning this swept frequency
power is coupled to the input of the Schmidt trigger.
with the 700 MHz Continuous Wave (CW) oscillator fre-
This signal synchronizes the sweep generator to the line
quency. The difference frequency is then amplified by the
frequency. In the MANUAL mode the SWEEP rate stage
video amplifier before being coupled to the monitor stage.
is bypasses, and a dc voltage controlled by the SWEEP
RATE ADJUST is present at the output. In the VARI-
a. CW Oscillator. The CW oscillator is a tuned line
ABLE mode a time constant controlled by the SWEEP
oscillator and has a freed output frequency of 700 MHz.
RATE ADJUST provides a 5 Hz to 60 Hz signal at the
b. VHF Mixer. The VHF mixer utilizes a diode bridge
input to the Schmidt trigger allowing for variable syn-
circuit arrangement with a low pass falter at the output.
chronization.
c. Video Amplifier. The video amplifier has four
similar gain stages. Each stage is of the common emitter
3-3 SWEEP DRIVE. The sweep drive stage provides a
configuration and the 0.5 to 300 MHz frequency response
non-linear drive signal which is used as the control volt-
is accomplished primarily by large amounts of negative
age for the UHF oscillator varactor CR 502. A linear
feedback in each stage.
and a non-linear amplifier operate from a drive signal sup-
plied by the sweep rate stage. A diode break circuit re-
3-7 MONITOR. The function of the monitor is the
shapes the drive signal supplied at the collector of the
distribution of the RF sweep. The monitor contains a
non-linear amplifier. When the diodes are reverse biased,
dividing network which channels the RF sweep to the RF
the circuit has a linear charateristic; but as the voltage
attenuator and the sweep sample divider/EXT marker.
on the collector increases, each of the diodes conducts
The monitor also detects a portion of the RF signal, pro-
in consecutive order; and the resultant waveform is re-
viding a dc voltage proportional to the RF signal ampli-
shaped. This reshaped waveform, together with the ampli-
tude. This voltage drives the RF output meter and is also
fied linear drive signal, is coupled to the SWEEP WIDTH
used as feedback by the leveler stage.
switch and potentiometer combination. The two signals
are combined and serve to maintain sweep linearity as
3-8 LEVELER. The function of the leveler stage is to
the sweep width is reduced. The resultant drive signal
maintain a constant RF amplitude at the RF output con-
is applied to the UHF oscillator varactor CR 502.
nector. The leveler stage operates in a closed loop sys-
tem consisting of the leveler stage, the device being leveled
3-4 FINE TUNING. The front panel FINE TUNING
(either the UHF oscillator or the VHF video amplifiler),
control provides a fine adjustment to the frequency dial
and the monitor. The monitor provides a dc voltage to
setting when the SWEEP WIDTH control is in the NAR
or CW mode. Adjustment of the FINE TUNING control
one input of a differential amplifier. The other input
varies the dc voltage at the base of an emitter follower
receives the RF output command from the RF output
which provides a low impedance source for the output
control. If the RF amplitude starts to decrease, the feed-
back voltage from the monitor starts to decrease. Since
signal. This signal is coupled to the SWEEP WIDTH
the command from the RF output control does not change,
control.
3-1
