TM 11-6625-2575-14
(1) Fixed-frequency oscillator. The 700
Q201 where the change in leveled B+ restores
MHz fixed-frequency oscillator consists of tran-
the rf output to its normal value. The leveled
B + is applied to the variable oscillator in the
sistors Q551 and Q552, a resonant line, a trimmer
HIGH band mode and to the first three stages
capacitor, and associated biasing circuits. The
oscillator output is loop-coupled to the mixer by
of the video amplifier in the LOW band mode.
resistor R554. One end of R554 is common to
c. Any change in rf output voltage is auto-
the case.
matically corrected by the ALC circuit. Since the
(2) Mixer and low-pass filter. The mixer is
rf output voltage at the monitor is constant for a
a double-balanced diode mixer with balanced
given voltage reference, this point is a zero-im-
transformer inputs from the 700-MHz fixed oscil-
pedance, constant voltage point, and R602 is
lator and the variable oscillator. Transformers
T551 and T552 are wound on toroidal ferrite
d. The DC reference point in the ALC system,
cores. The output of the mixer is coupled from
the base of Q207, can be adjusted from the front
the common point of the diode star (consisting
panel RF VERNIER control. By adjusting the
of CR551 through CR554) to RL551. Low-pass
RF VERNIER control, the leveled B+ is adjusted
filter FL551 couples the difference frequency of
and the rf output level may be continuously ad-
the two oscillators to J401, the input to the video
justed over a 10-dB range. The rf output indicat-
amplifier.
ing meter ( M601 ) is coupled through R611 and
(3) Video amplifier. The input signal is am-
R612 to the base of Q206.
plified by four cascaded, rf-coupled, common-
emitter amplifier stages. This four-stage video
e. Front panel switch S201 bypasses the in-
amplifier has a relatively flat response from 0.5
ternal monitor and couples the ALC circuit to the
MHz to 300 MHz. Capacitors C402, C408,C415,
front panel EXT ALC connector. The AN/USM-
and C422 permit adjustment of gain in indi-
203A may be leveled at a remote location if an
vidual stages to level the output throughout the
external voltage monitor is used.
low band. The first three stages are supplied by
f. The blanking circuit reduces the displayed
leveled B+. The fourth stage, with dual-emitter
vertical signal to a zero baseline during the
lead transistor Q404, has a higher power output
sweep retrace. Two blanking modes, rf and VI-
and is supplied by +50 volts dc.
DEO, are controlled from internal slide switch
d. Rf Detector Circuit. The rf detector circuit
S102 on the main printed circuit board. The
consists of diode CR603 and associated compo-
Schmitt trigger generates the blanking signal.
nents including R613 and R614. Diode CR603
In rf blanking, the rf output is blanked during
detects the signal from the' device under test and
retrace. Transistors Q204 and Q203 saturate,
couples a negative output voltage (when con-
Q201 is cut off, and the leveled B+ is blanked.
nected) to the video input connector.
g. In the VIDEO blanking position, the blank-
ing signal is coupled to the gate of JFET Q109,
an electronic switch which shorts the SCOPE
Blanking Circuits
VERT connector to ground during the retrace
(fig. FO-3)
interval.
a. The monitor senses the rf output signal at
the junction of CR601 and R602. The monitor
output, a negative detected signal, is coupled to
(fig. FO-3)
the base of transistor Q206, the sampling input
a. The AN/USM-203A is supplied with
of a differential amplifier.
a birdie-bypass marker system. A front panel
b. The base of Q207, the reference half of the
connector is provided for an external marker.
differential amplifier, is coupled to a stable, but
Internal receptacles are provided for eight plug-
adjustable bias voltage. Any difference between
in markers. Three internal markers of 1.0, 10.0,
the negative monitor sample and the reference
and 50.0 MHz are supplied with the AN/USM-
voltage appears at the collector of Q206 as an
203A. All markers are controlled independently
amplified, phase-inverted error signal. The col-
from front panel switches. In the marker system,
lector output of Q206 is further amplified and
a sample of the swept rf signal is mixed with the
phase-inverted by Q202. The collector output of
fundamental or a harmonic of the plug-in mark-
Q202 is coupled to the base of emitter follower
ers to produce beatnotes, or birdies. These birdies
