Section III
Paragraphs 3-18 to 3-20
329.3 to 335.0 mc. The r-f generator in the shielded
r-f assembly is crystal controlled by 38 crystals in
piston type and is mounted in such a fashion that its
small single-turn pickup loop may be moved with
two oscillators, the coarse frequency oscillator and
respect to the final output coil to vary the coupling and
the fine frequency oscillator. V-1501, the fine
thus the r-f output. This motion of the piston and loop
frequency oscillator, is a twin triode, the first section
is produced by a rack and pinion drive mechanism.
of which is connected as a Colpitts oscillator, while
the second section is a cathode follower acting as a
One end of the pickup loop is at ground potential. The
other end of the pickup loop is connected to a re-
buffer stage. V-1502, the coarse frequency oscillator
sistor of approximately 53-ohms contained within the
has 28 crystals with frequencies of 7375 to 9625 kc.
attenuator piston. This resistor in turn is connected
Crystal frequencies are tripled in the output of this
in s cries with the attenuator output cable. With the
oscillator, thus the r-f output is varied through the
NAV-GS switch in the NAV position, the r-f attenuator
range 22.125 to 28.875 mc in 28 steps as the outer
output is connected to the NAV output connector. Thus
dial of the MEGACYCLES control is turned. The out-
the NAV output connector provides an r-f voltage
put of the fine frequency oscillator with 10 frequencies
source with 53-ohm internal impedance. Since one
in the range 4875 to 5100 kc is added to the coarse
frequency oscillator output in mixer V-1503. The
side of this voltage source is at ground potential, there
exists what is commonly referred to as a 53-ohm un-
desired sum frequencies at the mixer output range
balanced source. With the NAV-GS switch in the GS
from 27.000 mc to 33.975 mc depending on the setting
position, output is obtained at the GS output connector
of the MEGACYCLES control. Output from the mixer is
from a tap on the univerter output coil. This also pro-
amplified in V-1504 and then fed into quadruple V-1505.
vides a 53-ohm unbalanced source of r-f voltage. Both
The quadruple feeds the final amplifier, V-1506. R -f
the NAV and GS output circuits are specifically de-
voltage appearing across the tank circuit of V-1506 is
signed for direct connection to radio receivers having
applied to the grid of demodulator V-1509, an infinite
an unbalanced input circuit of approximate ely 53-ohms
impedance detector in ones ection followed by a cathode
follower in the other section. The d-c voltage appear-
impedance. The SG-13/ARN MICROVOLTS dial is
ing at the cathode of the second section of V-1509 is
calibrated to indicate the r-f voltage induced in the
proportional to the r-f voltage on the grid of the first
attenuator pickup loop, i. e., the "open - circuit"
section. This cathode voltage is fed over into the
voltage of the signal generator. This means that
audio chassis where it actuates the metering circuit.
when the SG-13/ARN NAV or GS output jack is directly
The audio voltages obtained from the demodulator
connected to a receiver with a 53-ohm unbalanced
appear at the AUDIO OUT connector on the front panel
input, the MICROVOLTS dial will be direct reading in
of the instrument.
making all receiver sensitivity measurements. For
example, if a receiver test required a 2000-microvolt
Two con -
signal, simply set the SG-13/ARN MICROVOLTS
centric MEGACYCLES control knobs control the
dial to 2000. Radio receivers having a 95-ohm balanced
frequency selection and tuning of the entire r-f section .
input circuit may be tested with the SG-13/ARN if a
The large outer knob switches crystals in the coarse
UG-529/U adapter is connected between the NAV or
frequency oscillator to vary the r-f output frequency
CS jack and the receiver input terminals. This
in megacycle steps. The coarse frequency oscillator
adapter is used as an impedance-matching device to
crystals are arranged in two banks, one containing
match the 53-ohm output impedance of the SG-13/ARN
crystals for even megacycles positions, the other
to the 95-ohm input impedance of the receiver. The
containing crystals for odd megacycle positions. Turn-
adapter also has the effect of a 6-db attenuator pad
ingthe large knob rotates a shaft and selects a crystal
because it reduces the effective r-f output of the SG-
in one bank or the other depending on whether the
13/ARN by one-half. Under these conditions, the
selected megacycle frequency is odd or even. This is
MICROVOLTS dial is no longer direct reading in
accomplished by means of switch S-1503 which opens
For
making receiver sensitivity measurements.
relay K-1501 on odd megacycle positions and closes it
example, to set up a 2000 microvolt signal, the SG
on even megacycle positions. Tuning the large knob
13/ARN MfCROVOLTS dial must be set at 4000.
also actuates a slug rack which varies the tuning of the
seven coils in the r-f section. Powdered iron cores,
fastened to the slug rack, are moved in or out of the
RF coils in steps of approximately 0.015 inch for each
switch, the output of the r-f attenuator is switched by
megacycle change in the setting of the large knob.
means of coaxial relay K-1401 to the input of the
Thus the tuning of all r-f coils is automatically varied
univerter. This input signal in the range 129.3-135.0
in approximately one-megacycle steps to maintain
mc is coupled into the input coil and applied to mixer
proper tracking as the frequency is varied in one-
megacycle steps. The small inner knob on the MEGA-
V-1511. A 200-mc signal from injection oscillator
CYCLES control varies the r-f output frequency in 0. 1
V-1510 is coupled into the mixer through L-1524 in
-mc steps by switching crystals in the fine frequency
the mixer cathode circuit. These two frequencies are
oscillator. The small knob is mounted on a shaft con-
added in the mixer and fed to two stages of amplifi-
centric with the large outer knob and shaft. This
cation, V-1512 and V-1513. The coupling circuits
inner shaft goes directly to the fine frequency oscillator
between stages are overcoupled to have a broad
and switches in the desired crystal. The pass band of
response and pass all signals in the range 329.3 -335.0
the mixer and following r-f stages is sufficiently wide
mc equally well. The gain of the univerter is con-
so that no retuning of the slug rack is necessary over
trolled by varying the screen voltage on V-1512 with
the megacycle spread represented by the fine frequency
potentiometer R-1544.
crystals.
17
