TM 11-6625-2946-14/TM 08168A-14/1
CHAPTER
4
FUNCTIONING OF EQUIPMENT
4-4. Transmitter RF Power Output Test
4-1. General
Mode
The test set employs three basic circuits (fig. 4-l); a
In the transmitter rf power output (XMT) test mode, the
signal generator, a field strength detector, and a meter-
radio set rf output is connected to the test set ANTEN-
ing circuit. The circuits function to provide the four
NA connector. When the radio set is keyed, the test set rf
modes of operation required for operational testing of
input is attenuated 6 dB through a 50-ohm pad, then
the radio set at the Operator's/Organizational mainte-
detected to a dc current level by the rf attenuator and
nance level and isolation of faults to a module at the
detector assembly. The detected dc current is then fed
General Support maintenance level. The four modes of
through the rf power metering circuit to the meter. The
operation are:
meter zero is calibrated for a midscale deflection of 0.5
a. Battery test
watt transmitter rf output by the rf power metering cir-
b. Transmitter rf power output test
cuit; thus, transmitter output exceeding 0.5 watt will
c. Receiver sensitivity test
provide a reading in the green portion of the meter scale,
d. RF field strength measurement.
indicating adequate power output on the frequency
selected on the radio set. Transmitter power output can
4-2. Power Input
be measured at any radio set channel frequency.
The test set receives its power from a 14-vdc battery or an
4 - 5 . Receiver Sensitivity Test Mode
external 24-vdc source. When the battery is used, its con-
a. In the receiver sensitivity (RCV) test mode, the
nectors are mated directly with the BATTERY connec-
measurement of receiver sensitivity is attained by the
tors on the test set front panel. When the POWER switch
comparison of the noise level of the audio input (at
is placed in the BAT position, the battery voltage is fed
AUDIO connector) from the AN/PRC-68 receiver being
through the circuit breaker to the test set functional cir-
tested, with no rf input from the test set, and the level of
cuitry. When a 24-vdc external source is used it is con-
receiver quieting resulting from an rf input of 2.0 + 6
nected to the 24 VDC EXT connector on the test set
dB, 0 dB microvolt (ANTENNA connector). With
front panel. When the POWER switch is placed in the
the pushbutton SENS switch in the normal position, no
EXT position, the 24 VDC EXT input from the external
B + is applied to the rf signal generator; therefore no rf
power source is fed to a voltage regulator where it is
input is applied to the receiver. The volume control on
dropped to a 14-vdc level. The 14-vdc regulated voltage
the receiver is adjusted to set the noise level for a
is then fed through the circuit breaker to the test set func-
midscale reading on the test set meter. When the SENS
tional circuitry. In the transmitter rf output (XMT) and
READ switch is pressed, the rf signal is applied to the
receiver sensitivity y (RCV) test modes, the 14-vdc output
receiver (at ANTENNA connector) and the resultant
of the circuit breaker provides input power to the radio
audio quieting level output is indicated on the meter. A
set under test, through the RADIO output connectors on
reading in the upper (green) portion of the meter scale in-
the test set front panel.
dicates satisfactory receiver sensitivity y.
4 - 3 . Battery Test Mode
b. When the SENS READ switch is pressed, the rf
In the battery test mode, the battery is connected directly
MHz, rf output signal selectable in 50-kHz steps. The
to the BATTERY input connector on the test set front
desired frequency, corresponding to that selected on the
panel. The battery current is routed through the
AN/PRC-68 under test, is selected with the four fre-
POWER switch (BAT position), the circuit breaker, the
quency control switches on the test set. When the SENS
FUNCTION switch (BAT position), the battery test cir-
READ pushbutton is pressed, the synthesizer generates a
cuit, and the meter (dc ammeter) to ground, to provide
frequency reference voltage to drive the voltage con-
measurement of the battery voltage across a 44.2-ohm
trolled oscillator for the generation of a 42 to 68 MHz
load. The battery test circuit is calibrated to provide a
reference signal, 12 MHz above (for 30 to 54 MHz) or
midscale reading for a battery input level of 12 vdc and 1
below (for 54 thru 80 MHz) the selected frequency. The
volt-per-division, up to a full scale reading of 17 vdc, on
reference signal is amplified then combined, in a balanc-
the upper (green) scale. Since the end of battery life is 12
vdc, the meter provides a simple go (green) or no-go (red)
ed mixer, with the output of a 12-MHz stable oscillator
to provide a resultant rf output at the selected frequency.
reading to indicate the battery's serviceability status.
4-1
