TM 11-6625-1559-12
e. Observe the pattern on the oscilloscope
(2) Connect one lead to the center pin of a
screen; make adjustments and measurements as
UG-88/U connector.
described in 3-6.
(8) Connect the other lead to the shell of
the UG-88/U connector. Do not connect the
shield to any portion of the connector.
3-9. Testing Radiofrequency Amplifiers
(4) Substitute this special cable for the out-
put cable described in a(2) above.
a. Analyzing Amplifier Response. The re-
(5) Ground the shielding of the test cable
sponse curve of an rf amplifier operating within
to various points on the receiver chassis until a
the 500-kHz to 1,200-MHz range may be ana-
point of minimum hum pickup is found.
lyzed as follows:
(6) Ground the other end of the shielding
(1) Connect the RF OUTPUT jack of the
to various points on the sweep generator chassis
sweep generator to the input terminals of the
to see if further reduction of the hum voltage
amplifier under test; use a matching pad if the
can be effected.
input impedance of the amplifier differs from
(7) Use the combination of ground connec-
generator.
tions which results in the least amount of hum
(2) Connect the output of the amplifier
voltage.
under test to one of the DETECTOR jacks on
Note. Use all means available to reduce the hum
the front panel of the sweep generator; use a
rather than make the test with the receiver local oscillator
matching pad if the output of the amplifier is
disabled, because the local oscillator excitation affects
designed for other than 50 ohms.
the input impedance of the receiver mixer input stage and
(3) Terminate the other DETECTOR jack
alters the shape of the response curve.
with adapter-connector UG-1640/U, furnished
with the sweep generator.
3-8. Measuring BandPass of IF
Section of
(4) Perform the procedure described in
Receiver
The response curve of the IF portion only of
a receiver may be analyzed as follows:
measurements, make the adjustments and meas-
a. Connect the sweep generator to the receiver
(6) If the response curve has sharp changes,
b. Connect the RF OUTPUT jack to the
output of the receiver mixer tube for reasonably
b. Measurement of Amplifier Gain. The gain
uniform response without changing the output
of any portion of the response curve of the am-
impedance of the mixer state. A recommended
plifier under test may be measured as follows:
(1) With the equipment set up as described
OUTPUT jack of the sweep generator is con-
in a above, disconnect the test cables from the
nected to a 50-ohm terminating resistor at the
amplifier under test. Keep the matching pads at-
output of the receiver mixer stage.
tached to the test cables.
c. Couple the RF OUTPUT voltage of the
(2) Connect the two cable ends together,
sweep generator loosely to the receiver mixer
including the matching pads, so that the input
output to avoid changing the tuning of the
cable feeds through both matching pads into the
mixer plate circuit.
output cable.
Note. It is important to introduce the signal from the
(3) Set the ATTENUATOR db control on
sweep generator to the receiver mixer stage by a method
the sweep generator to 0.
that does not result in added overall feedback in the re-
(4) Adjust the RF OUTPUT control until
receiver IF circuits. Ground the outer shield of the test
the VOLTS meter reads 0 DBM on its bottom
cable from the sweep generator carefully to the chassis
scale.
of the receiver near the mixer stage. If the frequency is
above 10 MHz, attach the test cable to a connection which
brings it through the chassis, using the chassis as a shield
ING PULL switch pulled out, adjust the oscil-
against unwanted feedback fields.
loscope vertical amplifier gain control so that
d. Perform the procedure described in para-
the desired portion of the response curve is at
a convenient distance from the baseline (about
graph 3-6d through l, to obtain the best display
5 centimeters). Note this distance.