TM 11-6625-1559-12
its entire range while the pattern on the oscil-
(3) Pull the MASTER GAIN-PULL EXT
loscope screen remains stationary.
MKR switch/control and adjust it, in conjunc-
tion with the output controls of the external
center frequency marker source, until the oscil-
control to set the position of the baseline on
the oscilloscope for best presentation.
loscope display resembles that shown in B, fig-
j. Set the SWEEP WIDTH switch and the
(4) Adjust the output controls of the ex-
SWEEP WIDTH control just wide enough to
ternal one-half bandwidth marker source until
show the entire response curve of the receiver,.
the oscilloscope display resembles that shown in
Observe the phasing precautions in paragraph
3-2a(5), (6), and (7).
3-6. Measuring Bandpass of
Receiver
by increasing the ATTENUATOR db control on
the sweep generator by 10 decibels. Move the
Measure the overall bandwidth of a complete re-
ATTENUATOR db control to a setting 10 db
ceiver as follows:
higher than the one in use. If the shape (not
a. Connect the RF OUTPUT jack of the sweep
amplitude) of the pattern changes significantly,
generator to the input terminals of the receiver
overload distortion is indicated.
through a matching pad, balun, or dummy anten-
l. If distortion is present, reduce the input to
na as required.
the receiver and increase the setting of either the
b. Determine a suitable point in the receiver
PREAMPLIFIER GAIN control on the sweep
second detector circuit to obtain a rectified output
generator, or the vertical amplifier gain control
voltage. If this voltage is 0.1 volt peak-to-peak
cm the oscilloscope, or both, until the test in
or more, connect it directly to the vertical ampli-
k above indicates no distention on the pattern.
fier input terminals of the oscilloscope; observe
If necessary, insert an external 20-db attenuator
correct ground polarity. Connect a 50-micromi-
between the RF OUTPUT jack and the test
crofarad (uuf) capacitor, from the SCOPE VERT
jack on the rear of the sweep generator to the
ungrounded vertical amplifier input terminal to
m. Observe the pattern on the oscilloscope
the oscilloscope, to couple the frequency marker
screen; make adjustments and bandwidth measu-
signals to the oscilloscope.
c. If the receiver output voltage obtained as
n. Where sudden changes are present in the
instructed in b above is less than 0.1 volt peak-
response curve of the receiver circuit, such as
to-peak, connect it to the EXT DET jack at the
highly selective traps to remove specific frequen-
rear of the sweep generator. This action ampli-
cies, the sweep may move past these traps too
fies the receiver output signal and produces a
rapidly for the detector to follow accurately. For
readable oscilloscope pattern.
a more accurate display, decrease the SWEEP
d. If the receiver has automatic gain control
WIDTH control and shift the FREQUENCY
(agc), disable it and substitute a manually varied
MEGAHERTZ dial to cover only this small sec-
bias control. The exact procedure depends on the
tion of the response curve.
particular type of receiver being tested.
o. When the receiver circuit contains rejection
e. Turn on the receiver and allow it to warm
notches or other sharp changes in the response
up long enough to become stable.
curve, spurious component may be generated in
f. Set the FREQUENCY MEGAHERTZ dial
on the sweep generator to the same center fre-
quency as that of the receiver.
up as extraneous markers on the response curve
g. A pattern should appear on the oscilloscope
and can be distinguished from normal markers
by reducing the setting of the RF OUTPUT con-
ANCE control and/or DC LEVEL control until
trol. Normal marker signals will be reduced
a pattern does appear.
proportionally; extraneous markers will tend to
disappear completely. Whenever possible, use re-
(dc) by rotating the PREAMPLIFIER GAIN
duced RF OUTPUT control settings of the
control back and forth. Adjust the PREAMPLI-
FIER BALANCE control to keep the pattern on
tains sharp components.
the oscilloscope screen until the PREAMPLI-
p. When specific band-edge and band-center
FIER GAIN control can be rotated throughout
markers are required from an external source, re-
3-7