Connect the frequency to be measured to the ATTENUATOR INPUT
jack on the 540A.
Connect the VIDEO OUTPUT jack on the 540A to the vertical am-
plifier of the synchroscope. Synchronize the synchroscope with
the source of pulsed r-f energy.
Set all VIDEO RESPONSE, GAIN and INPUT ATTENUATION con-
trols to maximum, clockwise positions for maximum gain and
minimum attenuation and turn the FINE VERNIER control to the
center of its range (white dot straight up).
a. Adjust the frequency of the 540A as in steps a through e of
paragraph 2-4 for c-w frequency measurement. SinceT,
consists of pulses of r-f energy, the mixer output will con-
sist of the difference frequency signal, lasting the length
of the pulse.
b. Adjust the Transfer Oscillator to bring the difference -
frequency within the bandwidth of the video amplifier. When
the oscillator is tuned for a zero-beat with a pulsed r-f wave
applied to the system, the first presentation usually recog-
nized on the oscilloscope will be similar to that in Figure
lla. Note that about five cycles of difference frequency are
sh&m in the pulse envelope. For a 1 microsecond pulse
this would correspond to a difference frequency of about
5 mc.
NOTE
If too much input power is applied in
Pulsed R-F measurements, the detected
video pulse may obliterate the desired
beat frequency in the presentation. Use
only enough power to obtain an easily
read zero-beat.
C. Tune the 540A toward zero-beat. As the harmonic in the
Transfer Oscillator is tuned toward zero-beat, the number
of difference frequency cycles per pulse will decrease as
illustrated in Figure llb and finally become less than one
cycle as illustrated in Figure llc. When the beat frequency
is much less than one cycle, (very close to actual zero-beat)
a pattern like that in Figure lld will be obtained. Each of
the horizontal lines is now a segment of a sine wave, and
the lowest beat frequency is obtained when the pattern be-
comes a family of traces all having the same shape, as in
Figure lld. In Figure lld the beat frequency is about one-
hundredth>f a cycle per p<lse width.
2-13 |
ACEC
