noticed that, first, a low beat-frequency is approached,
then the exact zero-beat point begins to appear somewhere
on the screen. It moves about on the screen and then dis-
appears. If the frequency modulation occurs at 60- or 120-
cycle rate the PHASE control can be adjusted so that the
zero-beat first appears on one side and then disappears from
the other side, and the center frequency will be measured
by setting the zero-beat point to the center of the pattern.
Note the exact zero-beat point is where the lines in the pat-
terns become expanded horizontally and then reverse their
slope before reaching full amplitude. Also note: that the
zero-beat appears twice. This is because the line frequency
applied to the oscilloscope sweeps it in both directions and
that the zero-beat is crossed twice per cycle once in one
direction and once in the other. Either zero-beat can be
us e d.
If the pattern cannot be synchronized (the zero-beat stopped
in one place), the modulation frequency on the carrier is
different from the line frequency. If necessary, the 540A
oscilloscope can then be swept at the same rate by applying
the new modulation frequency to the HORIZONTAL INPUT
jack on the rear of the 540A chassis and switching the ad-
jacent toggle switch to EXTERNAL.
If the residual frequency-modulation is accompanied by
amplitude -modulation, the amplitude of the overall pattern
on the oscilloscope will be altered without affecting read-
ability or re solution. Amplitude modulation is indicated by
a difference in amplitude of the pattern at the forward and
backward traces on the oscilloscope. If the amplitude
modulation occurs at the 60-cycle power line frequency,
the phase control can be adjusted to superimpose the two
traces and produce the familiar trapezoid associated with
amplitude -modulation.
Frequency-modulated r-f signals are measured in exactly the same
manner as c-w signals and the step-by-step procedure in paragraph
2-4 is used for both types of signals. Step f of paragraph 2-4 de-
scribes in detail the effect that residual frequency-modulation has
upon the beat-frequency presentation of a c-w signal. The pres-
entation obtained when measuring a frequency-modulated carrier
is the same but the deviation is usually much greater and the zero-
beat point is much smaller in relation to the entire frequency :wing
(see Figure 9a - in contrast to 8).
2 -9 |
ACEC
