Sect. II Page 9
meter, the applied frequency of 50 KC in this ex-
ample would cause a half-scale reading on the 100
) KC range and therefore an average d-c output of
-10 volts or -0.2 volt d-c/KC. By now measuring
the peak-to-peak amplitude of the varying com-
ponent of the d-c output with an oscilloscope or a-c
voltmeter, the rt5 KC deviation in the signal will
be found to cause a measured value of 2 voltspeak-
to-peak. Figure 2B-6. Oscillogram, of Incidental FM
In practice, these voltages will all be affected incidental f-m modulated into a klystron oscillator
by the impedance of the filter used. The voltage from the heater circuit.
per cycle or per kilocycle out of the filter can
easily be determined by dividing the measured 28114 MEASURING KLYSTRON INCIDENTAL FM
d-c voltage out of the filter by the reading on the
frequency meter. Incidental f-m in the klystron output is trans-
lated to the range of the frequency meter by ap-
A filter suitable for the example above and most plying the klystron output to an -hp- 540A Trans-
applications is shown in Figure 2B-5A. The out- fer Oscillator? The Transfer Oscillator is tuned
put of this filter is down 3 db at 15 KC, 5 db at to produce a difference frequency of 70 KC which
18 KC, 10 db at 20 KC, and more than 55 db at also contains the incidental f-m. Test arrangement
23 KC and above. The filter need not always be shown in Figure 2B-7.
as elaborate as the one shown in Figure 2B-5A; in
some cases a single shunting capacitor will do. The The amplitude of the deviation is measured by
cut-off frequency of the filter may be adjusted to adjusting the oscilloscope gain to calibrate the
any frequency in the audio range, but it must scope graticule. In Figure 2B-6 each major di-
be higher than the modulation frequency and lower vision on the graticule is equal to 5 KC of devia-
than the lowest frequency of the modulated signal. tion. Total deviation represented by the waveform
can thus be seen to be 15 KC peak-to-peak.
In order to obtain a fair approximation of the mod-
1, ulation signal from the output of the filter, there The fundamental component of the modulation is
should be at least ten pulses into the filter for 60 cps which is combined with a large amount of
each cycle of the modulation frequency out of the second harmonic. If desired, an accurate mea-
filter. Thus, the average frequency of the modulated surement of each of the components could be made
signal applied to the 500B should be at least ten by applying the waveform to an harmonic wave
times the modulation frequency. For example, if analyzer (Figure- 2B-8). If deviations larger than
modulation components up to 5 KC are to be mea- 100 KC peak-to-peak are encountered, the -hp-
sured, the average frequency supplied to the input 520A 1OO:l scaler can be connected ahead of the
of the 500B should be not less than 50 KC.
1.
Figure 2B-6 is an oscillogram of a demodulated Dexter Hartke, ?A Simple Precision System for
f-m signal recovered by using the PULSE outpu? Measuring CW and Pulsed Frequencies Up to
of the frequency meter in the method describe 12,400 MC?, Hewlett-Packard Journal, Vol. 6,
in paragraph 2B-13. The waveform itself is the No. 12, August, 1955.
i -
Figure 2B-5A. Low-pass Filter Suitable for Most Applications
-B 00022-2 |
ACEC
