Fig, 2-25 Frequency modulated displays.
Fig. 2-26. Amplitude modulated displays.
Long term stability measurements require a recorder or a
Frequency Stability
The Type 491 con be used to measure long and short
term frequency stability, when the local oscillator is phase
locked to a stable crystal-controlled reference frequency.
See Stability in Characteristics Section.
Short term stability measurements apply to fast frequency
changes such as those caused by power supply noise and
ripple, vibration or other random factors. Fig. 2-24 shows
the random frequency modulation characteristics of a klystron.
series of photographs to show the frequency drift as a
function of time. Temperature compensation can be recomputed
by this process, since the amount and direction of the drift
may be graphically indicated by photographs.
Amplitude Modulation
Modulating frequency or frequencies and modulation per-
centage are the figures most often required from an AM
signal measurement. Fig. 2-25 shows two illustrations of
amplitude modulated signals, the methods to measure the
modulating frequency and modulation percentage.
2-22
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