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