Frequency Stability

Fig, 2-25 Frequency modulated displays.

Long term stability measurements require a recorder or a

Fig. 2-26. Amplitude modulated displays.

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