The Auto Correlation Measurement - HP 3563A Operating Manual

Using The Linear Resolution Mode
Selecting a Measurement and Display

The Auto Correlation Measurement

The auto correlation measurement indicates periodicity in time domain signals. It multiplies the
signal by a progressively time-shifted version of itself; this emphasizes periodic parts of the signal and
de-emphasizes non periodic parts. In the HP 3563A it is computed using the inverse FFT of the
power spectrum:
1
Rx(t)= FFT -
[Gxx]
where:
Gxx is the power spectrum
To avoid wrap-around error, the auto correlation measurement discards the last half of the time
record. The measurement display is therefore half the length of the original time record. If some
signal of interest is present in the last half of the record, use delayed triggering to shift the deleted
portion of the record. Refer to "Setting up Triggering" in chapter 7 for more information about
trigger delay.
Some uses of auto correlation are detecting periodicity and measuring impulsive signals, such as gear
chatter. Auto correlation is especially useful for extracting a periodic signal buried in noise when a
synchronized trigger is not available. To select this measurement, press
by the AUTO CORR softkey (see figure 3-7).
3-44
SELECT M EAS, followed