Stanford Research Systems SR785, Руководство по эксплуатации и программированию

Страница: 226 / 546

4-30 Display Setup Menu
SR785 Dynamic Signal Analyzer
The capture buffer is not a continuous representation of the input signal. The data is
sampled and has a time resolution of 1/(sample rate). High frequency signals will appear
distorted in the time record. However, ALL of the spectral information (up to 1/2.56
times the Sample Rate) is preserved by the Nyquist theorem as long as the value of each
sample is accurate.
Amplitude calibration is performed in the frequency domain. Hence, the captured time
data amplitudes are not calibrated.
User Functions
A User Function displays the results of a user defined math function. User Functions
defined within the FFT Measurement Group may include FFT measurement results. Use
the [User Math] menu to define a function.
A User Function may not be selected as the measurement if it uses a Trace which is
empty or which contains data taken in a different Measurement Group.
See ‘User Math Functions’ in Chapter 2 for more.
Measurement (Correlation)
Select the Measurement of the active display when the Measurement Group is
Correlation.
Each Measurement has an associated View. Changing the Measurement changes the
View to the View last used with the new Measurement.
Command: MEAS(?) d {, i}
The following Measurements are available in the Correlation Measurement Group:
Auto Correlation
Auto correlation is a single channel measurement. In the time domain, it is a comparison
of a signal x(t) with a time shifted version of itself x(t-
τ
) displayed as a function of
τ
.
This is useful for detecting similarities which occur at different times. For example,
echoes show up as peaks separated by the echo time. Sine waves appear as sine waves
and square waves appear as triangles. Signals which do not repeat or are completely
random for all time appear only at
τ
= 0.
Averaging
Auto Correlation = invFFT( FFTuN • FFTN* )
RMS or Peak Hold Averaging
Auto Correlation = invFFT( <FFTuN • FFTN*> )
Vector Averaging On
Auto Correlation = invFFT( <FFTuN> • <FFTN*> )
where N is Channel 1 or 2. FFT is a windowed FFT, FFTu is an un-windowed FFT
(uniform window) and invFFT is an inverse FFT.