2-50 Swept Sine Measurements
SR785 Dynamic Signal Analyzer
Cross Spectrum
The swept sine cross spectrum is a two channel measurement defined as
Cross Spectrum = conj( Spec1 ) • Spec2
The cross spectrum contains both magnitude and phase information. The phase is the
relative phase (at each frequency) between the two channels.
The magnitude is simply the product of the magnitudes of each spectrum. Frequencies
where signal is present in both spectra will have large components in the cross spectrum.
Frequency response
The swept sine frequency response (sometimes called frequency response) is a two
channel measurement defined as
Frequency response = Spec2 / Spec1
The frequency response contains both magnitude and phase information. The phase is the
relative phase (at each frequency) between the two channels.
Frequency response measures the response of a network or device under test . The
reference channel (1) measures the signal at the input to the device and the response
channel (2) measures the device output. The result is the complex frequency response of
the device.
Normalized Variance
Normalized variance is a single channel swept sine measurement similar to the coherence
measurement of the FFT measurement group. The normalized variance is an indication
of the signal to noise ratio after integration over the specified number of cycles is
performed. Normalized variance, like coherence, has a value between 0 and 1. A value of
1 indicates that all noise has been removed from the signal. Values less than one indicate
that there is noise remaining. Increasing the Integration Time or Integration Cycleswill
move the variance closer to 1.
User Function
User Function displays the results of a user defined math function. User Functions
defined within the Swept Sine Measurement Group may include swept sine measurement
results. Use the [User Math] menu to define a math 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’ later in this section for more.
Averaging - Settling and Integration
At each frequency point in a swept sine measurement, the inputs measure the amount of
signal at the source frequency. This is done by multiplying the input data by sin(
ω
t) and
cos(
ω
t) and averaging the results over an Integration Time. As with the FFT, real (in-
phase) and imaginary (quadrature) signals are measured yielding both magnitude and
phase frequency responses.
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