Lambda SR1, Operation Manual, Page: 150 / 317

SR1 Operation Manual 150
© 2014 Stanford Research Systems
averaging successive power spectra each bin of the new spectrum is compared to the current buffer— if
the value in the new spectrum is greater the value in the buffer is replaced. This hold the maximum value
in each bin and is useful for detecting unwanted transient events.
The "Clear" button clears the average buffer and re-starts averaging . The "Avg Done" indicator lights
when the required number of averages have been accumulated.
The FFT2 Meas2 Panel
This panel contains additional configuration controls for the FFT2 analyzer.
Window Selection
It is well known that the application of a window function is typically necessary to obtain maximum
dynamic range for FFT measurements. The discrete Fourier Transform calculated by SR1 calculates a
spectrum assuming the time record repeats continuously. Thus, if the signal being analyzed is not
perfectly repetitive in the time record interval the the calculated spectrum will include the discontinuities
between the beginning and end of the time record which show up in the frequency domain as wide skirts
and a high noise floor around the actual spectrum. The first spectrum shown below is the unwindowed
spectrum of two sinewaves of arbitrary frequency. The spectrum is completely distorted by the artifacts
associated with the time record discontinuities.
Unw indow ed Spectrum