Lambda SR1, Operation Manual, Page: 134 / 317

SR1 Operation Manual 134
© 2014 Stanford Research Systems
term or the Blackman- window are the best windows to use. (The Blackma window is a
proprietary improved version of the traditional Blackman Harris window.) These two windows have
sufficient dynamic range for most measurements. The other windows are included for historical interest
and comparison purposes. The FFT chirp and multitone sources are synchronous with the FFT time
record and have different frequency content over different parts of the time record. Therefore, the
Uniform
window should be selected when the FFT analyzer is being used with the FFT chirp or Multitone source.
The Variable Time window is "on" (1) during the portion of the time record specified by the "Variable Time
Window Limits". Outside of that interval the window is 0. The transitions between the off and on
segments of the window are done using a raised cosine function to minimize disturbance in the
frequency domain. The Variable Time window is useful for isolating analysis to a particular region in the
FFT time record. Obviously, the time record needs to be repetitive, or the
Analyzer Trigger needs to be
used to obtain meaningful results.
Phase Controls
The linear spectrum has phase as well as magnitude associated with it. Checking "Unwrap Phase"
instructs SR1 to "unwrap" the phase by adding or subtracting 360º at appropriate break points to create
a continuous phase curve.
Phase Spectrum of 1 kHz LPF Without Unw rapping
Sam e Spectrum w ith Phase Unw rapping On