Stanford Research Systems SR780, Operating Manual And Programming Reference

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FFT Time Record 2-9
SR780 Network Signal Analyzer
baseband sample rate. In comparison, the baseband 0-51.2 kHz span has a sample rate 1/2
of the full span baseband sample rate. This is because the baseband time record is all real
and the zoomed time record is complex.
Further filtering and down-sampling reduces the span even further. At each span, the
zoomed time record is complex and has half as many points (half the sample rate) as the
corresponding real baseband time record.
The Time Record Display
The baseband time record display resembles a digital oscilloscope display. Signals at
frequencies above the span have been filtered out. The anti-aliasing filters (both analog
and digital) have a steep but finite roll-off at their cutoff frequencies. Signals which are
just above the cutoff frequency are outside of the FFT span (not in the displayed
spectrum) but appear attenuated in the time record.
Baseband time records are entirely real, they have no imaginary part.
The zoomed time record display does not resemble the original data. The data has been
frequency shifted. Signals at the center of the span appear at DC while frequencies at
both edges of the span appear as high frequencies. The anti-aliasing filters have a steep
but finite roll-off at their cutoff frequencies. Signals which are just outside of the span are
not displayed in the FFT but appear frequency shifted and attenuated in the time record.
Zoomed time records are complex, they have both a real and an imaginary part. You can
display the magnitude and phase as well as the real or imaginary part. The sampling rate
is always half of the equivalent baseband span.
Why Use The Time Record?
The time record display can be useful in determining whether the time record is triggered
properly. If the analyzer is triggered and the signal has a large component synchronous
with the trigger, then the signal should appear stationary in the time record. If the signal
triggers randomly, then the time display will jitter back and forth.
Remember, the time record has a resolution of 1/(sample rate). A triggered time record
will always jitter by 1 sample. This jitter is removed in the computation of the phase of
the spectrum relative to the trigger.
Watch Out For Windowing!
The SR780 can display both the time record and the windowed time record. Most
window functions taper off to zero at the start and end of the time record. If a transient
signal occurs at the start of the time record, the corresponding windowed time record and
FFT may not show anything because the window function reduces the transient to zero.