The time record
A time record is the amount of time-domain data the analyzer needs to perform one
FFT operation. The time record and its FFT are the building blocks the analyzer
needs for all subsequent measurements.
Why is a time record needed?
Essentially, the time record is a block of time-domain sample points. Since the
actual Fourier Transform does not have explicit time or frequency references (it
simply operates on a sequential collection of points), FFT analyzers must assign
arbitrary start and finish times for data to be transformed. These blocks of input
data are called time records.
For example, with the default display resolution of 401 frequency points, the 89400
series vector signal analyzer takes up to 1024 samples of time data to produce 512
points of frequency domain data. The analyzer usually displays the first 401 points
of this data and discards the rest (this accommodates the anti-aliasing filters, but
that’s beyond the scope of our current discussion).
The time record can be described by both a length and a size. The time record
length is the amount of time required to acquire a time record and is altered by
changing resolution bandwidth, window, main length, or gate length. The time
record size is the number of time points in the time record and is dictated by the
time record length in combination with the sample rate (and sample rate, in turn, is
directly related to span). More detail on these interactions can be found later in
this chapter.
Time record size refers to the number of time points (samples) in a time record. Time
record length refers to the amount of time needed to acquire a time record.
Fundamental Measurement Interactions
19 - 16
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