3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
case it is cleared out when you start sweeping again. Restart clears out all
spectrogram traces and start over. The Waterfall display is also cleared on exit from
the Spectrogram View, so every time you enter the Spectrogram View, the Waterfall
window is empty.
The colors in the Spectrogram represent signal amplitude. The key to these colors is
displayed next to the Y Axis in the upper window. By changing the Y Axis
parameters, you can change the scaling; that is, by changing the Reference Level or
Scale/Div, the colors remap to new Amplitude values. Note that this does not restart
the Spectrogram, unless "Attenuation" on page 1954 changes.
As this is swept spectrum analysis, each horizontal line in the Waterfall window
represents a single trace, and the vertical axis represents time. You might thus
expect each line to slope upwards from left to right to more correctly represent the
point in time at which each point in the trace was taken. However, the lines are
horizontal, so the display represents each trace as representing a single time, which
is in fact its start time. If this distinction is important to you, use FFT sweeps (with an
FFT Width greater than your span, of course) to ensure that each trace point in a line
better represents the same moment in time.
If Display Trace = 0, the data for Trace 1 is written into the trace as the data is
acquired, just as in Normal view. So, you see the data as it is acquired; for a slow
sweep, for example, you see the trace fill as the points are taken. For any other value
of Display Trace, Trace 1 appears static, as it represents an historical trace. As the
traces roll up, the value of Display Trace does not change, so you see a different
trace in Trace 1 every time the live trace finishes. To freeze the spectrogram, put
Trace 1 into View, or put the instrument into Single sweep (note that unless
Average/Hold Number = 1, putting the instrument into Single does not freeze the
Spectrogram until the number of traces specified by Average/Hold Number have
been taken).
When returning to the Normal view from the Spectrogram view, Trace 1 holds
whatever data was in Display Trace 0 on exit.
Differences with Spectrum window
While in Spectrogram View, all functions and settings work as normal, except as
noted below.
–
The Single key behaves differently than it does in Normal view. In the
Spectrogram View, Single causes a specified number of traces to be read into
the spectrogram from Trace 1, after which the acquisition stops. The number of
traces to be read into the spectrogram is controlled by "Average/Hold Number"
on page 381 in the Meas Setup menu. For example, if you set Average/Hold
Number to 5, then every time you press Single, it takes 5 sweeps and puts the 5
traces one by one into the Spectrogram; then it stops sweeping. Note that you
can set Average/Hold Number to 1 to capture a single trace into the
196 Spectrum Analyzer Mode User's &Programmer's Reference
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