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Chapter 5, FRONT PANEL OPERATION
5-38
The Spectral Display menu shows the result of a discrete Fourier transform (DFT) of
the signal at the input, following amplification and filtering by the line frequency
rejection and anti-aliasing filters but prior to the demodulators. Its principal function
is to allow the user to determine the relative spectral density of the total signal plus
noise being measured, so that changes can be made to the reference frequency to
avoid particularly noisy regions.
Note that the Spectral Display menu does not have a calibrated vertical axis and
hence is not intended for measuring or comparing signal amplitudes.
The central section of the display shows a plot of spectral density (vertical axis)
versus frequency (horizontal axis). The measurement frequency resolution is
adjusted by using either the Res.
to bin widths of 0.2 kHz, 0.4 kHz, 1 kHz and 2 kHz, as indicated by the control
annotation. In figure 5-24 above it is set to 0.4 kHz.
The measurement resolution sets the overall frequency range of the X-axis. At a
resolution of 2 kHz, the nominal range is 0 kHz to 458.0 kHz, although frequencies
above 250 kHz are not usually of interest since they lie outside the frequency range
of signals that the 7270 can measure. With the finest resolution of 0.2 kHz, the
display range is nominally 45.8 kHz.
When the resolution is set to 1 kHz or finer then the
Lower and Upper frequency limit indicators at each of the X-axis can be used to
scroll through the whole of the available frequency range. When either limit is
changed, the other one changes by the same amount, thereby maintaining the
frequency resolution.
LOG SCALE / LINEAR SCALE
The
keys at the top right-hand corner of the display change the vertical
display between linear and logarithmic calibration, with the presently active
selection being shown.
RUN FFT
When the
key adjacent to the Run FFT control is pressed, the instrument
acquires a new set of data, performs an FFT on it and displays the resulting
spectrum. A change in the reference frequency also causes the displayed spectrum to
be updated.
LIVE FFT
When the
key adjacent to the Live FFT control is pressed, the instrument
acquires data repeatedly, performs an FFT on it and displays the resulting spectrum
until the Stop key is pressed.
NOTE: Live data acquisition is not available at 0.2 kHz and 0.4 kHz measurement
resolutions.
Cursor
The
and
cursor movement keys move the display cursor from side to side. The
cursor takes the form of a short vertical line that is positioned just above the top of
the data curve at the selected frequency, which appears just below the X-axis. In
figure 5-24 the cursor can be seen positioned at 100 kHz. The cursor makes it
possible to obtain an approximate frequency for any peak on the display, thereby
possibly assisting in identifying its source.
keys or by the
or
keys adjacent to the
keys and can be set
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