Ametek 7280 Instruction Manual page 86

Chapter 5, FRONT PANEL OPERATION
5-40
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.
widths of 3 kHz, 5 kHz, 10 kHz and 15 kHz, as indicated by the control annotation.
In figure 5-23 above it is set to 5 kHz.
The measurement resolution sets the overall frequency range of the X-axis. At a
resolution of 15 kHz, the nominal range is 0 kHz to 3.43 MHz, although frequencies
above 2 MHz are not usually of interest since they lie outside the frequency range of
signals that the 7280 can measure. With the finest resolution of 3 kHz, the display
range is nominally 687 kHz.
When the resolution is set to 10 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.
Figure 5-24, Spectral Display Menu
keys or by the or keys and can be set to bin
keys adjacent to the