USER’S MANUAL__________________________________________________________________
152 _________________________________________________________________ M211322EN-D
dB can be viewed at once. The horizontal lines also contain major and
minor tick marks to help calibrate the frequency axis. Major marks are
small downward triangles that represent integer multiples of 5 MHz; minor
marks are in between and represent 1 MHz steps. The power spectrum
example in Figure 27 on page 149 is from a system with an intermediate
frequency of 30MHz. Thus, the left edge of the plot begins at DC, and the
graph is centered on the sixth major tick, that is, 30 MHz.
Two types of spectra can be plotted on the screen: the frequency response
of the FIR filter and the frequency content of the burst pulse itself. The
burst spectrum is computed by first applying a Hamming window to the
raw samples. You may choose to view either plot individually, or both at
the same time.
Figure 27 on page 149 is an example of a single filter response plot,
whereas Figure 28 on page 160 shows a combined display of both spectra.
The combined display makes it easy to compare the filter being designed
with the live waveform that it is intended to selectively pass. Note that the
filter's frequency response is always drawn with its passband peak
touching the top of the plot. The vertical height of the burst spectrum,
however, will vary with signal strength but can be adjusted using the Z
subcommand.
The horizontal line at the top of the plotting area is also marked with an
upward pointing major and minor tick. These indicate the present value of
the burst pulse frequency estimator. The major tick is a triangle whose
position along the horizontal axis corresponds directly to the estimated
frequency. It should always be positioned directly over the main lobe of
spectral power. The minor tick gives finer scale resolution by indicating
the fractional part of each 1 MHz multiple.
It is helpful to read the minor tick relative to the ten horizontal division
lines that are present on most scopes. Motion of the minor tick is apparent
even with very small changes in burst pulse frequency; a change of just 5
KHz can easily be seen. This means that you can observe the frequency
drift of the magnetron in great detail, and also watch the AFC behavior in
real-time.
The horizontal line at the very top of the display (above the spectra plot)
serves to indicate the present value of the AFC control voltage. The line
contains an upward pointing major and minor tick, similar to the ones used
to represent the burst frequency estimate on the line below. However, the
horizontal axis now represents voltage rather than frequency, and the
overall span is the complete range of the AFC's digital-to-analog converter.
The major tick will move from the left edge to the right edge as the AFC
varies from its minimum to maximum value. The minor tick will traverse
the screen at ten times this rate.
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