Burst spectra and AFC plot display
Horizontal and vertical axis: frequency and amplitude
The horizontal axis of the spectrum plot represents frequency. The overall span from the left edge, set at DC or 0 MHz, to the right edge is half the acquisition system clock frequency selected in the Mc menu. The frequency span is printed on the TTY when the command is first entered.
The vertical axis of the spectrum plot represents amplitude. The scale is logarithmic, and is marked with faint horizontal lines in 10 dB increments. An overall dynamic range of 70 dB can be viewed at once.
Horizontal lines
The horizontal lines 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 in the example is from a system with an intermediate frequency of 30 MHz. The left edge of the plot begins at DC, and the graph is centered on the sixth major tick, that is, at 30 MHz.
Horizontal line at the top of the plotting area
The horizontal line at the top of the plotting area is marked with an upward pointing major and minor tick to 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 10 horizontal division lines visible 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 watch the AFC behavior in real-time.
AFC line
The horizontal line at the top of the display indicates the value of the AFC control span.
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 here represents where in the total control span the AFC is currently set, and the overall span is the complete range of the AFC's digital-to-analog converter. The left edge would represent -100% and right edge +100% of the controllable span.
The major tick moves from the left edge to the right edge as the AFC varies from its minimum to maximum value. The minor tick traverses the screen at 10 times this rate.