Figure 27 Example of a Filter With a Poorly Matched Filter
The exact endpoints of the plot depend on which alias band the radar’s intermediate
frequency falls in. For example, with a 72 MHz acquisition clock, a 30 MHz IF would imply a
horizontal axis range of DC to 36 MHz, whereas a 60 MHz IF would make the range 36 MHz
to 72 MHz. The frequency span is printed on the TTY when the command is
first entered.
Since the left edge of the spectral plot always represents an integer multiple of 36 MHz,
either the left side or the right side are always a multiple of 36 MHz. This is important to
remember when designing the matched
filter, since fixed DC osets in the A/D converters
appear aliased at these 72 MHz multiples.
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.
Horizontal Axis
The horizontal axis of the spectrum plot represents frequency.
The overall span from the left edge to the right edge is half the acquisition system clock
frequency selected in the Mc menu.
Vertical Axis
The vertical axis of the spectrum plot 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.
Chapter 6 – Plot-assisted Setups
139
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