6. Optimize the performance of the FIR filter.
The filter should not pass any frequencies that do not contain useful information from
the original transmitted pulse. If anything, choose a filter whose width is slightly
narrower than the bursts spectral width.
The previous figure shows an example of a filter that is poorly matched to the pulse.
Although the filter has fairly good DC rejection, it passes frequencies that are outside of
the transmitter's broadcast range. These frequencies contribute nothing but noise to
the synthesized I and Q data stream.
You can optimize the FIR filter manually or automatically:
• Manual Method
Defining a nearly optimal filter requires a few minutes of hunting with the I, W, and N
keys. Each time you press any of these keys, RVP900 designs a new FIR
filter from
scratch, and displays the results.
Even though you must still control two degrees of freedom (length and bandwidth),
the RVP900 design calculations perform several hundred iterative steps each time,
which preferentially select for the best
filter. Because the FIR coecients are
quantized in the
filter chips themselves, the process of finding an optimal filter
becomes quite nonlinear.
• Automatic Method
Type the $ command and let the RVP900 do all of the work.
See 6.6.2 Ps Subcommands (page 140).
Chapter 6 – Plot-assisted Setups
149
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