USER’S MANUAL__________________________________________________________________
150 _________________________________________________________________ M211322EN-D
the triggers are set properly, you may at first see nothing more than a flat
line of misplaced A/D samples. However, after a few moments of hunting,
the burst pulse should appear on the display screen. Fine tune the triggers
so that the burst envelope is centered in the window, and adjust the
amplitude zoom for a comfortable size display.
The clean center portion of the burst pulse should then be isolated to a
narrower subwindow of the overall FIR interval. Use the A and S
commands to change the aperture and start of the narrowed region from
which the AFC frequency estimators data will be derived.
Check that the burst pulse signal strength is reasonably matched to the
input span of the IFDR A/D converter. The maximum analog signal level
is +8 dBm. Exceeding this level produces distorted samples that would
seriously degrade the algorithms for phase locking and AFC. However, if
the signal is too weak, then the upper bits of the A/D converter are wasted
and noise is unnecessarily introduced. Vaisala recommends a peak signal
level between -3 dBm and +4 dBm, that is, a signal that might be viewed
at x2 or x4 zoom. Take note of the burst energy level reported on the TTY;
it helps decide the minimum energy threshold for a valid burst pulse, which
is needed in Section 4.2.6 Mb — Burst Pulse and AFC on page 126.
5.4 Ps — Plot Burst Spectra and AFC
Once the transmit burst pulse has been captured the next step is to analyze
its frequency content and to design a band pass filter that is matched to the
pulse. In a traditional analog receiver the matched filters use discrete
components that can not be adjusted, and the transmit spectrum can not be
viewed unless a spectrum analyzer is on hand. The RVP900 eliminates all
of these restrictions via its Ps command, which plots the burst spectrum,
designs the band pass filter, plots its frequency response, and also helps
with alignment of the AFC.
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