3. Adjust the plot time span to view a small region around the sample window, and set the
initial amplitude zoom to x16.
This assures that the plotted waveform is still noticeable, even if the burst signal
strength is very weak.
4. Verify that the transmitter is radiating, and observe the burst pulse samples on the
display.
Use the L and R commands to shift the timing of all 6 triggers relative to range zero.
This moves the burst sampling window relative to the transmitted pulse.
Depending on whether 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.
5. Fine tune the triggers so that the burst envelope is centered in the window, and adjust
the amplitude zoom for a comfortable size display.
6. Isolate the clean center portion of the burst pulse isolated to a narrower sub-window 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 are derived.
7. 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.
See 5.2.1 Mb — Burst Pulse and AFC (page 93).
6.6
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.
The Ps command plots the burst spectrum, designs the band pass
filter, plots its frequency
response, and helps align the AFC.
6.6.1
Interpreting Burst Spectra Plots
The following figures show an example of two plots from the Ps command. The display
screen is divided into 2 areas:
• The top portion (single line) shows the present AFC level.
• The lower portion shows power spectrum plots of the burst pulse and/or the matched
filter response.
Chapter 6 – Plot-assisted Setups
137
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