4.7 RF filter sweep
QDX contains its own internal signal generator, which can be used to sweep the receiver input
Band Pass Filters, checking its response and center frequency. Though the difference in
performance is small, the perfectionist may wish to adjust (by squeezing turns) the Band Pass
Filter inductor L12 to optimize the center frequencies of the filters.
Connect a dummy load for best results.
On entering the application, a sweep automatically commences. The percentage completion of the
frequency sweep is indicated in the bottom right corner of the screen.
The center frequency and sweep width are defaulted appropriately per band, to show several MHz
of frequency sweep cleanly, and are not adjustable. The vertical axis shows audio decibel (dB)
level. The offset is arbitrary. Gridlines axis are displayed in blue. Vertical gridlines are every 20dB,
and horizontal axis gridlines as appropriate to the band. Gridline labels are displayed in blue; the
minimum and maximum audio level (in dB) displayed in white.
A vertical line is shown at the position corresponding to the default operating frequency in the
current band. Ideally the peak of the response (line of yellow asterisks) should coincide with the
center frequency of the band. The filters are not very very sharp so the performance degradation
by being slightly off frequency is not severe. In the example screenshot (40m), the center
frequency is incorrectly somewhat below the 40m operating frequency.
The – and + keys can be pressed to move down and up to adjacent bands as defined in the Band
configuration screen.
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