4.6 Audio filter sweep
QDX contains its own internal signal generator, which can be used to sweep the audio passband
of the receiver, checking the audio response and the unwanted sideband cancellation.
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 sweep starts at 100 Hz (injected signal is 100 Hz higher than the “USB Dial Frequency”, then
proceeds in 50 Hz steps up to and including 3850 Hz. The RF frequencies (“USB Dial Frequency”)
used for bands are defined in the Band Configuration screen of the QDX. For example, for 80, 40,
30 and 20m are 3.573, 7.074, 10.136 and 14.074 MHz respectively. 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 every 1000 Hz. Gridline labels are displayed in blue; the
minimum and maximum audio level (in dB) displayed in white, and the maximum audio sweep
value also displayed in white.
The chart contains two lines, the line of yellow asterisks is the Upper Sideband (USB) and the red
line is Lower Sideband (LSB). In the default demodulation mode (USB), we expect to see the
yellow line nice and flat between the filter cut-off points 150 Hz to 3.2 kHz. The red line is the
unwanted sideband and we expect to see it around 60dB below the wanted sideband.
The – and + keys can be pressed to move down and up to adjacent bands defined in the Band
Configuration screen. Generally the result should not differ much from one band to the next.
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