User Manual

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The second mixer again uses a DDS with a PLL to mix the 45 MHz signal
down to the last intermediate frequency, which is 12 kHz.
Both DDS circuits derive their reference frequency from a 20 MHz reference
oscillator.
The 12 kHz IF output is then fed to the right channel of the Line input of the
PC sound card. You can hear what it sounds like if you use the sound card
mixer panel to listen directly to this input (rather than using the G303
Demodulator software).
The AGC is performed in the first IF stage, based on the level of the last IF
output (at 12 kHz IF). As the IF bandwidth of the first IF stage is 15 kHz, the
AGC action is delayed until the dynamic range of the first IF stage is fully
utilized – this is in order not to cause desensitization of the receiver in the
presence of neighboring strong signals, falling within the 15 kHz IF
bandwidth. The resulting variation in audio output is then compensated for in
software, using Audio AGC in the software demodulator.
The final IF bandwidth is then adjusted entirely in software. If the
Professional Demodulator is used, this bandwidth is continuously variable
from 1 Hz to 15 kHz.
Did you know?
The largest shortwave transmitters nowadays operate with 250,000 –
500,000 Watts of power. Using the CW mode, and in suitable atmospheric
conditions, radio amateurs make regular contact around the world with only
a very tiny fraction of this power: reports exist of round-the-world
communications achieved with less than five Watts of power.