User Manual

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The demodulator structure windows include two spectrum analyzers, making
it possible to view signal spectra in real-time. Each analyzer can be
"connected" to any of the test points shown as green dots in the diagram. To
connect the left spectrum analyzer to a particular test point, left-click on the
green test point. Its color will change to red. Right-clicking on a dot will
connect it to the right analyzer, and the color will change to blue. If both
displays are connected to the same test point, the point color will turn
magenta.
Within the displayed spectra, a red color frequency cursor can be manually
dragged, using the left mouse button, over a particular spectral component.
The two Vector Voltmeter displays labeled Left/Right indicate the relative
amplitude and phase difference between the two spectral components at the
cursor frequency.
There are two additional tools provided in this window: THD (total harmonic
distortion) and SINAD (signal-plus-noise-plus-distortion to noise-plus-
distortion) meters. They can be used to test the overall receiver performance
as well as provide indication of the sound card quality. To use these
facilities, a pure tone-modulated signal must be applied to the receiver
antenna input, and the frequency of the modulating tone must be equal to
the Test freq setting.
When either of the THD or SINAD buttons is activated, the right-hand
spectrum display will be automatically connected to the end of the
demodulator chain (i.e. right-clicking on any other test points will not work
until both the THD and SINAD buttons are released).
For synchronous demodulation of amplitude modulated signals (the AMS
mode) without carrier or with a fluctuating one, the G303 Professional
Demodulator uses a digital carrier recovery technique based on the so-
called Costas loop:
Synchronous AM Demodulator Structure