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8.6.3
Image sweep
QMX implements a conversion to baseband, sampling quadrature I and Q channels at 48ksps to a
24-bit stereo ADC then the rest of the receiver is done in Digital Signal Processing in the
microcontroller – a Software Defined Radio (SDR). The SDR implements a superhet with 12kHz
IF. Like any superhet, this means that there is an image response on the other side of the IF
frequency. In this case, that means the image response is 24kHz down the band. The amount of
the image rejection depends on:
Amplitude balance between the I and Q channels
•
Any phase error in the 90-degree quadrature relationship
•
In QMX to date, no attempt is made to compensate for these errors. The image rejection has been
found to be adequate without it. A future firmware version will likely include automatic adjustments
to improve image rejection.
In the image rejection screen, the VFO to the band center frequency as defined in the Band
Configuration screen, then sweeps the signal generator input from -30.5kHz (below) to +7kHz
(above). An example of the resulting sweep is shown below:
QMX operating manual; firmware 1_00_012
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