In the first scenario, the LO is set at 4745MHz, so the radio can select a channel at the 5745MHz RF carrier. This is called low side injection since the LO is a lower frequency than the desired carrier. The mixer output is a sum and difference of the RFIN and LO frequencies. The sum of the two frequencies is 10,490MHz. The IF filter is centered at 1000MHz. The 10,490MHz output is attenuated by the IF filter and is not passed to the VGA and ADC. The difference frequency is 1000MHz. The 1000MHz output is not attenuated by the IF filter so the ADC can capture the 20MHz wide channel present at 1000 MHz IF.