GENESIS G40: Assembly Help File - RX Image Rejection Adjustment (Receiver)

Start by powering up your SDR software. In this example we are using M0KGK. Alternatively for the purpose of image rejection adjustment programs like Power SDR SR40 or Winrad are equally suitable. Set LO or center frequency to 7,047 MHz. Do not connect G40 to sound card input yet! Note a strong 'spike' signal right on the 7.047 Mhz. This unwanted product 'signal' is generated by our sound card. In this particular case we are using computer integrated 48K sound card of rather poor quality for SDR purpose so the spike signal is relatively strong. A better sound card would produce lower amplitude spike - and high quality card would produce signal that is almost undetectable. Since centre frequency signal is unwanted property of your PC and sound card, image rejection adjustment will have no effect to it's amplitude.

Connect G40 to sound card input and connect 40m antenna. Tune your big HF radio to 7,057MHz. Connect dummy load, reduce output power to 1W and start transmitting.

Now observe the signal spectrum. Note 3 signals: centre frequency spike at 7047, siglanl genrated by your HF transmitter at 7057 and it's mirror reflection at 7037.

With image rejction adjustment, our aim is to reduce the amlitude of this unwanted IMAGE signal on 7037.

In build phase 3, trimer potenitometer Y501 (500 Ohm) was pre-adjusted to resistance of 150 Ohm. While observing the image signal, fine tune this trimer pot again until image signal is recuded for further 35-40dB.

In this particular case it was necesaary to turn trimer brass screw 3-4 turns. ( Post-adjustment resistance was measured and found to be 140 Ohm.)

Practically-acheivable RX image rejection of G40 is 35db to 55dB, typicall 40dB. Note: if during the image adjustment procedure (as described above) you've notice that instead of rejecting the image (7,037) you are actually rejecting the signal (7,057) then you have swapped I and Q channels. Open the audio plug which plugs into G40's RX connector (or LINE IN plug that goes into your sound card) and swap wires for L and R channels.

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