GenesisRadio G3020 Xcvr (BETA) 05_RX Mixers/OpAmps

Image Adjustment

Test Setup

At this stage it is absolutely essential to ensure that both I and Q channels are working properly and image rejection is adjusted (-40dB or better).

"Images" are a common artifact of the Direct Conversion receiver which is the basis for this SDR. They are encountered in the display when:

• the phasing of the I and Q signals is not just right; OR
• when only one of the two signals is present, either because of a bad connection or a mono soundcard input).

Images are pretty obvious on the spectrum display of the SDR, looking like mirror inage signals on either side of the center frequency. One of these is the desired signal; the other is an image that should be rejected. For example, if the center frequency is 10.125 MHz and your desired signal is 10.135 MHz (10 kHz above the center "zero"), then there may be a mirror image of that signal at 10.115 kHz (10 kHz below the center "zero"). Adjusting for image rejection causes these unwanted images to disappear down into the noise level.

To begin, Download and setup Winrad to serve as a SDR receiving program (you may prefer, instead, to use the GSDR program - Hans, LA2MOA's setup instructions are here.

Image adjustment will require a running SDR program tuned to one of the center frequencies and a signal source that is slightly above or below that center frequency. For this example, we will use the 30m center frequency of 10.125 MHz and a signal source of 10.115 MHz (10 kHz below the center). Preston, WJ2V, has found that if the test signal was too large, it was picked up in the unshielded G3020, and it would not suppress! Use a small signal to do these tests. When Preston got the signal down to proper levels for receive testing, the supression worked like magic.

Adjustment Procedure

The G3020 has four pots that must be adjusted to balance the audio lines for proper operation.

Set Up Sound Card

• Connect line In of PC to the "To PC Soundcard" jack on the G3020 board
• Do not connect the soundcard's line out to the G3020 yet

Signal Source

You will need a source of low signal in either the 30 or 20 meter bands. You must keep the signal level quite low or the Genesis will be overloaded, and you won’t see the nulls you are looking for. Builders have reported inability to find a null when the signal strength is too high. For example, you may have trouble with a standard 100 watt rig connected to a dummy load—the escaped signal may still be pretty high. If necessary, move your G3020 away from the signal source. Of course, a signal generator would be ideal, but it isn’t necessary.

Adjust Carrier Trim Pots

Now, look at the above graphic. If you printed this out in advance, it will be easier than trying to look at your computer while you work. Note there are two 5k (Y503) trim pots labeled "TX Carrier Suppression". These pots must be pre-adjusted. Locate them on your board, so you are oriented.

• Power the board so you can make some voltage measurements with respect to ground (WRTG)
• You will find that the trimmer pot at the top of your board has a 1K resistor right below it. The right side of this resistor (red dot) is connected to the wiper of this trimmer. Adjust that trim pot to exactly 2.50 volts (WRTG) with power on the board.
• Now go to the second Carrier Null trimmer. The wiper of this trimmer is connected to the left side (red dot) of the 1K resistor just to the right of the trimmer. Adjust this second trimmer for 2.50 volt (WRTG) as well.
• Note that although these two pots are ultimately used to null out the carrier, they must be very close to equal, or you will not be able to suppress the image in the receiver.

RX Image Adjustment

Next trimmer is the 500 ohm (Y502) receiver image adjustment trimmer, for which you will need a small signal. Staying with our 30m example, your G3020 uses a local oscillator at 10.125 MHz. If you generate a signal at 10.115, which is 10K below the LO center, the image will be on 10.135, 10K above. If you generate a signal at 10.135, the image is on the other side, i.e. 10.115.

So, now look at your SDR software's spectrum display onm the PC and note the two signals, the correct signal, and the wrong side image.

Turn power OFF. Referring to the trimmer adjustment graphic find the receive image suppression pot. With power off, pre-set it so that the resistance across the 330 ohm resistor (to the right of the pot) is 150 ohms, as shown in the above graphic

Once your image suppression pot is pre-adjusted, you can carefully turn it until the image starts to drop. It should show at least 30dB, more likely 40dB of null. If the wrong signal is dropping, go in to the your SDR software's setup, and reverse the receive I/Q lines to correct this problem. Now, the image should be suppressed.

Turn the power on, fire up your SDR software for the G3020 and start up your signal source. Then, adjust the image reject trimmer while observing the "image" signal on the spectrum display. Refer to the procedure outlined for the G40 kit's image adjustment to see what the image rejection process looks like. You will be using different input frequency with your G3020, but the rest of the instructions apply. You can also get a very good idea of how this works by viewing the You Tube video by Josh, Vk2FJDX (fast forward to minute 4:40).

Also, you might find that the Genesis software is easier to use than others.

Watch One Work

Test Setup

Here is a Youtube video of Brian KJ4OOS' Genesis G3020 receiving SSB signals at the end of this Phase: