soldered, then you will be able to ensure perfect alignment. Use the eight 6mm M3 screws, and four
12mm nylon hex spacers, to bolt the PCB to the LCD.
The combined 16-way plug and socket when
mated together, have a height of a little under
12mm. Therefore it is necessary to have a
gap somewhere.
My suggested method is illustrated to the right
here. I suggest closely mating the plug and
socket. Leave the gap between the male header (plug) and the U3 PCB. I have found this method
to work successfully.
4.3 Module assembly
First the microcontroller IC1 should be inserted in its socket. Be sure to align the microcontroller chip
correctly, the dimple on the chip must be at the same end as the dimple on the socket, and the dimple on
the silkscreen legend on the PCB.
Next, plug together the three modules as shown in the following photographs.
Ensure that the DDS module is inserted the correct way round. The Low Pass Filter kit module should be
plugged in with the “Out” legend aligned next to the RF Output connector of the main PCB (see photo).
Important!
Upon power-up, you will need to adjust the contrast potentiometer R1 to view the LCD
properly. Turn it fully clockwise to start with (before applying power). Then turn it gradually anti-
clockwise until the displayed text looks correct.
4.4 Notes concerning the DDS module
a) You will notice that the DDS module has a square, blue plastic adjustment preset potentiometer. This
adjusts the threshold for the DDS chip comparator that turns its 1V p-p sine wave into a square wave to
drive the microcontroller. Adjusting it will change the duty cycle of the DDS square wave. You should not
need to adjust this potentiometer, as I have already adjusted it individually, when I tested every DDS
module before putting it in a kit. However, some builders have noted that the potentiometer needs to be
adjusted slightly for best output waveform on 10m (28MHz). The adjustments needed are extremely slight,
the potentiometer setting is very sensitive.
b) The module uses the AD9850 DDS chip. The module is officially rated for output from DC to 40MHz.
You may want to test it at higher frequencies than that. In theory the chip could output up to 62.5MHz (half
the 125MHz reference clock frequency), but the output amplitude and spectral purity will deteriorate.
8
