A simple DDS for 136kHz

Posted on Feb 7, 2014

The Hex switches are connected to J2 as follows: pin 1=common of both switches (+5V) 2=LSB up to 5=MSB of first switch. Then 6=LSB of second switch up to 9 which is MSB. I modified the oscillator slightly to get my cheap Farnell crystal to start reliably (Philips 74HC00 chip). Increase R2 to 1 Meg-Ohm add another C of about 200pF from pins 9, 10, 11

A simple DDS for 136kHz
Click here to download the full size of the above Circuit.

to earth. Ensure you decouple the VCO (4046) chip properly and choose a 5V regulator with a slightly high output (it MUST be fed from a regulated supply), it didn`t seem to like 4. 9V! The other chips will stand up to 6V. You will need a buffer stage to level-shift up to 12V if you want to drive the G3YXM TX with it. I used a single common-emitter NPN stage; 10k from input to base, 4k7 from base to ground, 1k from collector to 12V, emitter grounded. The output comes from the collector. This isn`t great but it will drive down a metre of coax!

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