This is my first attempt at Direct Digital Synthesis.  What I want to do as the end result of all this is to use a cheap microprocessor and a R2R Ladder (network) to generate various tones into order to produce CW, DTMF, Packet (300 and 1200), RTTY and PSK31.

The hardest part in doing this is generating the tones.  For my experiments I am using an AT89S8252-24PC microprocessor with an 11.0592 MHz crystal.  The 8252 is a modern 8051 variant.  For digital to analog conversion I  am using a 4-bit R2R ladder network made from 1/8 watt 10.2 K-Ohm resistors, using two in series for the 2R rungs of the ladder network.

In this first step I am just trying to generate a 1,000 Hz sine wave using a 16-step sine wave table and the calculated phase increment value for a 1,000 Hz signal.

Here is a link to the assembler source code.  There is some unused code in the source code that I use in a lot of my programs.  Ignore it for now.  Below are the results of my first try at generating a sine wave.  The results are very encouraging.

(Click on an image for a larger view.)

Here is the 8252 and R2R ladder network on the proto board.

The 4-bit R2R ladder network put together from10-K Ohm resistors.

Here is a 1,000 Hz sine wave generated by an external oscillator and used for reference.

Another look at the 1,000 Hz sine wave generated by an external oscillator.

Here is a digitally produced 16-step 1,000 Hz sine wave.  You can clearly see the steps.

The same sine wave viewed with a different sweep time. It doesn't look too bad.

The waveform after adding a .01 uF capcitor after the R2R ladder network.

The waveform after adding a .01 uF capcitor after the R2R ladder network.

A view of my "workbench."

An overhead view of the protoboard.