Mini DDS circuit

Posted on Feb 6, 2014

A simple function generator. Just to generate a certain frequency. While waiting for the AD9832 chip to arrive, I came up with a very simple version of the DDS synth, using just the 2313 and a resistor network. It`s controlled over RS232 from a small Windows program, and can generate Sine-, Sawtooth-, Trangle- and Sqare-waves ranging from 0. 07 Hz to about 200-300 kHz in 0. 07 Hz

Mini DDS circuit
Click here to download the full size of the above Circuit.

steps (depending on your crystal). I wont go into details about how a DDS synth works. Maybe later. The code is pretty simple so you should be able to understand how it works by just reading through it. There`s not much to say about the schematic. It`s as simple as can be. Just 4 major parts. A voltage regulator/switch, a RS232 interface chip, the 2313 and the R2R resistor network. The R2R network is connected to PORTB on the 2313, making it a simple D/A converter and makes it possible to output 256 voltage levels. Neither the resistor network or port drivers of the 2313 is of perfect linearity, but it works pretty well anyway. But you`ll probably need a buffer stage as the output impedance is rather high (tens of kOhms in my case). The MAX603 handles the voltage regulation as well as the powerup/shutdown function, and is controlled by the DTR signal on the serial interface. So when you shut down the control program on the PC, the miniDDS will be shutdown, saving battery. The phase accumulator uses 24 bits, which determines the resolution of the output frequency. Maximum available frequency and resolution is also dependent on your crystal frequency :

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