[sdiy] Direct Digital Synthesis (was: integrator / capacitor leakage)

[ryan williams]

hi,

I have tried this method. I used two AD9850 DDS ICs to make a quadrature 
oscillator. You say that it is too expensive, but for me, it was free 
(samples). I had built a FM transmitter for school and we used these. I 
had some left over and really just wanted to see how it would work. My 
application is a frequency shifter anyway.

In school, I clocked the thing at 100MHz, and then used an image 
frequency to defeat the 1/2*Fs limit. For the quad. oscillator, I used a 
1MHz clock and control by an Atmel Atmega8 uC. There is a 16bit ADC for 
CV input. It seems to work ok, but there's $40 just in the DDS chips. 
The problem they have is that there is only 10bit DACs on them. You 
really pay for the high freqency. There are cheaper ones ($5), but they 
still have 10bit DACs. What kind of resolution is really needed for 
frequency shifting?. It runs fine down into LFO frequencies, and when 
the CV goes negative, I just use a negative tuning word to reverse the 
direction.

My plan is to build the rest of the frequency shifter and use this to 
test. In the end, I expect I'll build some new oscillators using an AVR 
+ DAC for the NCO. Atleast this way I can get more than 10bits.

-ryan williams