Re: PIC + DDS = Frequency Synthesizer?

From: Robert Gush (robert_at_suesound.co.za)
Date: 01/03/04 

Date: 3 Jan 2004 02:38:49 -0800

russ@sotainstruments.com (RussT) wrote in message news:<e5f6ed39.0401022012.3972ddfb@posting.google.com>...
> Thad Smith <thadsmith@acm.org> wrote in message news:<3FF5F65E.83A97FEF@acm.org>...
> > RussT wrote:
> > >
> > > Thad Smith <ThadSmith@acm.org> wrote in message news:<3FF4DCD9.BFC46FC4@acm.org>...
> > > >
> > Just set one output line at a time, looping through all the bits you
> > want to send.
> > In general, make sure that you program all the required registers.
>
> ***That should work. I will roll up my sleeves and see if I can get
> the DDS to accept some data.
>
> >
> > > Let's say I use a 4MHz oscillator on the DDS,
> > > and want a maximum 50kHz output.
> >
> > That's the third number I have seen for the upper frequency limit.
> > Sounds like a little stability in requirements is called for. ;-)
>
> ***Yup. Sorry, but I got thinking that DDS opens the doors so much I
> felt I could get even higher frequencies without a problem. I
> originally thought 20kHz would be good, but when I see DDS chips
> generating far higher...I got hungry for more. It is more of a
> convenience thing to get the higher frequencies. The application is to
> pulse a bank of LEDs at high rates. The higher I can pulse them, the
> more experimentation can be done. The LEDs are going to be used in a
> study to determine cellular response (ATP production) of tissue cells
> when subjected to a pulsed monochromatic light source. I just want to
> be able to vary the frequency in 1Hz steps up as high as I can. 20kHz
> is great, but if I can get more that's even better.
>
I think (I don't know the application) that 1Hz resolution at 20 Khz
would be overkill. Maybe if you specified the resolution in terms of
percentage of the actual frequency (say 1%, ie 200Hz at 20Khz) then it
would be fairly easy to code using the PWM generator or even in
interrupts.
If the PIC is running at 20MHz then you have 5 mips or so or a 100
counts on one of the timers at the rate of 50 K timeouts per second.
It would be easy to get 25KHz with a resolution of on percent.

Regards
Robert

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