Re: PIC + DDS = Frequency Synthesizer?

From: RussT (russ_at_sotainstruments.com)
Date: 01/04/04 

Date: 3 Jan 2004 18:57:55 -0800

jim granville <no.spam@designtools.co.nz> wrote in message news:<SQHJb.15615$ws.1692493@news02.tsnz.net>...
> RussT wrote:
> <snip>
> > ***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.
>
> Given the actual usage, you should allow for control of pulse width
> (LED On Time) as well as repetition rate. (not quite DDS anymore)
> Also, since the tissue is likely to be time-based, not frequency based,
> you might like to think in TIME rather than frequency.
> I would imagine 1Hz steps would be getting coarse, for slower tissue
> reactions - you are going to get 1000ms/500ms/250ms/125ms - better
> might be to run > 10Mhz, and define to 100ns (jitter free)
> ON time, and OFF time for your LEDS.
> Also LEDs have thermal effects, and some time-degrade, so depending on
> your target precision, you might like to add a (fast) light-sensor, to
> confirm the actual exposure given.
> Most uC can use capture/compare channels, to give 15-16 bits of time
> step. The better ones include prescalers on the capt/comp to allow very
> wide set ranges, close to 32 bits, but still with one part in 2^15-2^16
> region.
> If you need finer resolution than 2^15-2^16 region, or >> 10MHz
> then you are moving into CPLD/FPGA solutions.
> CPLD/FPGA could be designed for decimal setpoints, even from
> thumbwheels, if you want a PC-free setup :)
>
> Cygnal uC have very fast cores, and 25/50/100MHz clocks to the
> timers, so you could use those to push the uC solution.
>
> -jg

***Thanks jg, you've made some very interesting points. The current
research shows that at low enough frequencies from DC-5kHz, the
effects of the pulsed LED light (660nm and 880nm wavelengths) vary
quite a bit. DC seems to sedate, 73-584Hz appears to speed tissue
regeneration, and 4-5kHz are good for pain relief. I wanted to see if
there are other effects at much higher frequencies. Incidentally,
there is evidence showing that the cells can become acclimatized to
the stimulus. That might occur over time (dosage) or at certain higher
frequencies whereby the cell can no longer respond and sees the
stimulus as steady DC in a way. I built a prototype unit and treated a
rotator-cuff injury I have nursed for over 3 years. It has really
affected me - especially my golf swing! I tried 660nm at 584 Hz (a
bank of 60 x 3,000 mcd LEDs) for an abitrary treatment time of 20
minutes. To my utter amazement, I had nearly completely cured my
injury. I would say 95% positive effect. The range of motion is nearly
back to normal and the pain has all but vanished. I even have a better
golf swing. :>)
Russ :>)

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