I think perhaps instead of DAC (Digital to Analog Converter) you mean
DDS (Direct Digital Synthesizer). I say this because varying the clock
of a DAC does not necessarily vary the frequency of the signal it
outputs.

Sean


On Tue, Sep 1, 2009 at 2:38 AM, Marechiare<marechiare@gmail.com> wrote:
>>> ... generated by 100 mHz DAC.
>>
>> Huh? =A0100 millihertz is way way too low for the tiny systems the OP was
>> describing. =A0Think of the size of the transducers you'd need to get any
>> meaningful amount of air to move when one cycle takes 10 seconds.
>
> That was a typo, sorry. I meant that the DAC would be clocked at 100
> megahertz. And its output at, say, 1000 steps would be 100 kilohertz
> sine wave. At the sound path equal to 10 wave lengths the 10% change
> in the DAC frequency (and also in wave lengh) would produce full
> period phase shift on the received signal in regard to the reference
> signal.
>
> The change in the distance to the object of 0.1 wavelength would cause
> the phase shift 0.2 period. To get the phase shift back to full period
> he would change the number of clocks per DAC period by 20%, that is
> 200 steps. Thus the 1 step change in number of DAC clocks per period
> would correspond to 0.1 * 3mm / 200 =3D 0.0015mm change in distance.
>
> Also he can fix the number of DAC steps per period and vary the DAC
> clock frequency. What's better depends on the app specs.
>
> The idea, indeed, depends heavily on the speed of sound in the local
> air, but for measurements of low frequency variations of distance to
> some object it should work well, say for measurement of vibrations of
> concrete under heavy truck moving over it.
>
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