> I seriously doubt you're going to get 2.5um resolution from
> ultrasound. =A0The transducer alone will react slower than that.
> Even if you sent a continuous signal and measured phase
> relative to a reference, I doubt you would get that kind of
> accuracy. =A0Unless these are very very special ultrasound
> transducers operating at a unusually high frequency, this
> isn't going to work.

He didn't say the frequency is constant. Thus he could vary the
frequency of the sine during the mesarument, get the analog difference
signal, digitize it, and then using math, based on many points,
calculate the frequency of ZERO PHASE SHIFT.

Let's assume  the signal is 100 kHz sine (3 mm wave) generated by 100
mHz DAC. The frequency increment would be 1/1000 during the
mesarument.  This corresponds to 0.001 * 3 mm =3D 3 um path distance
change, or 1.5 um distance to the object change.

Zero phase shift detection could be done much easier than calculating
the phase shift. Just make the signals of equal amplitude (frequency
is the same already) and get them through some sort of heterodyne.

-- =

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