Mike said:

>Offhand I think of using two balanced demodulators, one inphase and the
>other quadrature.  The two DC ouptuts, which vary in cosine-sine fashion
>with the distance, then go to ADCs and standard resolver math.  This is
>standard radio hardware that isn't very expensive, though not rock-cheap
>either.  It would probably require factory calibration of several
>adjustments in each unit to reach one degree accuracy.
>
>Shifting the carrier frequency slightly and looking at the resulting
>phase change would give some idea of the coarse range.  It would take
>some more math to see if this is a viable technique to determine which
>cycle is in use.  I suspect that at long range (hundreds to thousands of
>3m cycles), a wide range of carrier frequencies would be needed.   The
>processing circuit may need to convert everything to an intermediate
>frequency so that the same phase detectors could be reused.


This type of circuit will work, but the errors are a pain. There isn't any
way to avoid the fundamental need for precision and stability in the
picosecond range, no matter how the circuitry is arranged. Constructing a
novel (or just different) circuit topology is, in fact, a transform of the
original requirement, and the limitations follow along to the new model.

There are attempts to do just what you describe every so often; the problem
is that the phase detector is not bad for decoding analog signals but gives
poor digital results. The numbers just don't get better than about +/- 0.25
inch over short times and short distances (say, less than 50 feet). Making
this type of circuit stable and accurate for measurement purposes is a
tricky proposition. Like I said before, the resulting box is usually
expensive.

There are several commercial laser ranging systems that exhibit about the
above mentioned stability and precision and cost anywhere from a few hundred
to several thousand dollars. Leica makes a neat one for about $2K. These
things are usually limited in distance unless a special target is used (or a
corner prism, as in surveying instruments).

--TR