Rick Dickinson wrote:

> Currently, I see three sections:
> - Analog circuitry (transducer drive and echo detection)
> - Ranging Unit (PIC program to pulse transducer & time return of
> pulse)
>   - Communications Unit (PIC program to handle serial comm/protocol)

  If, as I gather, the timing resolution is to be in the order of
milliseconds, or hundreds of microseconds, might it not be appropriate
to make the split between tasks at the point of timing; that is, one PIC
generates the pulses, controls receive gain ramping, correlates the
received signals and issues an "echo" pulse, while the other times the
echo pulse and formats serial data.

  The latter chip in this scheme would be very negotiable; MANY users
would choose to do their own timing.  It would simplify inter-chip
interfacing extremely; to two wires.  Protocol would go like:

 1) Co-ordinator waits till "receive" line is idle, asserts "send" line
to start a cycle.

 2) Ranging unit starts a send cycle and asserts RX line.

 3) On receiving echo, it clears RX line so that the pulse length on
this line is exactly the round trip time (with any corrections to make
this the case being the responsibility or the Ranging unit).  There are
possibilities here for multiple echoes to be measured.

 4) Clearing the TX line resets the Ranging unit.  Note that if a
further "ping" is requested before the Ranging unit is ready, it simply
declines to assert the RX line until it actually IS ready and has
started its "ping".

  AGC:  You may be aware that gain ramping (the exact term eludes me at
present) is an essential part of ultrasonic imaging, used perhaps most
extensively in medicine.  I suspect this is one of the design tricks of
the Polaroid system.  If you run at full gain to start, with or without
clipping, you are really likely to be troubled by ringing of the
transducer, mounting, etc.  And if you DO use gain ramping, you can
certainly consider a single transducer design.

  Most of the popular Doppler alarms seem to use rubber grommets to
mount the transducers anyway, presumably isolation is one reason.
Again, the main reason thy use two transducers is that the transmitter
is continuous.

  There must be a wealth of design information in the marine depth
sounder field.  They mostly use single transducers too, and the 4:1
scaling factor makes them very comparable to the presently considered
dimensions in air.  There are analog ICs designed for this purpose.

  The idea of damping or active damping of the transducer is
fascinating.  I suspect the marine ones probably do this too.

  Cosine error:  Probably not going to trouble anybody in practice.  For
close ranging you would have the transducers (all assuming you use two)
close anyway.  If you had to make corrections from a table and/ or
interpolation for these distances, it shouldn't be difficult.  I thought
the "ringing"/ gain consideration was more important.

  Cheers,
        Paul B.