Check out section 2.2 of the article "The production of acoustic impulses in
air" by Ian Marshall, published in the J. Phys. E (European Journal of
Physics E, Measurement Science and Technology 1 1990 413-418). This
describes a "step" method of driving a ringing transducer, which is what you
are trying to describe in your post.

--Tom Rogers  VP-R&D  Time Tech Inc.


-----Original Message-----
From: Robert Nansel <bnansel@NAUTICOM.NET>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Wednesday, November 12, 1997 12:26 AM
Subject: Re: Serial Sonar Unit (SSU)


>Some thoughts on the analog side:
>
>Use a transformer to match impedances, for sure. Also, what I've seen of
>ultrasonic transducers, especially the piezo variety, is that they are such
>high Q resonators that you don't even really need to drive them with an
>accurate frequency. A friend of mine in the Seattle Robotics Society built
>a system where just hitting the transducer with a pulse was enough to
>elicit an ultrasonic pulse (much like ringing a bell with a hammer tap).
>He used three op amps (actually, a TL082 dual op amp and an LM311
>comparator) for the receive function, if I recall correctly. It didn't use
>AGC, but had a very simple timed gain control, so that the longer you
>waited for the pulse to return (and therefor the weaker the pulse you are
>looking for), the more gain. I can dig up the schematics if anyone is
>interested.
>
>One thing that always intrigued me about Keith's design was how simple it
>was: he used just a transistor switch (TIP122, darlington configuration), a
>few resistors & caps, and a 1:5 impedance matching transformer (a TK2002, I
>think) to drive the transducer. There was a 1500 pF cap in parallel with
>the transmit transducer to make it behave like a resonant tank circuit.
>
>I remember Keith told me that he could watch the transducer ring decay on
>his scope; the decay period was long enough that it was this that limited
>the close-in range. It occurred to me that, since this design was entirely
>software driven (with some artful analog electronics where needed), that if
>you paid very careful attention to the frequency and phase of the forcing
>function that it should be possible to get both a good loud chirp _and_ a
>quick decay time. It would go something like this:
>
>        1) Bang on the transducer for N pulses, where N < 10, with a pulse
>           frequency selected to match the 40 KHz center frequency.
>
>        2) Wait an integral number of cycles (however long you want the
>           chirp to be) plus 1/2 cycle.
>
>        3) Bang on the transducer again for M pulses (M<N), but now you
>           are 180 degrees out of phase with the transducer oscillation,
>           so this has the effect of _damping_ the oscillation, thus
>           giving a cleaner and quicker pulse.
>
>
>Now, I don't know of anybody who's tried this, but it sure sounds worth a
>try. Thoughts?
>
>--BN
>
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