As somebody else mentioned, a wave is a wave is a wave and the low frequency
of ultrasound makes it
much easier to work with and do neat stuff.

I presume that you are looking for an active sonar (both TX and RX) so one
thing you need to decide is are you going to form the phase array for both
TX and RX or only for RX. The RX side is easier since it does not involved
power and you can do everything with DSP. For example, some sonar systems
have at the very front an analog multiplexer that sample the hydrophones
signals (after the preamps) in a very high rate. This is then fed to a
digital processing unit that does the beam forming in all directions at the
same time using a single arithmetic unit. This way you don't need to have to
rotate the transducer array.

The beam forming itself is pretty simple, you take a bunch of hydrophone
signals, delay them proportionally to match the additional wave path in the
desired direction and then some them using predefined weights. The delay is
set such that when a signal arrives from the center direction of the beam,
the signal after the delays have 100% correlation. If you have the geometry
of the array it is pretty easy to calculate the gain of the bean in various
directions.

As for transmitting, the easiest is to send an omni directional signal. It
makes the power circuits simpler.

One of the things that you need to make sure is that you have a good
impedance between your circuits and the hydrophones and make sure to test it
in water since the impedance (if I recal correctly) changes from air to
water.

BTW, there is a cool sonar technique (don't know how it calls) that uses a
continious TX signal and provides a faster update rate. It transmitted
continuously an FM signal modulated by a slow saw tooth signal and
determined the distance of the target by the frequency difference between
the TX and RX signals. This allow to transmit continuously and provided
faster update rate. I don't know how this technique is called but probably
it has a name.

Another interesting 'sonar' systmes is a system that measures the sound
speed in the water. It has two small tranducers (one TX and one RX) and it
measure the time it take the signal to travel betwene them (you can use one
or two small 'mirrors' to extend the effective distance). The speed is
typically affected by physical properties of the water such as amount of
salt and temperture.


Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Brandon Fosdick
> Sent: Monday, June 03, 2002 12:18 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: Phased arrays, ultrasound etc.
>
>
> I'm looking for a new project and phased array antennas have
> always intriqued
> me. RF is a bit of a black art for me so I'm thinking about
> playing with sound
> instead. I have no idea what I'm going to make, this is still in
> the thinking
> stage. Maybe a locater beacon/tracker for model rockets.
>
> I understand the basics of phased arrays, but I'm not too clear
> on the details.
> Both google and amazon have lots of info on RF arrays, how much of that is
> applicable for sound? Since both sound and EM radiation are waves
> I would expect
> to find a lot of similarities. Some of the RF books on amazon are a bit
> expensive, and I'd hate to spend a lot of money on a book shelf
> ornament. Any
> recommendations?
>
> I did a quick Digikey search for microphones and everything that
> came up was
> limited to audible frequencies (<20kHz). Where do I find a good
> source of high
> frequency microphones/speakers?
>
> What frequency range should I be looking at? I have pets so I'd
> like to go high
> enough to avoid annoying them if possible. Any idea what range medical
> ultrasound devices use?
>
> I imagine I'm going to need some high speed ADC's. I've looked over the
> corresponding parts from TI and Maxim (any other manufacturers I
> should look
> at?) and there's a wide variety to choose from. What are ECL
> interface levels?
> I've never designed a digital circuit over 20MHz, any gotchas
> that I need to
> know about?
>
> --
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>
>

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