(Eric Thosteson) wrote:
> I am trying to find/design a driver circuit for use in a high
> frequency underwater ultrasonic transducer....The piezos I want to
> drive operate at 5 MHz  with an impedance around 5 ohms
> at that frequency. 

5 Ohms seems pretty low. Most transducers I have seen had some __ty
Ohms, but a few had internal matching networks. BTW, the actual 
resonance frequency may deviate +-10% from the nominal frequency 
for most heads and 10% away from resonance you get completely
different impedance values.

The transducers must have a quite good efficiency. If you measure
their S11 with a network analyzer you get a much smoother curve
if you press a finger against the active surface or if you wet the
surface with a drop of water.

> I would like to hit the crystals with
> around 100 volts with a short pulse width, around 10 
> microseconds.  

That's ok, but 10 usec seems unneccessarily long. It will 
probably be best to approximate a 5 MHz halfwave i.e.  100 ns.

> This poses an interesting problem,
> because the power requirements should be minimal.
> (100 volts across 5 ohms sounds large, but because of
> the small pulse width, the majority of the time, the
> circuit is idle.)  The precise pulse width provided by the
> circuit is of the utmost importance.
> I have seen the problem solved before by generating
> a 5 MHz waveform at low voltage, then using a hand
> wound toroid to bump up the voltage.  (Commercially
> available transformers with these demands seem
> to be nonexistant - but if anyone knows of any...)

Just take some Amidon cores and have someone wrap them.

The transformer itself is probably less of a problem than
the source that has to drive it. If you choose a voltage
step-up ratio os, say, 1:10,  the impedance that has to be
driven will sink to 5/3.3 Ohms.

> I would like to find a more elegant method of doing this,
> as hand wound toroids at this frequency have very
> few wraps, 

That's an advantage if you have to wrap them. :-)

> and bumping them totally changes their characteristics.

> If anyone can suggest an alternative, knows anyone
> who has attempted this, knows of a source for any

You may want to try an avalanche transistor. That's a 
normal low-power transistor, operated under unusual
conditions. (see the text books)

250 V (Yes!) --------------------*--------
                                 \  100K
                                 |     ----------------------
                                 |     ----------------------
            50pF   100R          C     |      some meters of
TTL-gate----||----\/\/----*----B      GND     coax cable
                          |      E
                          /      |
                    100R  \      |
                          /      |
                          \------*-------- out
                                 \  50R

(first time i used netscape as a CAD tool.)

The low cost 2N3904 is reported to work well as an avalanche
transistor. In the quiescent state, the collector will be at 
250 V and the coax line will be charged through the 100k resistor.
A rising edge at the ttl input will bring the transistor to 
immediate breakdown. The charge contained in the cable will
flow to the output. You will see there a positive pulse with 
150 V amplitude and a risetime of 1 ns. The pulse duration 
will be 2 times the propagation time through the coax cable.
When the cable has lost it's charge, the transistor will switch
off again and the cable will recharge slowly through the 100K-R.

If you don't need a rectangular waveform you can substitute a
capacitor for the coax cable. If you make the capacitor too
large you may burn the transistor.( > 5nF is risky.) The 
capacitor must have good rf properties.

There are way to cascade several avalanche transistors if you
want even more pulse power:

W. Pfeiffer: Erzeugung hoher Impulsspannungen mit 
Avalanche-Transistoren in Kaskadenschaltung
Internationale Elektronische Rundschau, November 1973
It's written in german, sorry.

Mister Pfeiffer produced pulses of upto 1.6 KV into a 125 Ohm load
and upto 4.2 KV into 20 pF. Risetime was abt. 5ns.

The pulse repetition rate is limited, but this won't hurt in
your application.


Date: 7 Jul 1996 17:38:40 GMT

Original Subject: Re: Ultrasonic transmitter

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