# building a sine wave oscillator

Discussion in 'Electronic Design' started by Joe, Mar 6, 2008.

1. ### JoeGuest

Hello to the group,

I am looking for some ideas on how to build a 24khz sine wave
generator to drive an ultrasonic piezo transducer.

Or, if someone knows of a way that I can change a square wave from a
555 to a sine wave, that would work too.

I have googled and found something called a wien oscillator, which
uses a light bulb as a non linear element. I am just wondering if
anyone knows of a way to do it without having to use a light bulb.

Any ideas, suggestions, links greatly appreciated.

TIA,

Joe

2. ### John PopelishGuest

The harmonics of the 24kHz do little except draw some
capacitive current. You can limit them with a series
resistor, or resonate the piezo with a series inductor and
really reduce the harmonics. I doubt there is a good reason
to produce a pure sine wave. A 555 timer produced square
wave may be good enough.

What voltage do you need driving the piezo?

What is the piezo capacitance?

3. ### AlGuest

Check this out for a phase shift osc.

http://en.wikipedia.org/wiki/Phase-shift_oscillator

That will be your starting point.

Al

5. ### D from BCGuest

I have an offbeat idea..

Use a computer sound card with 2 tone into a frequency mixer.
http://en.wikipedia.org/wiki/Frequency_mixer
Say..
10khz + 11 khz for 21khz
Perhaps use a multiplier chip..
Then filter everything below 21khz.

D from BC
British Columbia

6. ### Phil AllisonGuest

"John Fields"
Joe

** Betcha it's a " woofer stopper ".

....... Phil

7. ### CharlesGuest

The Wien bridge oscillator can be amplitude/gain limited using a diode/FET
combination to dynamically adjust the loop gain.

However, a square wave might be OK. Also, a low-pass filter might be OK if
the harmonics are going to cause problems for your application.

8. ### Guest

If you can live with low level harmonics, you could use a Baxandall
class-D oscillator, which has the advantage of being remarkably
efficient, if your piezoelectric transducer looks more like a
capacitor than a resistor (that is lightly damped rather than heavily
damped by medium into which you are driving your ultrasound).

I posted a discussion of the circuit on my web-site some time ago

http://home.planet.nl/~sloma000/Baxandall parallel-resonant Class-D oscillator1.htm

10. ### Fred BloggsGuest

I think the industry has since advanced to Class E operation, same idea
but with much better efficiency:
http://www.freepatentsonline.com/5179511.pdf
The OP is clearly musing over a from_scratch build of an ultrasonic
range finder, he would be better off acquiring a driver on the surplus
market.

11. ### JoeGuest

Hi John,

The piezo that I am working with is an "in air", ie, it is built for
in the air use. No specs as it was a bargain from Allelectronics.
Eventually, though, I will be working with an underwater 24khz
transducer (PZT) to transmit a 24khz carrier and digital modulation
across the depths of Boston harbor. I am practicing with the "in air"
model. You are right, a square wave from the 555 works OK, but my
understanding of piezo is that it expands and contracts as the
electric field across it changes, so to maximize the performance, a
sine wave would probably be better. The voltage I am using right now
is 12Vpk, but I know it can go up to 20Vrms, as most of them can. I
know because I tried using sine waves from my function generator, and
a power tranny with a cap (trial and error for the best output
signal), and when I cranked the supply up just over 28Vpk, it made a
popping sound, and some smoke was let out, and then it didn't work
anymore. I have no specs yet on the underwater transducer that I will
be using as I have not decided which company to go with. They are
difficult to find. Airmar makes them, and a company called sensortech
up in Canada makes them also.

12. ### JoeGuest

Hello John,

Right now, I am practicing, Eventually, this will be used in an
underwater acoustic modem.

Joe

13. ### JoeGuest

Hello D,

Actually, I am using the mpy634 multiplier chip (Burr Brown, now TI)
and a 555 generated carrier, and the modulation is coming from a
function generator meant to simulate audio data.
This will have to be a portable application eventually, and the
underwater transducers normally dissipate a couple of hundred watts
(low duty cycle though). It works ok, but I would like to maximize the
performance of the eventual instrument.

Joe

14. ### John PopelishGuest

Then you can lock the 555 very close to the LC resonance by
taping the piezo voltage with a high impedance RC low pass
filter (say, 100k, 100pF) and connect the capacitor voltage
to trigger and threshold. Pick the inductor to set the
frequency. Something around a few milli henries should work.

15. ### John PopelishGuest

I think you will find that the at resonance properties of a
piezo in air will be quite different than one in water.
The corners of the square wave require a lot of current from
the driver. Adding a series inductor reduces those corner
currents. Adding a resonant inductor, so that the
electrical resonance of the inductor and the capacitance of
the piezo approximately matches the drive frequency,
effectively removes most of the harmonic current from the drive.
The series resonating inductor will make the piezo voltage
higher than the square wave voltage.
Expect to have to make significant design changes when you
go from piezo in air to piezo in water. I think a piezo in
water has a significantly less mechanically resonant
operation, since water is a much better mechanical impedance
to the element, so it carries energy away from it more
efficiently. To get a similar electrical resonance, you may
need to parallel it with a good quality capacitor and lower
the inductance to see similar electrical resonant Q (at a
higher drive current).

16. ### VWWallGuest

The main problem will be driving whatever transducer you select. You
will need to match the transducer to the driving amplifier. For maximum
efficiency, the driving impedance needs to be the conjugate of the load.

You will need to measure the transducer characteristics under water and
driven at the carrier frequency and waveform you will use.

17. ### Guest

And a much better chance of melting the switching transistor. "Much
better efficiency" is an intersting claim - the Baxandall circuit
typically offers better than 90% efficiency - Jim Williams claims 92%
in

A resonant driver isn't all that appropriate in that context.

18. ### JoeGuest

Hi John,

Well, sheepishly, I must say, I screwed up my algebra again. The
capacitance is looking like around 82pf at 24khz.

Joe

19. ### JoeGuest

LOL, Joseph, so you caught my algebra goofup too. There is no excuse,
I know, so I am not going to try and make any.
I will not use a light bulb. There are other nonlinear elements I can
use. I DO know the principles that the wien oscillator works by.
Also, there ARE other oscillators I can use, which I am just now
discovering

Thank you for pointing this out.

Joe

20. ### JoeGuest

Dear M...

I already have a hydrophone. That is what I am going to use to receive
the 24khz. It will receive or transmit up to 25khz, the mfr says to
stay away from 25khz if transmitting, but I can receive at any
frequency within its range with no problem. Been there, done that.

As far as research, you might want to do a little yourself, start
here: http://www.massa.com/fundamentals.htm

The what you call 'loss' is proportional to 1/r^2, just like em waves.
Because of the spherical or hemispherical nature of the wave
propagating thru the water. There are also absorption losses due to
the manganese dioxide present in sea water. At 20khz, this amounts to
-3dB in a kiloyard (I know, unfortunately some folks still use the
imperial system). Now the 1/r^2 I am speaking of is the acoustic
energy flux density (aka intensity), NOT the sound pressure level. The
sound pressure level falls off as proportional to 1/r. I would refer
you to a great book on this very subject:
"Principles of Underwater Sound" by Robert Urick <sp>, and also
"Transducers and Arrays for Underwater Sound", by Sherman and Butler.
You can take a quick peek at them at amazon.

I know a little something about the physics of sound propagation in
different media.

My original question was about building a sine wave oscillator, which
has been answered, and I wish to thank everyone who replied. I know
what I am up against as far as sending sound through the ocean. I am
trying to design with those principles in mind, hence, the reason I
wanted to maximize the output from my multiplier.

Electronics has been a hobby for me, so that is why I come here and
ask questions on occasion. Most of the time, I just lurk and learn.
Oh, and screw up algebra.

Thanks again to all who replied. I believe I can make my way from
here.

Joe