building a sine wave oscillator

[Joe]

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

 

[John Popelish]

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.

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?

 

[Al]

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

Check this out for a phase shift osc.

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

That will be your starting point.

Al

 

[John Fields]

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.

 

[D from BC]

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

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
Canada.

 

[Phil Allison]

"John Fields"
Joe

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





....... Phil

 

[Charles]

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.

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.

 

[[email protected]]

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.

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

 

[John Fields]

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.

 

[Fred Bloggs]

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

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.

 

[Joe]

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?

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.

 

[Joe]

Hello John,

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

Joe

 

[Joe]

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
Canada.- Hide quoted text -

- Show quoted text -

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

 

[John Popelish]

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.

 

[John Popelish]

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.

I think you will find that the at resonance properties of a
piezo in air will be quite different than one in water.

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 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 voltage I am using right now
is 12Vpk, but I know it can go up to 20Vrms, as most of them can.

The series resonating inductor will make the piezo voltage
higher than the square wave voltage.

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.

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).

 

[VWWall]

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.

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.

 

[[email protected]]

I think the industry has since advanced to Class E operation, same idea
but with much better efficiency:http://www.freepatentsonline.com/5179511.pdf

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

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1033,P1093,D4154

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.

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

 

[Joe]

Hello John,

So today, because of the questions about capacitance, I rigged up a
sort of a voltage divider, with the 'top' resistor 5.07k and the lower
one (closest to ground) was the 'in air' transducer. I then applied a
24khz sine wave to the Z1 (the input resistor), double checked the
amplitude was 6Vpk. Now when I measured the Vout at the intersection
of the 2 impedances I measured 6.4Vpk.

I guess in terms the engineers use, I then applied the transfer
function (Vout/Vin)=Z2/(Z1+Z2). Using 5070 ohms for Z1, I came up with
a negative impedance (assuming this is all capacitive). To compute the
capacitance, I used Z2= 1/(wc), which gave me a negative 1.87 Farads!

I probly made a mistake somewhere, but I double and triple checked the
Vout was > Vin. From what I've read, (specifically in Horowitz and
Hill), this shouldn't be unless Z2 has a negative resistance, which to
me makes no sense. What did I do wrong? I understand that the
impedance of a capacitor is i/wc, (or j for the engineers), but from
my understanding of physics the i (or j) is imaginary and represents
phase information. Another thing I thought about was that since it is
at resonance, I should expect a higher voltage, but I am not too sure
about that. Can you clarify or point out what I did wrong? Oh, and I
got the same identical results whether I used a square or sine wave.

Yes, I know that working in the water is a whole different ballgame.
The acoustic impedance of water is about 2 orders of magnitude greater
than in air.

Joe- Hide quoted text -

- Show quoted text -

Hi John,

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

Joe

 

[Joe]

When you can do the math to explain how to select the light bulb, you
won't have any problem designing your oscillator.- Hide quoted text -

- Show quoted text -

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

 

[Joe]

What you want is a hydrophonehttp://en.wikipedia.org/wiki/Hydrophone

You should do a bit more research before start this project. For
practical audio frequencies, the attenuation of air is uniform with
frequency. My recollection is water falls off at either 6dB or 12 dB
per octave. So the loss at 24KHz will be severe.- Hide quoted text -

- Show quoted text -

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