impedance measurement

[epilot]

Hello.

need to know the impedance of my T&R ultrasonic transducers,
any method to measure it?

It seems I am not able to measure their resistance with an ohmmeter!


By the way any method or formula to measure the beam seperation too?

thanks a bunch

 

[Nigel Goodwin]

Signal generator, scope (or AC millivoltmeter - NOT a multimeter), and a resistor - and apply ohms law!.

Be aware that the transmitter is series resonant, and the receiver parallel resonant.

 

[epilot]

Signal generator, scope (or AC millivoltmeter - NOT a multimeter), and a resistor - and apply ohms law!.

Be aware that the transmitter is series resonant, and the receiver parallel resonant.

Thanks Nigel,
Yes you are right.
But won't you like to talk more about the trick?!

 

[Nigel Goodwin]

Place resistor between signal generator and transducter, use scope to monitor signal level across transducer. For transmitter adjust frequency for minimum signal. Measure signal BOTH sides of resistor, you can then work out the current through it, and from that the impedance of the transducer - simple ohms law.

Do the same with the receiver, but adjust for maximum, and use a much higher value resistor - to be honest, there's no real need to measure the receiver!.

 

[epilot]

Place resistor between signal generator and transducter, use scope to monitor signal level across transducer. For transmitter adjust frequency for minimum signal. Measure signal BOTH sides of resistor, you can then work out the current through it, and from that the impedance of the transducer - simple ohms law.

Do the same with the receiver, but adjust for maximum, and use a much higher value resistor - to be honest, there's no real need to measure the receiver!.

I am not able to produce a sine wave at the moment,my oscillator can generates a square wave, so how to CALCULATE the current of an square wave?(I am not sure if my multimeter has this ability)

 

[Nigel Goodwin]

I am not able to produce a sine wave at the moment,my oscillator can generates a square wave, so how to CALCULATE the current of an square wave?(I am not sure if my multimeter has this ability)

The waveform doesn't matter very much - you're not looking for an accurate measurement of the current, or the voltage - just two voltage readings to calculate the current through it. Essentially all you're doing is comparing the impedance of the transducer with that of the resistor. Personally I would use a scope (because I have one) - but, like I said above, a multimeter ISN'T suitable, they only rarely work at the frequencies required.

Use a scope or AC milli-voltameter, and NOT a multimeter.

 

[audioguru]

A multimeter is made to measure the voltage at the mains frequency accurately (50Hz and 60Hz). The response at 1kHz and higher will probably be attenuated differently on each scale. Very few multimeters can measure a voltage at 40kHz.

Your ultrasonic transducers are probably piezo. A piezo transducer is a capacitor that bends its plates when a voltage is applied. Most piezo transducers have resonant frequencies when they are very sensitive and other frequencies where they barely work. Their impedance will vary with frequency the same.

 

[epilot]

Nigel,
See if I am right?:
At first I connect a resistor in series with thransducer while they are connected to a 40khz square wave oscillator.
Then I must measure the voltage across the resistor and then i'll have the cureent of the circuit with ohm's law,
again I measure the voltage across the transducer and divide this voltage by the current to obtain the impedance ,yes?

2 questions:
What is the reason of adjusting the freq to the minimum for the transmitter and max for the receiver?

Why I must consider this:
transmitter is series resonant, and the receiver parallel resonant. ?

 

[Nigel Goodwin]

Nigel,
See if I am right?:
At first I connect a resistor in series with thransducer while they are connected to a 40khz square wave oscillator.
Then I must measure the voltage across the resistor and then i'll have the cureent of the circuit with ohm's law,
again I measure the voltage across the transducer and divide this voltage by the current to obtain the impedance ,yes?

Measure the voltage either side of the resistor, that gives both readings you need - you can then either calculate the impedance by the relationship between the two readings, or use ohms law to calculate the current, then the impedance.

2 questions:
What is the reason of adjusting the freq to the minimum for the transmitter and max for the receiver?

Because that's where you have to measure it - if you don't measure at resonance you're completely wasting your time!.

Why I must consider this:
transmitter is series resonant, and the receiver parallel resonant. ?

As I said in the measuring process, a series resonant circuit resonates at MINIMUM and a parallel one at MAXIMUM - you need to adjust for those two points, they will probably be very sharply defined!.

 

[audioguru]

The datasheets for the ultrasonic transducers will have their impedance listed and how irregular and narrowband is their response. If you don't have their datasheets then the transducers might not be able to do what you want (wideband FM?).