Using Ultrasonic Transducer on an Oscilloscope - Page 1
Forum section: electronics / projects

author: manytoolsmike
author_rank: Contributor
timestamp: April 24, 2023, 05:39:06 am
content: Im hoping to tap into the EEVblog Brain Trust here,I use "Acoustic Emissions" transducers to measure structure borne ultrasound in the 18 kHz to 50 kHz range. These are Piezo-electric transducers optimized for measure ultrasonic signals in metal or concrete structures. I connect the transducers directly to the scope and typically measure signals in the 100 mV to 50 V range. My concern is that I get different frequency responses depending on which scope I use; I suspect it is due to the high input impeadance of the scope. My Rigol is 1 Mohm (13 pF), our hand held Siglant is 1 Mohm (18 pF), I have guys in the field using the cheap DIY scopes.My question is would a lower impedance terminator give more consistant measurements across my instruments? The transducer is a "charge- scourse" and is very sensitive to loading. The high input impeadance of my scope results in very high voltages, but I worry that a 50 ohm terminator on the scope will squash the lower amplitude signals.your advise and experience is appreciated,Mike

author: Berni
author_rank: Super Contributor
timestamp: April 24, 2023, 06:10:59 am
content: Piezo transdicers produce charge, not voltage or current.The voltage you see is just the produced charge distributing itself over the circuit. So for this reason the impedance of whatever the piezo is driving is very important. Even the cable used to connect it could affect its response.Lower impedance helps make it more consistent since various parasitic impedance are a smaller contribution, but this does indeed wreck the low frequency response as the produced charge quickly leaks away into the load.The ideal thing here is to build a high impedance amplifier and place it as close to the transducer as possible. This lets you present an even higher impedance to the piezzo of >10MOhm while the output of the amplifier is strong enough to drive a wide range of impedances without affecting the signal.

author: Marco
author_rank: Super Contributor
timestamp: April 25, 2023, 06:37:52 am
content: AFAICS the standard AE amplifiers for passive sensors are capacitive dividers with the sensor capacitance at these frequencies (50 Meg input resistance, so cable capacitance will dominate at frequencies of interest).At the cost of a little sensitivity you can do the same with the oscilloscope, put say 220pf across the scope BNC connection. That should drown out both the effect of input resistance and input capacitance variation.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2023, 06:38:25 pm
content: Acousic emissions can produce quite strong signals - depending on that is done to the sample and how the sensor is coupled. I have once learned it the hard way and destroyed a sensor - at least the scope survived (maybe due to the tiny sensor, I may have had extra protection).Added capacitance to swap variations in the scope is likely a good idea. 220 pF would be some 2.2 m of normal coax.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 25, 2023, 07:16:11 pm
content: Piezo tranducers are usually very high impedance capacitive sources. The will be heavily loaded by the cable and the scope input capacitance and resistance. A low impedance load will swamp the cable and scope impedances and give a more repeatable signal at the cost of a significant reduction in signal amplitude. You don't want to go as low as 50 ohm, that will reduce your signal by several thousand and you will be swamped with noise. A better option is to use a pre-amp with a consistent impedance, ideally close to the piezo so you can reduce the cable capacitance.

author: manytoolsmike
author_rank: Contributor
timestamp: April 26, 2023, 04:25:06 am
content: Thanks guys, You have given me some good ideas to test out.I have a couple of different amplifiers/signal conditioners that I sourrced from the AE sensor manufacturer. (photo attached). These amps cant handle the high level signals I am measuring. The dynamic range of my signals is a beast (100 mV to 40 V). I will test the the 220 pF idea.Thanks again for the help.Mike

author: Someone
author_rank: Super Contributor
timestamp: April 26, 2023, 04:37:07 am
content: Quote from: manytoolsmike on April 26, 2023, 04:25:06 amIf your scope is relatively low noise, consider going the other direction and terminating the sensors into a lower impedance at their end (attenuate the signal) for coupling into the coax and onward to the scope. This is how scope probes have a smaller tip capacitance than the scope input its self.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2023, 08:56:36 am
content: A normal scope is not made for a large dynamic range. Because of the loading / clamping it may also be tricky to use 2 scope channels with different gain in parallel. The high gain channel could act as additional load and this way effect the other channel for the larger signal. It may still be good enough, as amplitude accuracy is usually not that important.The given frequency range is quite small. The interresting parts may happen at higher frequencies, possibly in the range of acousic resonances of the probe (e.g. the reflected wave coming back). So one may want to look at the acoustic signal also to higher frequencies to maybe the low MHz range (depending on the sample size).