Ultrasonic transducer frequency responce - Page 1
Forum section: electronics / beginners

author: misho
author_rank: Newbie
timestamp: January 21, 2015, 07:54:44 pm
content: Hello guys,I am currently working on a school project regarding ultrasonic "triangulation". It is different from commercial ultrasonic sensors in the way that the transducer's location has to be determined via an array of microphones or other ultrasonic receivers, whose position is known. If anyone is curious the exact method is called multilateration. Basically I want to measure the phase shift of the signals coming from the different microphones to measure the time delay.The circuit works perfectly when the signal is fed with a FG. The problem comes either from the ultrasonic transmitter or the receiver. I am feeding the ultrasonic transmitter with a function generator. The settings are the following: AM with a sine carrier of 40kHz and a square modulator of 2.5kHz square wave and a modulation factor of 1.0. Vpp 10VIdeally I would like to receive the same shape signal but either the transmitter or the receiver is screwing things up. I have tried using a matching pair transmitter and receiver:http://nl.farnell.com/multicomp/mcusr10p40b07ro/receiver-38-3khz-10mm-plastic/dp/2362668?ost=2362668%E2%80%8Bhttp://nl.farnell.com/multicomp/mcust10p40b07ro/transmitter-40khz-10mm-plastic/dp/2362667?ost=%E2%80%8B2362667and I got the result you can see in the attached picture.The signal measured at the terminals of the transmitter is at the top. Signal is coming from the FG with the settings mentioned above. The signal from the receiver is seen at the bottom and as you can see is quite distorted.I have also tried using a different transmitter but with the same results.https://www.conrad.nl/nl/piezo-luidspreker-pct-geluidsontwikkeling-90-db-283-vrms-25-60-khz-inhoud-1-stuks-710991.htmlI have also tried using an electret microphone for a receiver but still the same (I dont have a link for the microphone, it was generic)I have tried terminating the receiver with different value resistors (from 50 ohm to several kilo ohm) with no improvement whatsoever.The transmitter and receiver were spaced apart about 15cm.Is anyone familiar with this phenomenon and how to eliminate it? Does it have anything to do with the nature of ultrasonic transducers or is it some accoustic phenomenon?If you have a solution to this problem I would really appreciate it!!!Thank you

author: dmills
author_rank: Super Contributor
timestamp: January 21, 2015, 08:40:33 pm
content: There is something funky going on there as your received carrier appears to be about twice the frequency of the transmitted one.....Fix tis first, then address the other problems:You are running a ~10 cycle burst at @40KHz, so will need at least a few KHz of transmit bandwidth, very unlikely with a thin disk in air (And tricky in water), try sweeping the frequency and plotting either a received signal strength Vs frequency or (better) the transmitter impedance Vs frequency, I suspect you find it is horribly narrow band.Personally were I trying this I would go for a CW tone modulation scheme at maybe a few hundred Hz then do synchronous demodulation of the resultant am at each receiver, then some phase measurements and linear algebra.Reflections will however kill you with this scheme where a leading edge scheme is somewhat immune (But a much bigger ask of the transmitter).Regards, Dan.