Piezo disc impedance matching - Page 1
Forum section: electronics / projects

author: dimbmw
author_rank: Regular Contributor
timestamp: June 14, 2020, 09:36:43 pm
content: HelloI am building a piezo transducer and have some questions on matching.I am using radial resonance of the pzt-5a disc.There are resonance and antiresonance frequencies.I use the same piezo for transmit and receive. I use burst excitation. It is a range measuring application.Max response frequency is somewhere between resonance and antiresonance frequencies.Are there any hints on what is the best way to do impedance matching and if it is useful at all if i am not planning on using long cables.I tried to do the matching by using series inductor and parallel capacitor (calculated via smith charts) but wasn’t impressed with improvement of pulse echo response. Also it is hard to match without using adjustable inductors and capacitors as this thing is pretty sensitive to particular values... it will be nearly impossible to use standard L and C values in production.Does anyone have similar experience and some hints?Thanks!

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2020, 11:21:05 pm
content: you tend to use impedance analyzers for piezos devices, I don't think the VNA works well at their load impedance$$$Have you considered using tunable inductors and capacitors to at least determine what the optimum effect is? What do your values required look like? Not many people will want to get a impedance analyzer. I don't think its very useful or standard.

author: dimbmw
author_rank: Regular Contributor
timestamp: June 15, 2020, 12:54:48 am
content: Quote from: grouchobyte on June 14, 2020, 11:18:57 pmBob,I am using impedance analyzer in Analog Discovery 2, it is good enough for my frequencies.For driving, i use siglent sdg1025 generator, it has burst mode i need.There is no datasheet for the piezo transducer, as I am the one who tries to design it, so the datasheet should follow, lolProblem is - matching does not seem to bring any significant improvement in perfomance. I am puzzled with that actually.

author: dimbmw
author_rank: Regular Contributor
timestamp: June 15, 2020, 01:01:43 am
content: Quote from: coppercone2 on June 14, 2020, 11:21:05 pmyea, i use impedance analyzer in analog discovery, and then throw Rs and Xs values (they are like Rs=500ish ohm, and Xs =-750ish ohm) , throw them in SimSmith software where I get values of L and C for matching network. For my particular case I am getting about 300 uH series inductor and about 3nF parallel capacitor... but even slight changes do change the tuneup dramatically..... so not a great setup overall. i bought some variable Ls and Cs on digikey, but it isn’t practical for production anyways, hence my post here.... maybe I need something completely different like matching transformer? feel like brain stuck

author: coppercone2
author_rank: Super Contributor
timestamp: June 15, 2020, 01:19:27 am
content: well you use the variable parts to see what matching actually does, then if you like it you figure out how to match it cheaply (i.e. parallel/series components)How are you measuring performance of the device during match tuning? I suspect your IA will have a difficult time (not sure how well it can mimic a real Keysight IA).You should be able to produce a graph of output power vs tuning some howI do see values of nF for capacitor, but I don't know about series L. I only worked with one piezo device and I never finished the project.Not sure if you want to say tuning/adjustment is bad for production, its best avoided.. but it might be unavoidable. Plenty of good profitable things get adjusted. You need to test products before sale typically, especially if they do a process (you can't just plug it into a card, you need to make sure the transducer works and is in spec), so having the test technician trim something is not outlandish.I assume you will have to test a glue bond to the device being stimulated anyway. Don't pull a harbor freight and sell a ultra sonic cleaner which has the horn fall off and melt the chassis and try to light tables on fire..............

author: dmendesf
author_rank: Frequent Contributor
timestamp: June 15, 2020, 03:38:41 am
content: First let's remember what impedance matching brings to us: when a source is matched to a receiver then you have the biggest transfer of POWER possible (and it's 50%). Question is: you don't want the power. You want the signal. So you need to maximize the signal to noise ratio, not the power coupled.

author: dimbmw
author_rank: Regular Contributor
timestamp: June 15, 2020, 06:03:29 am
content: Quote from: dmendesf on June 15, 2020, 03:38:41 amI need maximum response (transmit->reflection from target->receive). I guess you can say that it's better to have max power.On a side note, I don't think it has to be 50%. I can have the output impedance lower than 50 ohm, let's say 10 ohm, and can have better %.

author: dmendesf
author_rank: Frequent Contributor
timestamp: June 15, 2020, 01:15:24 pm
content: Power is V*I. If you put a voltage buffer at the sensor output then as long as you have enough current to drive the resistors and Op Amp input then you want to maximize the voltage, not the product V*I, so perfect power matching isn't what you need.

author: dmendesf
author_rank: Frequent Contributor
timestamp: June 15, 2020, 01:18:23 pm
content: Ok I think I got it: I'm talking about the receiver, you're talking about the transmitter. For transmission you want good matching, for receiving it's not important.

author: dimbmw
author_rank: Regular Contributor
timestamp: June 15, 2020, 08:20:27 pm
content: Quote from: dmendesf on June 15, 2020, 01:18:23 pmIn my case the same transducer is transmitting and receiving, it is a pulse - echo setup.