Piezo element impedance matching - Page 1
Forum section: electronics / rf-microwave

author: tbw2
author_rank: Contributor
timestamp: December 05, 2023, 09:45:36 am
content: I'm trying to impedance match a PZT-5A transducer with a small PCB, and having no success. However, using the same value Axial leaded components, it works great.Tearing my hair out trying to understand the results, as they seem completely random and don't match simulated Smith charts.Info below - if i'm missing anything please let me know & I will update this post.Raw CrystalMatched with series 2.7uH and Q-damped with parallel 10RMatched with PCB using SMT 2.7uH and shunt 10R

author: JohnG
author_rank: Frequent Contributor
timestamp: December 06, 2023, 10:50:05 pm
content: I have worked on a couple circuits using a resonant piezoelectric actuator that operated at a few watts of real mechanical output power. The application was actually a synthetic jet fan.The first versions of these used a series inductor to cancel the capacitive reactance of the actuator. The challenge was that the actuator's resonant frequency and Q changed substantially as the drive was increased, so I measured it using an AP300 low frequency network analyzer. I used the analyzer's output to drive an amplifier capable of driving the actuator to full power, and measured voltage and current directly to compute impedance vs. frequency. I did this at several power levels over the range of operation, so I could see the impedance plot change with power level. Also, when the actuator ran at the higher power levels, it got warm and the impedance would also change. Fortunately, the Q got lower at high powers, so and inductive match got more broadband.In the end, I used a kind of PLL to feed an amplifier that drove the actuator at resonance, tracking the resonance over temperature and load. I still used an inductor that provided a better match when the power was low, and when the power increased, the reduction in Q meant that the impedance stayed closer enough to real that I could still get a good enough power flow into the circuit.Hope this helps,JohnI'm not sure this is the issue you are seeing, but it might be worth checking

author: tbw2
author_rank: Contributor
timestamp: December 08, 2023, 01:39:04 pm
content: Hi John,Thank you for the reply.I am curious now if my NanoVNA-H4 is unsuited to measuring the crystal. From your experience, I can assume that my low-power VNA is going to see a different impedance than the high-power UT Test rig that we use to benchmark the crystals, would you agree?

author: Marco
author_rank: Super Contributor
timestamp: December 08, 2023, 02:00:36 pm
content: Doesn't explain why your PCB is measuring a near short, the shunt seems like it's 0.1 instead of 10 Ohm. The antiresonance impedance for the raw crystal is awfully low.The Co seems huge, how large is that thing?

author: RoV
author_rank: Regular Contributor
timestamp: December 09, 2023, 10:34:46 pm
content: I find the graphs quite confusing.Is the Smith Chart display normalized respect to Z0=50 ohm? That would explain the 3rd chart (impedance close to real part 0.2, or 10 ohm, mostly due to the shunt resistor). It would also explain the effect of the 2.7 uH (67.8 ohm @ 4 MHz, or 1.35 in normalized units) in graphs 1&2, because the change in imaginary part from chart 1 to 2 is close to 1 in the inductive direction. However, I don't see at all the effect of the resistor in graph 2. In a case you say "parallel resistor", in the other "shunt resistor": what are the actual schematics in the two cases?So, please tell us more details: the reference impedance; the frequency sweep range; the schematics.You should also detail the kinds of inductors used in the two cases. I suspect the SMD inductor had an insufficient Q, causing a high series resistance.

author: mawyatt
author_rank: Super Contributor
timestamp: December 10, 2023, 12:42:19 am
content: Would think a modern DSO with Bode capability might help find the resonate areas.You could use a buffer amp like the inexpensive Junket DPA-2698 (25VPP 0.5A DC-10MHz) to boost the power/voltage from the AWG, and make a custom transformer if higher voltage is required.Best,

author: Marco
author_rank: Super Contributor
timestamp: December 10, 2023, 07:41:40 pm
content: Quote from: RoV on December 09, 2023, 10:34:46 pmNot unrealistic it seems,this paperhas an alternative way to measure impedance of piezo crystal and they use 4 MHz discs with ~0.2 Ohm impedance at resonance.BTW I don't have one, but doesn't Joe Smith's software have an impedance measurement mode for the NanoVNA? (Shunt mode, so not very precise but still easier to read than a Smith diagram.)

author: coppercone2
author_rank: Super Contributor
timestamp: December 11, 2023, 01:14:54 am
content: capacitor loads make me crywhy oh why must the amplifier die

author: JohnG
author_rank: Frequent Contributor
timestamp: December 11, 2023, 02:44:40 am
content: Quote from: tbw2 on December 08, 2023, 01:39:04 pmI can only speak from my limited experience with the piezo drivers I was using. They consumed a real power of a few watts, and the drive voltage at the piezo terminals was a sign wave of over 100V amplitude. The resonant frequency would change by several percent and the Q would drop by a factor of 5-10 compared to a signal of a few hundred mV. So, for that device, the small signal measurement did not match the large signal measurement. This was expected because the mechanical load was not linear. Sorry I don't have exact numbers, but the details are at my former employer, and it was more than a decade ago.I know next to nothing about your load, and am not an expert at piezo drivers, either. I've dealt with a fair number of non-linear loads for power circuits, though.John

author: tbw2
author_rank: Contributor
timestamp: January 03, 2024, 01:14:21 pm
content: Thank you everyone for the responses. Back on this project now and will hopefully make some more headway. Will try and answer the questions below.Quote from: RoV on December 09, 2023, 10:34:46 pmYes the graphs are normalized to Zo=50 Ohm, sorry for the confusion. Schematic attached.Previously used inductor datasheet: SC302R7KThttps://www.farnell.com/datasheets/1732078.pdfQmin = 37 @ 7.9MHzSMD inductor used now: NLV32T-2R7J-PFhttps://datasheet.lcsc.com/lcsc/2304140030_TDK-NLV32T-2R7J-PF_C89017.pdfQ=~35 at 4MHzQuote from: JohnG on December 11, 2023, 02:44:40 amI now have access to a HP4815A Vector Impedance Meter. I will compare readings with the NanoVNA, but I hope that we will see some less puzzling results.

author: RoV
author_rank: Regular Contributor
timestamp: January 04, 2024, 09:21:50 am
content: Quote from: tbw2 on January 03, 2024, 01:14:21 pmHi, the two inductors seem quite similar. By extrapolating behavior of the THT model @ 4 MHz I estimate, @ 4 MHz:Z = 3.6 + j68 ohm (THT)Z = 1.9 + j68 ohm (SMD)If the circuit is the same in both configurations, also the measurements of graphs 2 and 3 shouldn't be very different, unless you have significantly changed the placement of the piezo (e.g. in water vs. in air, or with/without strong reflectors).I would tend to believe that the 10R in parallel to the piezo should "kill" it and generally produce a graph similar to your 3rd. Is it possible that the 10R was not properly connected during the masurement for graph 2?You didn't clarify the frequency sweep range used for the graphs and if it was always the same.

author: coppercone2
author_rank: Super Contributor
timestamp: January 04, 2024, 10:15:12 am
content: i am curious about the application if you can say anythingthey are interesting but I don't know what I can try to make with one. ultrasonic soldering iron is probobly too hard and the cleaner already exists and seems good enough/reliable, and the cutter is a bit scary for me