Stepdown transformer for signal generator? - Page 1
Forum section: electronics / projects

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 28, 2025, 07:39:26 pm
content: I'd like to drive a small capacitor, say 250 nF, at a low voltage at about 150 kHz. That's a Z of about 5 ohms, so a factor of ten less than my 50 ohm signal generator output. I'm only looking for 0.2 V or so. Is a transformer a reasonable way to go and could I wind something on an appropriate toroid core. Maybe a powdered metal like Micrometals MP? This is to measure capacitor (PZT) resonance at around that frequency so I'd actually like to keep the voltage reasonably constant by keeping the impedance low. Or, is there some much better way?

author: TimFox
author_rank: Super Contributor
timestamp: July 28, 2025, 07:46:08 pm
content: How much voltage will your SG put into 50 ohms?If, say 5 V rms (0.5 W), you could use a 50 ohm and 2 ohm external resistor divider to get 0.2 V.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 28, 2025, 07:50:59 pm
content: Actually, that will work.Still, I'm curious about the transformer.

author: moffy
author_rank: Super Contributor
timestamp: July 28, 2025, 09:17:43 pm
content: A transformer would work just fine, it's a modest frequency should be quite straight forward. The only issue might be parasitic resonances with the inductance of the transformer.P.S. those resonances might get in the way.

author: mawyatt
author_rank: Super Contributor
timestamp: July 28, 2025, 10:18:39 pm
content: Use a resistive series divider with the AWG source impedance since this avoids any resonances the transformer may introduce.The simplest is just use the 50Ω AWG source and adjust the AWG amplitude to get the desired voltage across the DUT.A better solution is use the highest voltage available from the AWG and add a series resistor to achieve the desired DUT voltage which provides higher source effects isolation from the DUT.Since the OP intent is to measure the PZT resonance the higher the source impedance the better. If a Buffer Amp (like the Junket DPA-2698) is available then a higher effective source voltage and higher value series resistor can be utilized further improving the source isolation and measurement results.Best

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 28, 2025, 11:11:01 pm
content: Sure, a transformer would work if it's designed correctly. An audio transformer might work well (if its coupling is good), though it will probably have some weird resonances somewhere.Resistive divider/attenuator is a more straightforward option, if it's able to give you the signal level you need.

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 12:33:05 am
content: PZT, like quartz crystals, will show series and parallel resonances.Series is best seen with low source and load, parallel with high impedances.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 29, 2025, 11:50:56 am
content: To mimic the way it's driven (from a low impedance), I drive the same way and usually look at the PZT current with a very small sense resistor. You'd normally expect a nice big resonance and anti-resonance, but these devices are highly damped, so the waveforms are not as "dramatic". I'm going to try the transformer, but I bet I end up with the resistive solution as being cleaner. Thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2025, 04:23:34 pm
content: You could use the Bode Function in your DSO and AWG for this measurement.https://www.eevblog.com/forum/testgear/admittance-measurements-with-dso-awg-with-bode-function/msg4491952/#msg4491952https://www.eevblog.com/forum/beginners/impeadence-measurement-with-scope/msg5570419/#msg5570419Best

author: RoGeorge
author_rank: Super Contributor
timestamp: July 29, 2025, 04:46:56 pm
content: Quote from: Conrad Hoffman on July 29, 2025, 11:50:56 amMy guess is the resonance in a PZT is a mechanical property only, so it should be specific to the device, and should have little to do with the impedance of the driver. So, no need to drive it the same as in the application circuit. For test, and for finding the resonance, I think the PZT should be driven from a large impedance, so to easily observe the resonance(s).If the generator has very low resistance, that low resistance of the generator will appear in parallel with the PZT, which will dramatically lower the observable quality factor, Q. Q is the ratio between stored and dissipated energy at each oscillation cycle. If the generator has very low output resistance, its internal resistance will waste a lot of energy (probably much more than the energy lost inside the PZT itself), and thus making the overal Q very small, and making the resonance peaks to spread, and become harder to observe, or not visible at all.I would add a series resistor to the 50ohms generator, to make it much bigger (random guess, 1k, but I don't know the expected resistance of a PZT at resonance). Then browse the sinusoidal frequency from the generator, while keeping the oscilloscope probe in parallel on the piezo element (so after the series 1k added resistor). The larger the resistor, the more localized the resonant peak.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 29, 2025, 07:16:06 pm
content: Found one of the piezo discs (a buzzer from a singing postcard), and I was curious how much of a difference a series resistor would make.This PZT is of about 4-5cm diameter (1.5 inch), and measures about 54nF. The T4 universal tester shows 54nF and 17ohms, but I don't know how it measures the ESR.Tried the PZT directly at the 50 ohms generator (no series resistor), then in series with a 1k, then in series with 10k. The series resistors also attenuates the signal seen by the PZT, so each time, the amplitude was increased from the generator, such that the signal on the oscilloscope remains about the same: 50mVpp for no series R, 300mVpp for 1k series, and 2Vpp for 10k series.In all 3 captures, the oscilloscope was in parallel with the piezo disc.The frequency sweep was linear, between 40kHz and 80 kHz, sinusoidal signal. The blue falling edge is a fixed marker (at 58.2kHz in all 3 captures), to observe if there are any shifts in the resonant peak.The resonant frequencies doesn't seem to change with the generator impedance. For this piezo disc the resonance is clearly visible even at 50ohms (no series resistor), but the easiest to spot is with the highest series R=10kohm.

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2025, 07:46:19 pm
content: The "Dip" preceding the "Peak" is likely the transducer series resonance.Best

author: johansen
author_rank: Super Contributor
timestamp: July 29, 2025, 07:56:08 pm
content: Quote from: Conrad Hoffman on July 28, 2025, 07:39:26 pmjust about any small common mode choke will handle this without weird resonances.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 30, 2025, 02:03:12 am
content: In a previous life I've done a lot of those sweeps. They provide a crack test as any defects will show up as a bunch of extra peaks and dips.Here's a piece of PZT trivia that's not often appreciated. Let's say you have a resonant based actuator. Being an actuator implies that it's going to do work of some sort. Now, a pure reactance can't dissipate any power. None. You have to have a loss term if you're going to do work with it. Thus, whatever frequency it's operated at, resonance or a bit off, there will be a significant dissipation factor if it's going to move anything. There's no free lunch.And yes, I'm using my Siglent scope and arb gen to do the bode plot!

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 04:40:35 am
content: Quote from: RoGeorge on July 29, 2025, 04:46:56 pmDoh, I've browsed a couple of AN about quartz. If I got it right, the voltage dips corresponding to a series LC resonance is usually calledresonance, while the voltage peak corresponding to a parallel LC resonance is often calledantiresonance. So my guess was wrong.A high impedance generator would be favorable to detect theantiresonance(parallel resonance), while a low impedance generator would ease to spot theresonance(series resonance).BTW, one of the AN mentions the possibility ofspurious resonance, page 11 of 15, chapter "1.4. Unwanted resonances of quartz crystals", Figure 16 Series resonant quartz crystal with spurious resonance" inhttps://www.geyer-electronic.de/wp-content/uploads/2022/11/GEYER-Quarz-Tutorial_e_07_22_V1.0.pdfAgain guessing, I think that is why the smaller dip in the measured voltage, that would be a (series)spurious-resonance, followed by the main voltage dip, the (series)resonance, followed by the voltage peak, the (parallel)anti-resonance.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2025, 06:04:57 am
content: PZT ceramics has quite strong piezo electric effect compared to quarz. So the electric loading can effect the resonance quite a bit more than the weak trim via the load capacity in crystal oscillators. Resistive loading tends to broaden the peak as it adds loss. Both very low and high resistance would be low dampingHow the part is hold can also add loss to the resonances and broaden the peaks and sometimes add extra resonances. For a bare part one may want 2 or more runs with different holder.

author: moffy
author_rank: Super Contributor
timestamp: July 30, 2025, 10:26:16 am
content: You could always use, say a 25nF capacitor, to couple between the signal generator and the PZT, as a capacitive divider.

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 02:49:56 pm
content: Quote from: RoGeorge on July 30, 2025, 04:40:35 amDon't recall the general use of "antiresonance", more just Series and Parallel Resonance to identify which type.Best

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 02:54:59 pm
content: Quote from: mawyatt on July 30, 2025, 02:49:56 pmIn physics, the term “resonance” is used for LC series resonance, and “anti resonance” for parallel resonance.

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 03:00:01 pm
content: Quote from: TimFox on July 30, 2025, 02:54:59 pmWe're coming from the Electrical Engineering sideBest

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 03:39:26 pm
content: Same here, I knew about series and parallel resonance modes in quartz, not resonance/antiresonance.But by the "I was wrong part" I meant not the names, I meant I was giving advice about finding the parallel resonance, while the request in the opening post is about looking for a way to find/measure the series resonance. At first I didn't register this aspect (the interest in series resonance mode only), only later, after the following posts about mechanical work and about insisting to drive the PZT from a low impedance.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 05:11:06 pm
content: Quote from: mawyatt on July 30, 2025, 03:00:01 pmI come from the other side.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 05:39:00 pm
content: That reminds me about "charge amplifier" vs "transimpdance amplifier", which are the same thing.But such example is still not that bad. Once I was talking to a friend about electron-volts (eV), and he start laughing. My friend was a photographer, and I was telling him about my first DSLR camera, and those eV were not electron-volts from physics, as I wrongly assumed, but eV (exposure-value) from photography.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 30, 2025, 06:04:43 pm
content: EV numbers for exposure, darn near obsolete today, but if you're talking about sensor noise, we measure that in electrons. After enough years in electronics and having to work with physicists and other disciplines, I'm not to fussy and the brain just adjusts in the background. Alas, the one thing that drives me crazy is when people use V in a formula for voltage. I was taught E (for EMF) and E it will ever be.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 06:12:59 pm
content: We were thought here (Eastern Europe/Romania) not with V, but with U. E was for energy.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 08:23:06 pm
content: Quote from: RoGeorge on July 30, 2025, 05:39:00 pm“Transimpedance amplifier” usually means current-in, voltage-out, where the ratio is impedance.Charge amplifiers use a capacitance (not a capacitive reactance) to convert charge to voltage.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 08:28:00 pm
content: Quote from: TimFox on July 30, 2025, 05:11:06 pmThe physics terminology is analogous to node and antinode for standing waves on transmission lines or vibrating strings, which show multiple modes of oscillation or vibration. Node is at a minimum amplitude, and antinode is at a maximum.