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The real device is this.
The LT SPICE model is here.

The schematic of the driver part of the circuitry (omitting power supply circuitry and relay circuitry that basically switches the driver between the 2 outputs):

Schematic of the driver and transducer equivalent circuit

Trying to understand particulars of this circuit besides that it has positive feedback and resonance producing oscillations.

One question that I have is how to model the effect of the oscillations at the transducer somehow coupling into the input (base of Q1) (I think modelling this and then playing with the simulation may help to understand other parts of the circuit better).

The measured oscillations at the transducer (unloaded) and at the input look like this (CH1 - input, base of Q1; CH2 - output to the transducer, collector of Q3):

Oscillations and coupling to the input with the transducer unloaded

When the transducer is loaded (palm pressed firmly against it) the coupling to the input almost disappears:

Oscillations and coupling to the input with the transducer loaded

The simulation with the linked SPICE model doesn't show this kind of output/input coupling, the input is just a clean 5V step.

What do I need to add to the SPICE model to get the kind of output/input coupling observed on the real device?

I tried:

  1. Giving the voltage sources series resistances
  2. Connecting all ground points to the common ground through resistances

Note: the transistors in the simulation don't match the real transistors, LTSPICE doesn't have the transistors the real device has. The real transistors are Q1 - KTC9014, Q2 - KTC9012, Q3 - KSC2073TU.

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  • \$\begingroup\$ Your link to ebay requires access. The schematic shows some Rser and Cpar to the 2.2mH inductor which I didn't include. \$\endgroup\$ – a concerned citizen Apr 22 '18 at 16:34
  • \$\begingroup\$ Edited the ebay link. \$\endgroup\$ – axk Apr 22 '18 at 17:05
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Here's an attempt at modeling (inductive) coupling for the base of Q1:

test

The oscillations seem to go on, which is no surprise. No idea what coupling you meant, or mean, it's some brute-force weak coupling with a low inductance (secondary). It could be capacitive, in which case you could try using the behavioural capacitor, but it's safer to just insert a capacitor from the load to the base (though not very "realistic").

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  • \$\begingroup\$ Thanks! I'm not sure which type of coupling it is, I'm just observing it in the measurements. Calling it coupling for the lack of a better word, maybe there's a different term for this. \$\endgroup\$ – axk Apr 22 '18 at 17:03
  • \$\begingroup\$ So you basically added an inductor between the voltage source and the base of Q1 and coupled it to the inductor L5 in the oscillator? \$\endgroup\$ – axk Apr 22 '18 at 17:04
  • \$\begingroup\$ @axk Yes, a simple trace parallel to another trace which happens to have an inductance on it. It's merely behavioural, in this case, but plausible, if you take time to actually measure the inductances and the coupling. It could also be coupling due to power trace being too close, or even pins that are too large and with high currents, add your own in the mix. \$\endgroup\$ – a concerned citizen Apr 22 '18 at 17:50
  • \$\begingroup\$ I wonder what happens when I press my palm against the transducer and the amplitude falls and the coupling is almost gone. \$\endgroup\$ – axk Apr 22 '18 at 21:30
  • \$\begingroup\$ @axk You influence the coupling (lower it). You also add the resistance of your skin to the circuit, and it can be as low as 10s of kOhms, depending on consitution, humidity, dirt, ..., which also acts as a damping factor. \$\endgroup\$ – a concerned citizen Apr 23 '18 at 5:26

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