Electronic – Ceramic resonator changes and maintains frequency when touched

harmonicsoscillatorresonance

I'm experiencing a weird phenomenon with a ceramic resonator test circuit, where the frequency of its output can be changed by touching the circuit with my finger. The circuit would maintain this higher frequency oscillation indefinitely as long as I don't disturb it again by touching or cycling power. It always starts up to the correct frequency on power up however.

This is what the output looks like on the scope. Note that both wave forms are stable even if left running for hours. normal wave form increased frequency

The resonator is a Kyocera KBR-480Y, specified to resonate at 480 kHz. I'm using two gates of a CD4011BE quad NAND chip as inverters to drive the circuit. Here is a hand drawn schematic: schematic

Circled in red is the portion of the circuit that is susceptible to touch. It's around the leads of the resonator, including the feedback resistors and loading capacitors.
bottom of circuit

Here is what the circuit looks like from the top. The small surface mount device is a ripple counter, which should have no effect on this situation. top view

My questions are:

  • How does this happen? What mechanism causes this to happen? I vaguely understand that there are multiple modes of resonance in crystal and ceramic resonators, but I'm not sure why touching it would create this effect.
  • Would it be feasible to utilize/exploit this property intentionally in designs? Could there be a reliable way to induce this effect to generate a higher frequency signal from a resonator that is rated lower?
  • I guess related to the second question, how would one go about suppressing this behavior? Having this happen on a production board would not be good.

Thank you for taking a look at this and apologies in advance for my lack of knowledge and experience.

Additional information – here is what the signal looks like on both legs of the resonator. The purple one is connected to the input of the first inverter, and the yellow one is the leg connected to the output of the gate. Both probes are Siglent PP215 set to 10x attenuation, if that matters.signal at both legs of resonator

Best Answer

As said in the other answers, ceramic resonator oscillator circuits are extremely sensitive as they have high input impedance and huge gain. The bias resistance in the circuit is about 1 megaohm and the load capacitances are about 100 pF. Poking it with a finger that can have roughly 100 kohm and 10pF of impedance so will have a considerable effect on the circuit operation.

But what is really the source of the problem is that the NAND IC type you are using is unsuitable for the circuit as drawn, or the circuit as drawn is unsuitable for the chip. It is revealed by the waveforms at resonator pins, as they are quite square-wavish instead of being nice sine-wavish, so they contain harmonics.

So there is too much gain in the system and the resonator is being overdriven. Usually these oscillator circuits use a simple inverter like 4069UB, where UB means Unbuffered. The chip you are using is 4011B, where B means Buffered. It is not a simple NAND gate, it has extra inverters at inputs and outputs, so it has more gain, and it can't be run stably in linear feedback amplifier mode, like a UB gate can be.

So due to huge gain and harmonics already present, that circuit is even more sensitive to to into weird oscillating modes when poking it with a finger.

To fix it, gain should be reduced. The buffered 4011B should really be changed to unbuffered 4011UB. Another option would be to add the series resistor between NAND output and resonator.

Using the higher frequency modes of oscillation on purpose is not what these resonators are designed for, they are meant to be used at the rated fundamental frequency. At least you can get crystals that are specifically intended to be run at the 3rd harmonic to get the rated frequency you want. But running these at higher harmonics mean also they might not be stable due to tolerances and the much larger input power and harmonic content that these are designed to be run. The mode of oscillation could be different from the intended mode and it might not be stable.