# quartz tuning fork and its series and parallel resonances

Based on the explanation here, I understand that if you electrically drive a tuning fork, you would first see a peak (resonance) and dip (antiresonance).

I just tested the frequency response of a tuning fork, which was designed with 10pF load capacitance in mind. I excited the tuning fork with and without a 10pF capacitor. I noticed the frequency position of the resonance did not change but the frequency position of the antiresonance increased, as would be expected from the wikipedia page linked above.

My question is the following: why do companies make the quartz tuning forks such that the tuning fork has antiresonance frequency of 32768Hz with the appropriate value of load capacitance?

Antiresonance shows up as a slight asymmetry in your amplitude plot, as can been seen in the black curve in the image below (B). Why wouldn't they design it such that the peak (resonance, marked as A in the image) is at the frequency of 32768Hz?

The resonance is definitely more pronounced than the asymmetry (antiresonance). Plus, it has the benefit of the frequency not changing due to load capacitance.

P.S. I just added screenshots of the plot of my actual measurement. You can see that the amplitude peaks at its series resonant frequency (also note the slight asymmetry)

Also note the dip of the parallel resonant frequency on log scale, and note the circuit diagram added.

• Series depends on low ESR driver and not C and consumers more power than uW parallel with rated C load at high Z Commented Aug 15, 2018 at 0:02

Your plots are not accounting for the impedances in circuit, you are just viewing it from the series resistance perspective. From the oscillators in circuit perspective parallel resonance is sharp.

Series resonance is not commonly used, because it requires an inductor in the oscillator circuit to cancel out the electrode capacitance of the crystal.

The simplest and cheapest oscillator circuits don't use an inductor, they use capacitors, and thus must be in parallel resonance. (the crystal is operated in the inductive region)

Series resonance is commonly used for overtone oscillators, as without an LC filter, the oscillator will usually prefer the fundamental.

Note that manufacturers can't/don't "make" the crystal for either resonance. All they can do is tune the crystal to frequency at a specified load capacitance. The crystal is identical, and oscillates the same. (just off frequency of you use different load C.)

In series resonance signal passes through the crystal. In parallel resonance, it is blocked. Which you see depends on how your test circuit is arranged.

simulate this circuit – Schematic created using CircuitLab

• Comments are not for extended discussion; this conversation has been moved to chat. Commented Aug 16, 2018 at 11:47

My question is the following: why do companies make the quartz tuning forks such that the tuning fork has antiresonance frequency of 32768Hz with the appropriate value of load capacitance?

Because it is easy to count a 2^15 value with a counter that is 15 bits and make a 1Hz or 1second signal with that counter.

• couldn't you have made the resonance frequency 32,768Hz instead of antiresonance ? Commented Aug 15, 2018 at 0:08