Hi Antknee,
I’ve done a lot of work on this circuit. For research of how it functions there are many good sources. A similar circuit is labeled: “Figure 6. Fundamental Mode Pierce Oscillator”
in a paper titled “Oscillator Design Techniques…” at
https://www.electro-tech-online.com/custompdfs/2010/02/saosc.pdf
Read crystal oscillator circuit design at:
https://www.electro-tech-online.com/custompdfs/2010/02/2001SEP06_AMD_ANPDF.pdf
These US patents describe the operation and design of piezo drive circuits very similar to the one under examination.
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Here’s one that describes a 100Khz circuit!
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Useful calculator sites:
Series / Parallel Capacitor Calculator
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The capacitance to calculate the resonant frequency is the series combination of these parts:
C6 – 1.5nF
C5 – 22nF
and the piezo (2.25nF) in series with the 0.1uF and both of these in parallel with the diode (15pF) = 2.215nF (effectively the piezo element itself).
The series combination of these = 859.4pF. [C6, C5 and piezo combo]
The inductor L1 = 60uH
The oscillator starts up at the LC tank frequency of 700 kHz. The piezo resonates at 1.67Mhz. It is modeled as a capacitor in parallel with a series combination of capacitor, resistor and inductor. It is positioned between the collector and the base of Q1 providing feedback. Once the circuit begins oscillating the piezo takes over as the crystal oscillator element. This is observed on the scope as a much larger amplitude wave at a 1.67Mhz frequency and the piezo element buzzes (the low frequency envelope is audible). The piezo operates as an inductive element and the circuit provides the capacitance.
Attached are some screen captures of the Q1 collector voltage at start up and after oscillating. Unfortunately, the device ran too long dry and the piezo element died. Now the circuit only resonates at the RC frequency (a learning opportunity…).
The component values are also chosen for their reactance values at the operating frequency of 1.67 MHz so that the transistor biasing is correct for stable oscillator operation. These are:
L1 – 60uH – 630 ohms
L3 – 3.4uH – 35.7 ohms
C6 – 1.5nF – 63.5 ohms
C5 – 22nF – 4.3 ohms
The patent writers of the listed Japanese companies seem to propagate an error in calling this a Colpitts oscillator circuit. The Colpitts design has one side of the resonator connected to ground. This circuit is a Pierce oscillator – (see Oscillator Design Techniques…).
A poem:
Ran the humidifier circuit dry
Turned the voltage up too high
Scope trace showed the how and why
I watched a piezo crystal die!
…and now I need another 1,666,666 Hz piezo resonator. Any good sources?
Eric