MC68HC908QT2, experiment of JFET crystal oscillator


9.8304MHZ CAN oscillator is not available at the moment, but few pieces of Xtal are handy to my desk now, so how to build my oscillator. ofcourse, to buy one CAN oscillator or use 74HC series invertor to build Xtal oscillator could be a deal. Anyway, I would like to do a little bit more experiment, in order to understand more about electronics & oscillator.

Pierce Crystal Oscillator

Xtal is the key of set frequency

quote ——-

Pierce Oscillator

Another common design of crystal oscillator is that of the Pierce Oscillator. The Pierce oscillator is a crystal oscillator that uses the crystal as part of its feedback path and therefore has no resonant tank circuit. The Pierce Oscillator uses a JFET as its amplifying device as it provides a very high input impedance with the crystal connected between the output Drain terminal and the input Gate terminal as shown below.

Pierce Crystal Oscillator

Pierce Crystal Oscillator

In this simple circuit, the crystal determines the frequency of

oscillations and operates on its series resonant frequency giving a low impedance path between output and input. There is a 180° phase shift at resonance, making the feedback positive. The amplitude of the output sine wave is limited to the maximum voltage range at the Drain terminal. Resistor, R1 controls the amount of feedback and crystal drive while the voltage across the radio frequency choke, RFC reverses during each cycle. Most digital clocks, watches and timers use a Pierce Oscillator in some form or other as it can be implemented using the minimum of components.

quote ——-

conclusion –

when Xtal’s frequency is going up, amplitude of output signal is narrowing down.

for example,

Xtal 10MHz, amplitude is around 5Vpp

Xtal 4MHz, amplitude is around 8Vpp

Xtal 20MHz, amplitude is around 2.5Vpp

ref:

http://www.electronics-tutorials.ws/oscillator/crystal.html

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