I have read the answers to the question,"How do crystal oscillator start to work" in this forum. And the common answer which I got is that, there are many frequency around the crystal. The crystal will pick up these all noise signal frequencies and send it to the internal amplifier and filter which will select the frequency and send it back to the crystal again which the barkhausan criterion being satisfied? Am I right? Please correct if I am wrong.

My doubt is that, I have a quartz crystal of 8MHz connected to an MCU. When I don't power the MCU, I am not getting any signal from the ends of the crystal (I probed using an Oscilloscope). I should get the noise frequencies at one end of the crystal,right? Since, the MCU (system) is powered down, I may not get the desired 8MHz with amplification. But I should get some signals at the ends of crystal,right? Or am I wrong? Please help. Thanks

 

The noise signals need only be very very tiny, less than you can see on your 'scope. Also when the chip is powered the oscillator circuit in the chip will produce its own noise.

 

This might be helpful -

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


Regards, Dana.

 

"Noise" is not the appropriate way to explain the operation of a quartz crystal oscillator.
You cannot measure the "noise" of a non-powered quartz crystal by probing with an oscilloscope.
A quartz crystal oscillator circuit is a resonant device, very much like a tuning fork, old-school bell, or a string on a guitar or piano. When such a device is struck it vibrates at its resonant frequency. It simply needs a kick start to get it ringing.

A quartz crystal works in the same way. The application of power gives the crystal a kick and sets it ringing. The additional components, resistors, capacitors, and amplifiers assist in ensuring that the quartz crystal continues to oscillate at the correct frequency. Note that you can trim the frequency of oscillation ever so slightly by adjusting these external components. An external trimming capacitor is often used to adjust the frequency when an accurate time-keeping function is required.

 

Hello,

There is a difference between quartz crystal and quartz oscillator:



The quartz oscillator is pictures at these pages of the wiki:
https://de.wikipedia.org/wiki/Datei:Integrierter_Quarzoszillator_(smial).jpg
https://commons.wikimedia.org/wiki/File:Crystal_oscillator_internals.JPG

Bertus