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| What determines the amplitude of a Crystal Oscillator? |
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| dicky96:
Hi, so the question is quite simple really. When I stick my scope probe on the legs of a 2 pin Crystal Oscillator in circuit (usually during diagnosis of a faulty device such as motherboard or embedded microcontroller) what determines the amplitude of the oscillation I should see? Sometimes it is quite large, greater than 2.5V, other times it can be small, say 60mV. One leg usually has a much larger amplitude than the other. Is this a function of the device the crystal is attached to or is there such a thing as a faulty crystal that does oscillate at the correct frequency but with too low amplitude to drive the device? Thanks |
| Whales:
I believe it entirely depends on the drive circuit. Crystals are not useful on their own. Sidenote: MCUs with the ability to directly-attach crystals still have drive circuitry, just internal. |
| ataradov:
The "large" leg is Xout - the driving output. 2.5 V is a very large amplitude. Are you sure it is a crystal and not a CMOS generator? MCUs often have automatic gain control. Visually pins should have undistorted sine waveform. And crystal manufacturers specify crystal drive power, and it should be followed for optimal performance. Overdriving the crystal may shorten its life. |
| RoGeorge:
The amplitude seen on the oscilloscope is irrelevant. The parasitic capacitance of a passive oscilloscope probe is very big for a quartz oscillator. It is so big that can stop perfectly stable oscillators, or can make defective ones to start oscillate. |
| dicky96:
--- Quote from: RoGeorge on September 06, 2019, 09:03:00 am ---The amplitude seen on the oscilloscope is irrelevant. The parasitic capacitance of a passive oscilloscope probe is very big for a quartz oscillator. It is so big that can stop perfectly stable oscillators, or can make defective ones to start oscillate. --- End quote --- Then how to reliably test if the Quartz crystal is actually any good? |
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