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How to test crystal oscillator with an oscilloscope

Published time: 2018-09-13

Sometimes we have such doubts: When we observing the waveform of the crystal oscillator pin with an oscilloscope, the waveform is not visible or the waveform of the oscilloscope is displayed. Can the 200MHZ oscilloscope not measure the crystal oscillator of 10MHZ?  Jotrin Electronics Limited will solve this doubts for everyone.

The common types of crystal oscillations 

First, let's briefly introduce the crystal oscillator. The crystal oscillator can be roughly divided into two categories: passive crystal oscillator and active crystal oscillator.

1,What is the passive crystal oscillator

A passive crystal oscillator is a non-polar electronic component that requires a clock circuit to generate an oscillating signal and cannot oscillate itself. 

The Passive crystals have no voltage requirements, and the signal level is variable, that is to say, passive crystal oscillator is determined by the start-up circuit. 

The working principle of passive crystal oscillator as follows: electrodes are plated on both sides of the quartz crystal wafer, and a certain voltage is applied to the two electrodes. 

Because the piezoelectric effect of the quartz is formed, the voltage is naturally formed, a sinusoidal waveform formed.

passive crystal oscillator

Figure 1 Passive crystal oscillator

2,What is the active crystal oscillator

The active crystal oscillator is a complete oscillator with a transistor and a RC component except the quartz crystal. 

The active crystal oscillator does not require an internal oscillator, the signal quality is good, stable, and the connection method is relatively simple, and does not require complicated configuration circuits.

Active crystal oscillator
Figure 2 Active crystal oscillator

The Analysis of crystal vibration waveform

The crystal oscillator waveform is generally a sine wave or a square wave.When the output waveform is a square wave, the general rising edge is steep and contains more high-frequency signals. 

At this time, it is necessary to ensure that the test of bandwidth is sufficient. 

The theoretical value of bandwidth is twice as high as the frequency of signal that be measured, the actual test square wave bandwidth should be 10 times the frequency of the measured signal.

In addition to the bandwidth, when testing the crystal oscillator, Jotrin Electronics Limited reminds everyone that it should be paid attention to: 

The crystal oscillator is sensitive to the capacitive load, and the probe capacitance is relatively large, which is equivalent to a very heavy load connected in parallel in the crystal oscillator circuit, which easily leads to the circuit stop the vibration and get the correct measurement results. 

Therefore, when performing the crystal test, it is necessary to ensure sufficient bandwidth and a small input capacitance.

The correct way to test the crystal oscillator by oscilloscope.

First of all, let's answer the previous question. We can definitely test the waveform of 10M crystal with 200M oscilloscope, but why is the measured waveform the shape of Figure 3

Distorted crystal waveform

Figure 3 Distorted crystal waveform

That is because during the test, the probe selects the ×1 gear position. 

The ZP1025S probe of the ZDS2000 series has a bandwidth of 10MHz in the ×1 gear and an input capacitance of 55 pF±10 pF, which causes distortion of the waveform.

We adjust to the probe gear position to ×10. At this time, the probe bandwidth is 250MHz and the input capacitance is 13pF±5pF. Let's take a look at the waveform at this time.

correct crystal waveform
Figure 4 correct crystal waveform

In order to improve signal fidelity, you should also use the grounding spring that comes standard with the probe instead of grounding the alligator clip to ground.

Standard grounding spring

Figure 5 Standard grounding spring

Finally, Jotrin Electronics Limited reminds everyone that use the oscilloscope for testing, futher more,not only pay attention to the oscilloscope settings, but also pay attention to the current probe gear position, different gears correspond to different parameters, the right is the best!

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