Oscilloscope Practice Special 14: Crystal Oscillator Measurement

When using an oscilloscope to observe the waveform on the crystal oscillator pin, the following situation often occurs: the crystal oscillator waveform cannot be seen on the screen or the characteristics of the waveform are incorrect. This article will explain to you the correct method of measuring the crystal oscillator waveform with an oscilloscope through actual operation.

 

We already know that using the ×10 block of the probe can increase the measurement range of the oscilloscope, so what other uses does this function have?

 

When using an oscilloscope to observe the waveform on the crystal oscillator pin, the following situation often occurs: the crystal oscillator waveform cannot be seen on the screen or the characteristics of the waveform are incorrect. It is very likely that you did not use the probe ×10 block!

 

 

Figure 1 Crystal oscillator waveform observed using ×1 gear

 

The crystal oscillator is sensitive to capacitive loads. When using the ×1 gear, the probe capacitance is relatively large, which is equivalent to a heavy load connected in parallel in the crystal oscillator circuit, which can easily cause the circuit to stop oscillating and fail to obtain correct measurement results.

 

 

Figure 2 The result of measuring the crystal oscillator waveform using the ×10 range

 

The correct way should be to use the ×10 range for measurement. At this time, the capacitance in the probe is relatively small, which can reduce the load effect introduced by the test. At this time, the measured crystal oscillator waveform is accurate! In order to improve signal fidelity, the probe's standard grounding spring should be used instead of the grounding alligator clip to ground nearby.

 

Figure 3 Probe using the standard ground spring

 

On the other hand, the output edge of the crystal oscillator is generally steeper and the rise time is shorter. In fact, the output of the crystal oscillator contains more high-frequency components, which should be treated as high-frequency signals and measured at full bandwidth. The bandwidth of the probe ×1 block is limited, while the probe ×10 block is full bandwidth, so the ×10 block must be used for measurement.

 

When measuring signal waveforms, only by using the correct method can you accurately measure the signal and the conclusions drawn will be meaningful!