When you are testing a crystal oscillator circuit with an oscilloscope probe, the oscillator may stop oscillating or the waveform may be severely distorted. Why?
Every probe functions as an external circuit connected to the device under test. Each probe has its own input resistance, capacitance and inductance, imposing additional load to the DUT. Connecting a probe to the oscillator circuit (or any electronic circuit) adds extra load to a signal or may distort the waveform displayed on the oscilloscope thanks to the loading effect. Therefore, probing an oscillator circuit requires special care, as the oscillator circuit is highly sensitive to capacitance.
Figure 1 Connecting a probe to the oscillator circuit adds extra loads to a signal
There are two main factors to consider in choosing a probe for oscillator testing. The first is that the oscilloscope probe is adding capacitance to the existing load capacitors (C1 and C2 in fig 2). The load capacitance is a parameter for determining the frequency of the oscillation circuit. It is important to note that the change in the value of the load capacitance may result in changes in the output frequency of the oscillator or at worst case, it may stop the oscillation. The second factor is that the probe is introducing resistive loading to the oscillator circuit. Both factors can be significant enough to keep the oscillator from working. At DC or low frequency ranges, probe loading is mostly caused by the resistance of the probe, and as the frequency goes up, capacitive component of the probe becomes the dominant factor in the loading effect.
Figure 2 A circuit diagram of a typical oscillator circuit
A solution is to use a probe with high input resistance and low input capacitance in order to cause the lowest possible loading effect. In general, a passive probe with 100:1 attenuation ratio such as Keysight N2876A passive probe (with 2.6 pF of input capacitance) reduces capacitive loading significantly on the circuit, compared to a conventional 10:1 passive probe with ~10 pF of capacitive loading. Loading can be further reduced by switching the oscilloscope input coupling from DC to AC, as DC coupling mode on an oscilloscope presents additional loading to the oscillator and may cause it to stop.
Or, better yet, using an active probe with low input loading such as Keysight’s N2795A 1 GHz active probe or N2796A 2 GHz single-ended active probe may deliver better results. This active probe provides 1 Mohm input at DC and low frequencies for low resistance in parallel with <1 pF input capacitance loading to the circuits. Another good probe for oscillator circuit measurement is a Keysight InfiniiMax 1130A Series or N2750A Series InfiniiMode probe that presents even lower loading to the circuit.
Figure 3 Input impedance equivalent model of N2795A/96A single-ended active probe
Also, it’s important to note how to connect the probe to the DUT. If your probe connection has obviously longer input lead wires or a connector at the tip, you should suspect frequency response variation and degradation. In general, the longer the input wires or leads of a probe tip, the more it may decrease the bandwidth, increase the loading, cause non-flat frequency response and result in more variation in response. If at all possible, keep the input leads of the probe tip as small as possible, and keep the loop area of connection as small as possible.
Figure 4 Keep the input leads and the loop area of the connection as small as possible for more accurate results.
Figure 5 If your probe connection has obviously longer input lead wires or a connector at the tip, you should look for frequency response variation and degradation.