Becoming Familiar with your Standard Oscilloscope Probe
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Introduction
An often overlooked, important part of oscilloscope testing is the probe you use. The main role of a probe is to connect your target device under test (DUT) to the oscilloscope’s input so you can pick up the signal from the DUT and view the waveform on the oscilloscope screen.
But keep in mind that an oscilloscope probe is not just a piece of wire with a pointy tip attached to it. There is more to it that you need to know if you want correct testing results. The majority of oscilloscopes with ≤1 GHz bandwidth come standard with one high-impedance passive probe per oscilloscope channel. This is the probe the majority of people will use.
The probe tip resistance is typically 9 MΩ and will form a 10:1 resistor divider ratio with the 1 MΩ oscilloscope input. The input signal to the probe is attenuated by a factor of 1 MΩ/ (9 MΩ + 1 MΩ). For instance, if you probed a 10 V signal with a probe including a 10:1 attenuation ratio, you should have a 1 V signal coming from the probe that is fed to the oscilloscope’s input. The probe tip is followed by a high-impedance cable. A termination box or interface box at the end of the probe cable connects to the input of the oscilloscope.
The primary feature of this probe is its very high impedance. At DC, the probe’s input impedance has 10 MΩ of resistance, and as the frequency goes up, the input impedance of the probe falls off as the capacitive reactance of the probe gets larger.
The standard passive probe is the most rugged, flexible, and inexpensive probe that also gives a very wide input dynamic range - making this probe an excellent choice for general-purpose probing and troubleshooting.
Probe Compensation
Most standard passive probes have an adjustable compensation capacitor for matching RC coefficient to the input capacitance of the oscilloscope. The probe’s adjustable compensation capacitor can be adjusted to cancel the input capacitance of the oscilloscope. To compensate the probe, the oscilloscope probe is connected to the probe compensation terminal to make the square wave look as square and flat-topped as possible. Before making any oscilloscope measurements, you should connect your probes to the probe compensation signal on the front panel to ensure they are properly adjusted.
Dual Attenuation Ratio Probe
Most standard passive probes offer a 10:1 attenuation ratio. A standard passive probe usually provides a dual attenuation ratio - user-selectable 10:1 and 1:1 range. The 1:1/10:1 probe has a switch that inserts what’s usually a 9 MΩ resistor in series with the signal. In the oscilloscope, there’s a 1 MΩ resistor for forming 10:1 input attenuation ratio. In the 1:1 mode, there is no series resistor on the probe side and the total DC resistance seen from the probe tip is only 1 MΩ on the oscilloscope’s input.
The primary benefit of using a dual-attenuation ratio probe is it provides the convenience of both 10:1 and 1:1 attenuation. In general, the 1:1 mode of the probe provides lower noise, making it ideal for measuring low-level signals such as ripple and noise of a power supply. However, the 1:1 mode will produce a significant amount of capacitance appearing in parallel with the oscilloscope’s input, resulting in the reduction of the bandwidth down to approximately 25 MHz.
In the following example, a 10:1/1:1 probe is used to measure the power supply output noise with each attenuation ratio setting. At 1:1 mode, the measured noise is favorable with almost half the noise measured with the 10:1 mode.
