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Measuring Equipment

Oscilloscope

Probes

Oscilloscope probes connect the circuit under test to the oscilloscope. A typical probe consists of a probe head, flexible cable, and a BNC connector for oscilloscope input. The probe tip includes attachments like a spring-loaded hook for better contact and a ground connection (alligator clip).

Types of Oscilloscope Probes

Probes

  1. Passive Probes – The most common type, available in 1× (low-frequency, low-voltage) and 10× (high-frequency, high-voltage) configurations.
  2. Active Probes – Include transistors and amplifiers, requiring power. Suitable for high-frequency, low-capacitance applications.
  3. Differential Probes – Measure voltage between two points instead of ground. Ideal for floating measurements.
  4. Current Probes – Detect magnetic fields from current flow and convert them to voltage. Used for non-intrusive current measurement.

Passive Probes: Ideal vs. Real

An ideal probe would have infinite bandwidth and zero attenuation, but real probes have frequency-dependent behavior.

  • 1× Setting: Used for signals <10 MHz and voltages <1V. Low-pass filter effect due to oscilloscope input capacitance.
  • 10× Setting: Reduces amplitude by 10×, making it better for high-voltage, high-frequency signals. Compensation cancels out oscilloscope capacitance.

Probe Compensation

Probe capacitance must be adjusted to match the oscilloscope’s internal capacitance for accurate measurements. Poor compensation results in distorted waveforms.
Steps:

  1. Connect the probe to the oscilloscope’s compensation terminal.
  2. Adjust the probe’s capacitance using the small screw near the probe head until a clean square wave is displayed.
  3. Always compensate when switching probes or using different oscilloscope channels.

Probe Comp

Setting 1× & 10× Probes

Most measurements use 10× probes. Ensure the oscilloscope matches the probe setting:

  • Some models autodetect 10× if the probe has a metal detection pin.
  • Otherwise, manually adjust the oscilloscope settings.

Probe Loading Effect

Probes affect circuit behavior through input resistance and input capacitance:

  • Input resistance slightly reduces the measured signal’s amplitude.
  • Input capacitance loads the circuit, impacting rise times at high frequencies.

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