Oscilloscope Probes and Compensation

The probe is the connection between your oscilloscope and the circuit under test. It is the part of the measurement system most often overlooked and most often responsible for measurement errors. A scope that costs several hundred dollars can produce completely wrong results if used with a poorly compensated probe, a probe that does not match the oscilloscope’s bandwidth, or a probe with a long ground lead. Understanding your probes — what they do, how they work, and how to use them correctly — is as important as understanding the scope itself.

The probe that ships in the box with most oscilloscopes is a 10× passive voltage probe. This is the workhorse of oscilloscope measurement. Used correctly, it extends the oscilloscope’s useful range to high-impedance, high-frequency circuits with minimal circuit loading. Used incorrectly, it introduces errors that can make a good circuit look faulty or a faulty circuit look good.

Why a Probe Is Needed

You cannot simply connect a piece of wire from the circuit to the oscilloscope BNC input and make accurate measurements. There are two problems with a direct wire connection:

First, the oscilloscope input has a capacitance of 10–20 pF in parallel with its 1 MΩ input resistance. A wire adds its own capacitance — a 50 cm wire has roughly 50–100 pF of distributed capacitance. Adding this capacitance directly across the signal source loads the circuit, affects tuned circuits, changes the rise time of digital signals, and introduces measurement errors that worsen as frequency increases.

Second, a wire connected directly to the scope forms an antenna that picks up interference. Long test leads act as loops and can add 50 or 60 Hz interference, radio frequency interference, and noise from nearby equipment, all of which appear on the scope display as if they were part of the signal.

The probe solves both problems by using a combination of resistances and capacitances to reduce circuit loading, limit the effective length of the high-impedance connection, and reduce the overall input capacitance seen by the circuit under test.

The 1× Probe: Direct Connection

A 1× probe is essentially a shielded cable with a BNC connector. It passes the signal directly to the oscilloscope input with no voltage division. The measurement sensitivity is maximum: a 100 mV signal appears as 100 mV on the scope. However, the input capacitance seen by the circuit is essentially the full scope input capacitance plus the cable capacitance — easily 100–150 pF total. At 10 MHz, 150 pF has a reactance of only about 100 Ω, which loads even a modest-impedance circuit significantly.

The 1× probe is used when:

• You are measuring very small signals (below about 100 mV) that would be attenuated too much by a 10× probe
• You are measuring in a low-impedance circuit where the loading from the probe capacitance is negligible
• The signal frequency is low (audio range) where the capacitive loading does not matter

For most general-purpose measurements in medium and high impedance circuits, and for anything above audio frequencies, the 10× probe is the correct choice.

The 10× Probe: How It Works

A 10× passive probe uses a voltage divider to reduce both the voltage delivered to the oscilloscope and the capacitive loading presented to the circuit. The probe cable contains a series resistor — typically 9 MΩ — in the probe tip section. This 9 MΩ resistor combines with the oscilloscope’s 1 MΩ input resistance to form a 10:1 resistive voltage divider. Any DC voltage or low-frequency AC voltage at the probe tip is divided by 10 before reaching the oscilloscope.

The result: if you measure a 5 V signal with a 10× probe, the scope input sees 0.5 V. The scope display must be corrected by a factor of 10 to show the true signal voltage. Most oscilloscopes do this automatically when they detect a 10× probe (via a ring connector on the BNC that tells the scope to multiply by 10). Some older scopes require you to press a “10×” button in the channel menu to tell the scope which probe attenuation factor is in use.

The voltage division reduces sensitivity by a factor of 10, which is the tradeoff. Where a 1× probe can show signals down to a few millivolts on the most sensitive scale, a 10× probe with the scope at 1 mV/div effectively measures the circuit at 10 mV/div. For the vast majority of ham radio measurements, this is not a limitation.

The input impedance seen by the circuit is now 9 MΩ + 1 MΩ = 10 MΩ — ten times higher than with a 1× probe — which loads circuits far less. More importantly, the effective input capacitance at the circuit connection point is reduced to approximately 10–15 pF, compared to 100–150 pF for a direct connection. This lower capacitance dramatically reduces the effect of the probe on high-impedance and high-frequency circuits.

The Probe Equivalent Circuit

A real 10× probe is more complex than a simple 9 MΩ resistor. The probe cable has distributed capacitance (about 100 pF for a typical 1.5 m cable), and the oscilloscope input has 1 MΩ || 10–20 pF. If you simply connected a 9 MΩ series resistor, the cable capacitance and scope input capacitance would create a frequency-dependent impedance divider that would attenuate high-frequency signals differently from DC — causing the probe to give correct readings at low frequencies but wrong readings at high frequencies.

To compensate for this, the 10× probe includes a small adjustable capacitor (the compensation capacitor) in parallel with the 9 MΩ series resistor. This capacitor creates a capacitive voltage divider in parallel with the resistive divider. When the capacitive divider ratio exactly matches the resistive divider ratio (both 10:1), the combined probe has a flat frequency response from DC up to the probe’s bandwidth limit, passing all frequencies with the same 10:1 attenuation.

The compensation capacitor is adjustable precisely because the cable capacitance varies between individual probes and changes with cable length and temperature. The oscilloscope’s input capacitance also varies slightly between instruments. For accurate measurements across the full frequency range, the compensation must be tuned to match the specific probe-to-oscilloscope combination.

The equivalent circuit of a 10× passive probe. The probe body contains a 9 MΩ series resistor and a small adjustable compensation capacitor (C_comp, typically 10–35 pF) in parallel. At the oscilloscope end, the input presents 1 MΩ || C_in (10–20 pF). Correct adjustment of C_comp ensures that the capacitive divider ratio matches the resistive divider ratio (both 10:1), giving a flat response across the full bandwidth.

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Probe Compensation

Probe compensation is the process of adjusting the compensation capacitor in the probe tip until the probe has a flat frequency response. If the compensation is wrong, the probe will display square waves with distorted corners — either rounded (under-compensated) or spiked (over-compensated). Any measurement made with a mis-compensated probe is inaccurate for anything other than DC or very low-frequency signals.

Compensation should be performed:

• The first time you use a probe with a new oscilloscope
• Any time you switch the probe between oscilloscopes
• After significant temperature changes (temperature affects cable capacitance)
• Any time you are uncertain whether the probe is correctly matched to the scope

How to Compensate a Probe

1. Connect the probe to Channel 1. Connect the probe tip to the oscilloscope’s CAL (Probe Comp) output terminal, which is typically found on the front panel. Set the oscilloscope to DC coupling, 1 V/div (or 500 mV/div), time/div 200–500 µs. The CAL output is typically a 1 kHz square wave at 3 V or 5 V.
2. Observe the waveform. A correctly compensated probe shows a clean square wave with flat tops, flat bottoms, and sharp corners with no overshoot or rounding.
3. If the corners are rounded (low-frequency emphasis, high-frequency attenuation), the probe is under-compensated. Increase the compensation capacitance by adjusting the trimmer screw in the probe tip.
4. If the corners overshoot or spike (high-frequency emphasis), the probe is over-compensated. Decrease the compensation capacitance.
5. Adjust the trimmer — usually accessible through a small slot in the probe body near the BNC connector — with a non-metallic trimmer tool (never use a metal screwdriver, which can affect the adjustment by adding capacitance). Turn the trimmer slowly while watching the corners on the square wave. Stop when the corners are as sharp and flat as possible.

The compensation adjustment is intentionally fiddly: small movements produce visible changes. Take your time. The goal is a clean, sharp square wave with no sag, rounding, or overshoot on the corners.

Probe compensation states. Left: under-compensated — the compensation capacitor is too small, so high frequencies are attenuated relative to low frequencies, rounding the square wave corners. Center: correctly compensated — flat response across all frequencies, sharp clean corners. Right: over-compensated — the capacitor is too large, emphasizing high frequencies and producing overshoot spikes on the corners. Only the center state gives accurate measurements above audio frequencies.

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Ground Lead Effects

Every oscilloscope probe comes with a short coiled wire with a clip attached: the ground lead. This must be connected to the circuit ground near the measurement point. The ground lead closes the current return path for the probe measurement.

The ground lead has inductance — typically 20–200 nH for the 15–20 cm leads supplied with standard probes. At high frequencies, this inductance resonates with the probe’s input capacitance to form an LC circuit. The resonant frequency of this LC combination falls in the range of 30–200 MHz for a typical probe ground lead, depending on its length. Near this resonant frequency, the oscilloscope sees severe ringing that has nothing to do with the circuit under test. It is an artifact of the probe ground lead inductance.

The practical consequences are:

• At audio and low RF frequencies (below ~5 MHz), the ground lead length does not matter significantly.
• At HF frequencies (3–30 MHz), a long ground lead can cause visible ringing on fast signal edges.
• Above 50 MHz, even the supplied ground lead causes significant measurement error. The ground connection must be made as short as possible — by using a spring-loaded ground adapter that makes contact at the probe tip rather than through a long lead.

For ham radio HF measurements, the supplied ground lead is generally acceptable for audio and power-supply measurements. For RF waveform measurements above 10–20 MHz, use the shortest possible ground connection or a ground spring adapter. Many probes come with a short wire ground adapter specifically for this purpose.

Probe Bandwidth and Derating

Probe bandwidth is specified by the manufacturer and is typically matched to the oscilloscope’s bandwidth. A 100 MHz oscilloscope ships with 100 MHz probes. The −3 dB bandwidth of the complete measurement system (scope plus probe) is lower than either component alone, because bandwidth specifications combine roughly as:

For ham radio HF work, the bandwidth of the standard probe shipped with a 100 MHz scope is adequate. If you upgrade to a 200 MHz or higher scope and want to use it at its full bandwidth, invest in appropriately rated probes. Cheap probes bundled with entry-level scopes are often rated to 60–100 MHz; using them with a 200 MHz scope limits the system to 60 MHz performance regardless of what the scope itself can do.

Never use a probe rated below the frequency you are trying to measure. The probe will attenuate the signal and show a distorted, amplitude-reduced waveform with no indication that it is wrong. This is a particularly dangerous error because the reading looks plausible.

Active Probes

Passive probes present a fixed impedance to the circuit that worsens as frequency increases. For measurements above 500 MHz, or in circuits that are extremely sensitive to loading, the solution is an active probe. An active probe contains a small amplifier in the probe tip itself, typically a JFET or MOSFET source follower. This amplifier has an extremely high input impedance (often 1 MΩ || 1 pF or less) and low output impedance, allowing the probe to drive the oscilloscope input without presenting significant loading to the circuit.

The advantages of active probes are:

• Input capacitance of 1 pF or less, compared to 10–15 pF for a passive 10× probe
• Bandwidth of 1 GHz or more for most active probes
• No need for probe compensation adjustment
• Unity gain: the probe does not divide the signal, so sensitivity is maximum

The disadvantages are cost (active probes typically cost $200–$1000), limited voltage range (typically ±5 V to ±40 V), and susceptibility to damage from static discharge or exceeding the input voltage range. They also require power from the oscilloscope, usually supplied through the probe connector.

For most ham radio work below 200 MHz, a passive 10× probe is entirely sufficient. Active probes become necessary if you are working above 500 MHz, in circuits where even 15 pF loading changes the circuit behavior, or in very high-speed digital design.

Current Probes

Current probes measure current rather than voltage without breaking the circuit. They clamp around a wire or conductor and sense the magnetic field produced by the current flowing through it. Two types exist:

• Hall-effect current probes: Use a Hall-effect sensor in a magnetic core that clamps around the conductor. They measure both DC and AC currents, typically from milliamps to tens of amps. They connect to the oscilloscope and produce an output voltage proportional to current (for example, 100 mV/A).
• AC current probes: Use a split-core transformer. They only respond to AC currents, with the core saturating for DC. They are simpler and less expensive than Hall-effect probes and are suitable for measuring power supply currents, motor drive waveforms, and RF line currents.

For ham radio use, current probes are valuable for measuring transmit current draw during different operating modes, checking that power supply current is clean during transmit, and investigating whether RF is appearing on power or control leads (a common cause of RF interference to station equipment).

High-Voltage Probes and Safety

Standard oscilloscope probes are rated for maximum voltages of typically 300 V CAT II (IEC 61010 category II, which covers measurements within a fixed equipment). This rating is more than adequate for most amateur radio circuits. However, several areas of ham radio equipment contain voltages that exceed this limit:

• Valve (tube) amplifier plate voltages: Typically 2,000–4,000 V DC
• High-voltage power supply circuits: 400–2000 V depending on the application
• Antenna tuner high-voltage components: Can reach several kilovolts at high power

A high-voltage (HV) probe — sometimes called a 100× probe — incorporates a much larger voltage divider resistor (often 900 MΩ) and is rated for voltages up to 20 kV or more. These probes are essential for any measurement in high-voltage circuits.

For the typical ham radio station — solid-state transceivers, solid-state power amplifiers, switching power supplies — standard probes are safe for all accessible test points. Only vintage or homebrew valve equipment and high-voltage supply designs require HV probes.

Probe Care and Handling

Probes are precision instruments. Their accuracy depends on the condition of their internal components, and they are more fragile than they look. Follow these guidelines to keep probes in good working condition:

• Never coil the cable tightly. Coiling the coaxial cable tightly damages the inner conductor and changes the cable capacitance. Store probes hanging or loosely looped.
• Protect the probe tip. The hook tip and any collet-style probe tip accessories are easily bent or broken. Replace them if damaged; a bent tip makes it impossible to make accurate contact with fine PCB traces.
• Clean contacts. The BNC connector and the probe tip contacts oxidize over time and can introduce contact resistance errors. Clean them occasionally with contact cleaner.
• Do not pull by the cable. Always grip the probe body or the connector, not the cable, when disconnecting. Pulling the cable stresses the internal connections at the BNC end and can break the inner conductor.
• Re-compensate after storage. If a probe has been stored for a long period or exposed to significant temperature changes, re-check compensation before making precision measurements.
• Match probe to scope. Do not use a 10× probe rated for 60 MHz on a 200 MHz scope and expect full-bandwidth results. Use probes rated for the scope’s bandwidth.