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No—not for most oscilloscope measurements. Use the normal 1 MΩ input with a conventional 10× passive probe. Select 50 Ω when you are terminating a controlled-impedance coaxial signal, reproducing a source’s specified 50 Ω load, or using a probe that explicitly requires 50 Ω.
The wrong setting can change the circuit, distort a fast waveform, or make a generator’s reading appear wrong. Treat the probe, cable, source impedance and oscilloscope input as one measurement system.
What “50 Ω” means on an oscilloscope
The setting is the channel’s input termination. A 1 MΩ input is a high-impedance voltage load, usually shown as 1 MΩ in parallel with input capacitance. A 50 Ω setting connects a resistive load intended to match a 50 Ω source or coaxial transmission line.
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- Source impedance: the effective output resistance of a generator or circuit driver.
- Cable impedance: the characteristic impedance of the transmission line, commonly 50 Ω in laboratory RF systems (75 Ω is also used in video and other systems).
- Scope input impedance: the load selected at the channel.
- Probe input impedance: the additional load placed on the test point.
Even a nominally light 1 MΩ input can load a fast signal through its capacitance. At high frequency, probe geometry, ground inductance and cable length can matter as much as the resistance.
When to use the 1 MΩ input
Choose 1 MΩ for ordinary circuit work and whenever the source cannot safely drive 50 Ω:
- Standard 10× passive probes.
- Logic-level signals at circuit test points.
- High-impedance or weak sources.
- Signals with substantial DC voltage, after checking the probe and channel ratings.
- Unknown circuits where loading must be minimized.
Conventional passive probes are resistor-divider probes designed around a high-impedance scope input. A 10× probe increases the effective resistance seen by the circuit and reduces the capacitance at the tip, although it is not accurate to describe the scope itself as simply “10 MΩ.” Keysight describes these probes and their loading trade-offs in its probe data sheet.
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Select 50 Ω when the measurement arrangement requires a matched load:
- A 50 Ω source is connected directly through coax and its amplitude is specified into a 50 Ω load.
- RF, pulse or high-speed signals are traveling through 50 Ω coax.
- The oscilloscope is intended to terminate the cable and absorb the incident wave.
- An active or low-impedance (Z0) probe requires a 50 Ω input.
- Waveform integrity and reflection control matter more than light loading.
Keysight low-impedance resistor-divider probes use approximately 450 Ω or 950 Ω at the probe to form 10:1 or 20:1 attenuation with a 50 Ω scope input. They are intended for applications such as ECL logic, microwave devices and 50 Ω transmission lines. The Keysight N2874A is a 10:1 example specified for a 50 Ω input and low-impedance or 50 Ω-line signals.
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Why a generator can read twice as high in 1 MΩ mode
Many function generators display an amplitude calibrated for a 50 Ω load. If the generator is set to 1 Vpp into 50 Ω:
| Connection | Approximate scope reading | What it represents |
|---|---|---|
| Scope set to 50 Ω | 1 Vpp | Voltage delivered to the intended load |
| Scope set to 1 MΩ | About 2 Vpp, depending on source, frequency and calibration | Lightly loaded or near-open-circuit voltage |
This apparent factor of two is a load-condition difference, not necessarily an instrument fault. Generator menus may label the output load as 50 Ω, high impedance or similar; consult that instrument’s manual.
For a source with output resistance Rs, the approximate voltage across a 50 Ω input is:
Vscope = Vopen × 50 Ω ÷ (Rs + 50 Ω)
A 50 Ω source therefore delivers half its open-circuit voltage to the scope. A 1 kΩ source delivers only about 4.8% of its open-circuit voltage into 50 Ω.
What happens to fast coaxial signals in 1 MΩ mode?
A 1 MΩ input is a severe mismatch to a 50 Ω cable. A fast edge can reflect from the scope end, producing overshoot, ringing, doubled edges or other shape changes. This is governed by rise time and electrical cable length, not merely by the signal’s repetition frequency. A short cable carrying a slow waveform may show little visible effect; a long cable carrying a very fast edge can behave as a transmission line.
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Proper 50 Ω termination absorbs the arriving wave and usually gives a cleaner measurement. It does not, however, correct ringing caused by a long passive-probe ground lead; use a spring ground, coaxial connection, differential probe or suitable active probe when appropriate.
Does the probe determine the setting?
Standard passive probe
Use 1 MΩ unless the probe documentation explicitly says otherwise. Switching to 50 Ω changes the divider and compensation network, so amplitude, bandwidth and calibration can be wrong.
Low-impedance or Z0 passive probe
Use 50 Ω. These probes rely on a 50 Ω coaxial path and are intentionally heavier loads than ordinary passive probes.
Active probe
Follow the probe manual. Some active probes require 50 Ω; others use a dedicated probe interface or adapter that supplies the required termination. “Active” does not automatically mean 50 Ω. Active probes are often preferable when a node cannot tolerate a 50 Ω load; Rohde & Schwarz reports typical active-probe input capacitance around 1 pF in its probe information.
Direct BNC cable
A BNC cable is not a passive probe. Connecting it to a 1 MΩ channel does not create a matched measurement. For a controlled-impedance signal, use the scope’s internal 50 Ω termination or an external rated terminator.
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If the oscilloscope has no 50 Ω setting
Use a 50 Ω feed-through terminator at the scope input when the source and measurement require a 50 Ω load. Rohde & Schwarz describes this option for scopes without native 50 Ω termination.
- Verify frequency, maximum voltage, power and connector ratings.
- Account for the voltage and power now delivered to the terminator.
- Do not assume the scope’s 1 MΩ input has become the termination.
- A purpose-built feed-through device is clearer than an improvised arrangement.
Do not stack terminations accidentally. If the scope is already in 50 Ω mode, adding another 50 Ω terminator in parallel creates about 25 Ω and changes the source loading and amplitude.
Loading and safety checks
Fifty ohms is a real load, not a “more accurate” mode. It can pull down a normal circuit, draw substantial current and dissipate source power. Rohde & Schwarz warns that permissible input voltage can be significantly lower in 50 Ω mode than in 1 MΩ mode; the exact limit depends on the oscilloscope model, frequency, coupling and channel configuration.
- Check source current and power capability before enabling 50 Ω.
- Check the channel’s 50 Ω voltage rating, including DC offset.
- Never use a 50 Ω termination as a substitute for a high-voltage probe.
- For high-voltage or unknown signals, begin with an appropriately rated high-impedance probe.
A practical selection procedure
- Identify the connection: standard passive probe, active probe, Z0 probe or direct coax.
- Read the probe documentation: confirm whether it expects 1 MΩ or 50 Ω.
- Determine the source convention: open circuit, high impedance or 50 Ω load.
- Check ratings: channel voltage, probe voltage, terminator power and frequency.
- Start at 1 MΩ for an unknown circuit with a suitable probe.
- Select 50 Ω for direct 50 Ω coax when the source and signal level permit it.
- Investigate a 2:1 amplitude discrepancy: compare the generator’s load setting with the scope termination.
- Investigate ringing together: check termination, cable length, rise time, probe ground and bandwidth.
Quick decision table
| Measurement situation | Scope setting | Typical connection | Reason |
|---|---|---|---|
| Ordinary circuit troubleshooting | 1 MΩ | 10× passive probe | Minimizes resistive loading |
| High-impedance or weak source | 1 MΩ | 10× passive or active probe | Avoids pulling down the circuit |
| Function generator specified into 50 Ω | 50 Ω when reproducing that load | BNC/coax | Matches the stated amplitude condition |
| RF or fast pulse through 50 Ω coax | 50 Ω | Direct coax | Terminates the transmission line |
| Standard passive probe | 1 MΩ | Probe tip and ground | Matches probe design |
| Low-impedance/Z0 probe | 50 Ω | Specialized probe | Required by probe architecture |
| Scope lacks 50 Ω mode | External 50 Ω terminator | BNC feed-through | Adds the missing load, subject to ratings |
| Unknown signal or high DC voltage | Start at 1 MΩ | Appropriately rated probe | Safer default while ratings are verified |
For additional probe-selection guidance, see Rohde & Schwarz’s oscilloscope probe tips and Keysight’s oscilloscope probe selection guide.
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