What a 10x Probe Does, How It Divides the Signal and When to Use 1X

What a 10x Probe Does, How It Divides the Signal and When to Use 1X

ByKen

What a 10x Probe Does, How It Divides the Signal and When to Use 1X

As an Amazon Associate we earn from qualifying purchases. This guide does not link to any products. A 10x probe is the most common passive oscilloscope probe. It reduces the voltage that reaches the scope by a factor of ten, so the scope can measure larger signals and the probe loads the circuit less. The trade-off is that the scope sees a smaller signal, which matters when the signal is already small. This guide explains how the divider works, what the scope has to be set to, and when a 1X setting is the better choice.

Quick answer

  • A 10x probe has a series resistor of about 9 megohms at its tip. With the scope’s 1 megohm input, it forms a 10:1 divider.
  • Keysight’s application note gives the example that a 10 V signal arrives at the scope as 1 V. The scope must be set to the 10:1 probe factor, or the readings will be wrong by a factor of ten.
  • Use 10x for general work and higher voltages. Use 1X for low-level signals, where the 10x loss of signal costs more than the extra loading.

How the divider works

A passive 10x probe is a voltage divider. Keysight’s application note describes the tip as having about 9 megohms in series with the scope’s 1 megohm input. The input signal is attenuated by a factor of 1 megohm divided by the sum of 9 megohms and 1 megohm, which is one tenth. A 10 V signal therefore arrives at the scope as 1 V. The scope then multiplies its reading by ten to show the tip voltage, provided it has been told the probe factor.

Tektronix’s ABCs of Probes primer gives the same arithmetic, with the probe input resistance set at about 10 megohms at DC. Because the 10x probe is designed for a 1 megohm scope input, the scope input has to be set to 1 megohm when a 10x passive probe is used. The primer notes this as an exception to the usual rule of matching the probe to a 50 ohm input, because a 50 ohm input would not produce the 10:1 ratio.

Teledyne LeCroy points out that the “10X” label is a little misleading. In its view the probe is a one-tenth attenuating probe, so the scope input sees one tenth of the voltage at the tip. Either name describes the same ratio, and the scope’s display setting is what makes the readout match the tip voltage.

Why the probe has a 10x setting at all

Tektronix’s probes FAQ gives two reasons. First, the 10x setting raises the voltage range. The ABCs primer compares the two settings: a 1X probe on a typical scope covers about 4 millivolts to 40 volts, while a 10x probe covers about 40 millivolts to 400 volts. Second, the 10x setting lightens the load. Because the probe’s impedance is multiplied by ten, it draws less current from the circuit under test. The ABCs primer also notes that probe cable length reduces bandwidth.

The Tektronix FAQ says most passive voltage probes are 10X probes. It also notes that some scopes detect the probe type and scale the reading automatically, while others need you to multiply by ten yourself. Teledyne LeCroy describes one scope that detects the attached 10X probe and corrects the displayed voltage to the tip voltage. Check your scope’s manual or settings before you trust a reading.

Compensation before you measure

A 10x probe has an adjustable compensation capacitor that matches the probe to the scope’s input capacitance. Keysight’s note says the probe’s RC network must match the scope’s input capacitance. Without matching, the probe does not give flat attenuation across frequency, and fast edges look distorted.

The check is simple. Connect the probe to the scope’s front-panel compensation signal, which is usually a square wave. Adjust the compensation capacitor until the top of the square wave is flat. Keysight recommends doing this before any measurement. A probe that is not compensated can look fine on slow signals and then show overshoot or rounding on fast ones.

The noise trade-off

The 10x setting costs signal. Teledyne LeCroy’s article makes the point most clearly. A 10x attenuating probe reduces the signal by ten, but the scope’s own amplifier noise stays the same. In the article’s words, you have “effectively given away 10X the signal but have the same amount of noise,” which costs about 20 dB of signal-to-noise ratio. The article recommends avoiding 10x probes for low-level signals or anything near the scope’s noise floor.

Keysight’s note describes the 1:1 setting as the way to recover that signal. In 1:1 mode the series resistor is removed, so the scope sees only its 1 megohm input. Keysight says this gives lower noise, which helps with low-level signals such as power-supply ripple. The cost is bandwidth. Keysight gives roughly 25 MHz for 1:1 mode, so the 1X setting is not suitable for fast edges.

Tektronix’s FAQ puts the choice the same way. Use 10x for most signals and for higher voltages, and use 1X for low-level signals, accepting that the probe loads the circuit more.

When to use 10x and when to use 1X

  • Mains, motor drives, or any signal above a few volts: use 10x. The range is what you need, and the loss of signal is small compared with the size of the signal.
  • Fast digital edges on a board: 10x is usually the better choice for bandwidth, and it loads the circuit less, provided you compensate the probe first.
  • Power-supply ripple or a small analog signal: consider 1X, because the lower noise matters more than the extra loading.

If the signal is low and fast, neither setting is ideal. Use a probe and scope pairing made for that job, and check the bandwidth and noise figures in the manual for the exact probe.

Common mistakes

  • Wrong probe factor: the scope shows the signal at one tenth of its true value if it is set to 1X while a 10x probe is connected.
  • Wrong input impedance: a 10x passive probe is designed for the 1 megohm input. Setting a 50 ohm input with it does not give the 10:1 ratio.
  • Skipping compensation: a probe that is not compensated distorts fast edges even when slow signals look correct.

Frequently asked questions

Is a 10x probe always better than a 1x probe?

No. It has a wider voltage range and loads the circuit less, but it cuts the signal by ten. For low-level signals near the scope’s noise floor, a 1X setting can give a cleaner reading.

Why do I need to set the scope to 1 megohm?

The 10x probe’s divider uses the scope’s 1 megohm input. The ratio of 9 megohms to 1 megohm is what gives the 10:1 attenuation. With a 50 ohm input, the probe would not produce the ratio it is labeled for.

Do all scopes scale the reading automatically?

No. The Tektronix FAQ says some scopes detect the probe and scale the reading, and others do not. Check your scope’s setting and make sure the displayed value matches the probe factor.

What we could not verify

We did not find a Keysight Learn page on 10x probes that we could open. The Keysight material used here is the application note 5992-2121. We did not verify the exact resistor and capacitor values for every probe model, because the sources give typical values only. The bandwidth figure for 1:1 mode is Keysight’s figure for its own probe and scope example, and it will differ with other probes and scopes. Check the manual for the probe you have.

How we chose these sources

We opened each source as a readable page before using it. Two are from Tektronix, one is a Keysight application note, and one is a Teledyne LeCroy blog post from a scope manufacturer. We did not use retailer listings, forums, or third-party manual mirrors for any claim in this guide.

Related reading on this site

For a broader look at the instrument a probe connects to, read the guide to digital oscilloscopes. For a scope that adds logic channels, read the guide to automotive oscilloscopes.

Sources

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