Attenuation and Vertical Scale

Vertical Scale Limits

User-facing vertical scale limits are determined by the oscilloscope hardware's vertical limits and the total external attenuation from probes and accessories. The following three cases are considered:

  • If no probe is attached, the vertical scale limits are determined by the oscilloscope's hardware.
  • For probes with a fixed attenuation, this is a simple calculation. For example, if the maximum vertical scale of the oscilloscope is 1 V/div, when a 10:1 probe is attached, the maximum vertical scale is 10 V/div. The offset limits are scaled in the exact same way: ±4 V offset becomes ±40 V offset. If the probe has a manually variable attenuation, it follows this same behavior using the current attenuation setting.
  • For probes with automatic variable attenuation, the vertical scale limits are determined by the probe's maximum and minimum attenuation. For example, consider a probe with a 1:1 and 10:1 attenuation. Even if the probe is currently in 1:1 mode, the probe can still be changed to 10:1 mode in order to achieve the 10 V/div vertical scale. So, the limit should be 10 V/div instead of 1 V/div. However, the offset limits are determined by the current vertical scale and the current attenuation.

After one of the above cases is determined, the vertical scale limits are further modified by multiplying any external attenuation from accessories or unknown user devices.

Changing Vertical Scale and Attenuation

When Attenuation is Set by a Probe Physical Switch

Many passive probes have only a fixed attenuation or a physical switch to change the attenuation. Even some of Keysight's newer active probes contain a physical switch to change the attenuation. In these cases, you change the attenuation, and the vertical scale is automatically adjusted to match the attenuation setting. The vertical scale does not control or affect the attenuation setting.

When Attenuation is Automatically set by Keysight Smart Probes

However, many Keysight smart probes have variable attenuation that is controlled digitally. In these cases, you control the vertical scale, and the attenuation is automatically adjusted to match the vertical scale. You cannot manually change the attenuation setting. Automatically in the background, the oscilloscope software determines the best attenuation setting to match the vertical scale and sends digital commands to the probe to change the attenuation.

The automatic attenuation algorithm seeks to use the lowest attenuation possible, effectively zooming in on the signal and achieving the best signal integrity. However, if the probe's Signal Range would be smaller than the vertical scale, the algorithm switches to a higher attenuation.

Default Vertical Scale

The default vertical scale for a channel is different when a probe is attached. Instead of the normal oscilloscope default range, the default will be to set the vertical range to the Signal Range of the probe's default attenuation.

Any time a new probe is plugged in or removed, that channel is automatically set to the new default vertical settings.

Viewing the Signal Range

In the graphical user interface, the Signal Range is shown:

  • As a bar on the right side of the waveform grid. See Setup.

  • In the Setup subtab of the Probe Setup dialog box. See Probe Characteristics.

In the SCPI remote programming interface, the Signal Range can be queried using the :CHANnel<N>:SRANge? command.