Configure Measurements (Thresholds tab)

Meas. mode:

Use the Thresholds tab of the Configure Measurements dialog to select the standard IEEE threshold measurement levels or define custom threshold levels (percentage or absolute voltage/power) for the rise and fall times of a waveform. If you want to assign specific amplitudes to the threshold levels, ensure the thresholds are within the limits of the top-base levels.

Measurements Affected by Threshold Settings
Instrument Mode
Oscilloscope Eye TDR/TDT
Average Duty Cycle Distortion Average
Duty Cycle Fall Time Fall Time
Fall Time Rise Time Rise Time
Rise Time   Δ Time
Frequency   Time at Edge
Preshoot   Time at Amplitude
+ Pulse Width  
— Pulse Width  
Δ Time  
Time at Edge  
Time at Amplitude  
Amplitude at Upper  
Amplitude at Middle  
Amplitude at Lower  

Threshold Definition

The standard IEEE threshold points are

  • 10%, 50%, 90% for typical waveforms
  • 20%, 50%, 80% for waveforms with excessive ringing or overshoot (often used for eye diagrams)

Threshold Reference (Eye/Mask Mode)

In Eye/Mask mode, the Threshold Reference selection is available and can be set relative to either the top (100%) and base (0%) levels or to the one and zero levels. Select the Top-Base Definition tab to change the definition of top and base. On the right side of the Thresholds tab dialog, a listing shows the measurements that are affected by the threshold settings.

Custom Threshold Definition

If you select the Custom threshold setting, click Select Source to select the source waveform (if more than one waveform is displayed), and % or Units to select the type of thresholds. In TDR/TDT mode, the available Source waveforms include channels (labeled Ports), T-domain parameters, and S-parameters. For parameters, the type of content window used is also indicated as shown in examples in the last column of the following table.

Selecting Units allows you to enter absolute amplitude levels, however, ensure that the thresholds are within the boundaries defined by the top-base levels. If the thresholds are outside the boundaries, the measurement results will report which threshold level is in question. You will then have to redefine the threshold levels and repeat the measurement.

Setting custom thresholds causes the measurement results table to display the text "user defined."

The following table shows the amplitude units for the various waveform content windows.

Units Used for Displayed Content Window Graphs
Name of Content Window Graphs Amplitude Units Example Source
Waveform Selections
Oscilloscope, Eye, and TDR/TDT Modes
Window Volts and Watts Channel 2B
Differential 1A
Port 1+ (1A)
TDR/TDT T-Domain Parameters
Time-Ohms Ohms T11 Ohms
Time-Volts Volts TCC21 Volts
Time-% Percent T21 Percent
TDR/TDT S-Parameters and FFT Math Operator Waveforms
Freq-Mag Decibels FFT[2A] (F1)
S21 Magnitude
Phase Degrees FFT[2A] (F2)
S12 Phase
GrpDelay Seconds FFT[2A] (F3)
SCD11 Delay

The Custom option is not recommended when measuring eye diagrams.

The 10% , 50% , 90% selection is often used for single valued waveforms. The percentage level is based on the measured or user defined values for Vtop and Vbase.

Selecting Volts as the units of measurement option will allow you to define absolute voltage values for the threshold levels. Top-base levels are independent of the measurement when Volts mode is selected for the threshold level. Therefore, the top-base levels are not used and do not need to be set. The instrument will only use the values defined for the threshold levels in the period measurement calculation. If you are measuring an optical channel, please note that the measurement result be displayed as Watts .

Rise time and fall time measurements do not require you to enter a value for the middle level threshold.

Top-base levels are not used in the fall time calculation when threshold levels are set to absolute voltages. Therefore, the top-base levels do not need to be set in this mode.