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Why your function generator outputs twice the programmed voltage

The default setting for Keysight function generators is to display the desired voltage as though terminated into a 50 Ohm load. When a high impedance device, such as an oscilloscope is used to measure the output of the function generator, the waveform appears to be twice the voltage set on the display of the oscilloscope.

Some oscilloscopes can change their input impedance from standard high impedance to a 50 Ohm termination. Another solution is to add a 50 Ohm feed through (Keysight part number: 0960-0301) to the end of the BNC cable.

Function Generator output impedance diagram

Other common impedances are: 25, 75, 93, 135, 150, and 600 Ohms -Video systems are most often 75 Ohms and many audio systems use a balanced 600 Ohm termination. If not terminating the output of the function generator into a 50 Ohm load, it may be necessary to adjust the output voltage to compensate for the different impedance. For a 50 Ohm source the desired voltage Vmeasured into an impedance R can be calculated as Vmeasured=V(R/(R+50)), where V=2*display or Vmeasured=display(2R/(R+50))

The Keysight 33220A and 33250A function generators have the ability to do this calculation for you and directly display the desired voltage. They include a feature that allows the output termination to be set to any impedance from 1 to 10 k Ohm, or infinite. For example if the output termination is set to 75 Ohms and then generator is connected to an oscilloscope with a 75 Ohm termination (or 75 Ohm feed through). The function generator display will match what is displayed on the scope.

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