NI-FGEN Output Signal is Doubled when Measuring the Signal

Updated Oct 14, 2019

Reported In

Hardware

  • PXI-5422
  • PXI-5401/5411
  • Waveform Generator Device
  • PXIe-5433

Issue Details

I set my function generator to output a sine waveform with 2 Vpp (peak to peak voltage). But, when the output signal is measured by my oscilloscope, a sine waveform with 4 Vpp is measured.
What is the problem?

Solution

The function generator have an output impedance of 50 ohm. The output voltage is defined as the voltage divider between the load impedance and the output impedance, as shown in the picture below. 
The relationship between the output voltage and the load can be calculated using the following equation: 

Vout = [RL /(RL RO )] × [VS ]

where,
Vout = the voltage level delivered to RL

RL = the load impedance in ohms

RO = the output impedance on the NI signal generator

VS = the voltage level generated by the source previous to RO

If the generated signal is defined as 2 Vpp, the actual output Vs is 4 Vpp

Therefore, to output the exact waveform you want, you should use 50 ohm load impedance.

Additional Information

By default, the FGEN will expect a high-impedance load.

The FGEN does not measure the impedance of the load, and must be told manually what it will be so it can compensate internally. This can be changed with a property node:
 

Note:

If you want to use the maximum voltage that the function generator can output, use very high load impedance or do not use load impedance.