How to find Internal source resistance practically

[ss_engg]

I would like to perform and experiment in which i want to measure internal source resistance of function generator (as you can say to verify the internal source resistance of function generator is 50 ohm) so i can perform it on bread board.

 

[betwixt]

1. Measure the output voltage with no load.
2. Wire a variable resistor of 100 Ohms directly across the generator output.
3. Adjust the value until you read half the off-load voltage.
4. Switch off, disconnect and measure the resistance with an Ohm meter.

The voltage will have dropped to half when the output impedance and the variable resistor form a 1:1 potential divider, the resistance you measure is therefore the same as the generator output impedance.

Brian.

 

[crutschow]

You can also measure the open circuit voltage and the voltage with a fixed resistor of about 50 ohms.
You can than calculate the internal resistance from the voltage divider equation for two resistors.

 

[D.A.(Tony)Stewart]

Do you want a linear spectrum display on scope of impedance of any Device under Test (DUT) vs f or just manual verify generator?

 

[KlausST]

Hi,

The solution with the external known resistor only works if the internal impedance is also purely resistive.

If you want to know the impedance, then you need a second function generator and connect both outputs together with a known resistor. Switch both function generators ON, but the DUT should actively drive zero. Let the second generator output different frequencies (or sweep) and measure both voltages before and after the resistor.
With these values you can calculate voltages, currents and phase shifts. All you need for calculating the impedance.

Klaus