A brief discussion on the relationship between function generator output impedance and output amplitude

As shown in the figure below, the left side of the dotted line is the internal equivalent circuit of the function generator, which is composed of a voltage source and an internal resistance generally 50Ω. The right side of the dotted line is the output terminal, connected to an output load; the electromotive force generated by the function generator is It is not completely added to the output load, but is equivalent to a voltage divider circuit. The formula is as shown below. Detailed introduction: Vgen is the actual output electromotive force of the function generator, Vload is the voltage distributed to the load, R0 is the internal resistance of the function generator, and Rload is the output load.

It is known from the formula calculation that when the output impedance is set to High-Z, the actual output electromotive force of the function generator is consistent with the Amplitude displayed on the screen; when it is set to 50Ω, the actual output value of the function generator is twice the Amplitude displayed on the screen. ,As shown below. When the output impedance is set to 50Ω, we want to output a waveform with an amplitude of 1.5V, but the measured amplitude of the actual output is about 3V. Why is this?

This is because when the output impedance is set to 50Ω, the function generator thinks that the output terminal is connected to an output load of about 50Ω, forming a voltage dividing circuit. In order to make the output reach 1.5V, the actual output electromotive force of the function generator must reach 3V. A connected 50Ω output load can distribute a voltage of about 1.5V. For this reason, we connect a load of about 51Ω at the output end, and the output waveform amplitude is consistent with the set Amplitude, as shown below. Therefore, under normal circumstances, the output impedance output load should be set to High-Z mode to prevent the output voltage from being too high and damaging the object under test.

Summary of how to set output impedance:

1. If a resistor is connected:

If you connect a 50 ohm load, you need to set the OutputChannel Load to 50 ohms.

If a high-impedance load is connected, the OutputChannel Load must be set to high impedance.

2. If it is connected to a lossless transmission line: it depends on whether it is terminated:

If a termination is added at the end, the output must be set to 50 ohms.

If the end is not terminated, high impedance must be set.

A lossless transmission line can be regarded as transparent. It just transfers voltage from one place to another.

This conclusion is based on the internal resistance of the signal source being 50 ohms. (i.e. source termination)

In fact, from the perspective of transmission line theory, lossless transmission lines are transparent for level transmission. Improper termination only produces overshoot and ringing. The final level is equivalent to directly connecting the destination end to source.