Question

The function generators in lab can be configured for a 50Ω or Hi‐Z loads. When in 50Ω mode, the output is doubled because the generator expects to drive a 50Ω transmission line, terminated with a 50Ω load so that the proper voltage is seen at the load. If a student were to mistakenly use a 75Ω coax in this mode, calculate the voltage that would be seen at the load if the function generator is set to 3V on its display:

a. For a 50Ω load connected.

b. For a high impedance load connected.

Answer 1

Here I am explaining the theory part first and then solving the problems that are given.

Using the AFG output:

AFG has an output impedance (ZS) of 50 ohm. Therefore the load impedance (ZL) forms a voltage divider, along with ZS. The voltage across the load (Vef fective) is as shown in Figure.

By default, AFG generates VS assuming that the load impedance is 50 ohm. Thus, for a voltage setting of 1V on AFG panel, VS generated is equal to 2V. And for ZL of 50 ohm, V effective is equal to 1V (as expected). Loading AFG output with much greater impedance than 50 ohm (such as connecting the output directly to an oscilloscope), will drop majority of the voltage across the load. Thus V effective can be up to double the panel setting. Whereas, loading AFG with ZL < 50 ohm, can cause V effective to be less than the panel setting.

AFG Load Impedance setting

AFG is often used to generate an input signal for amplifiers. Amplifier circuits (with high input impedance) load AFG with a large ZL. With default setting, AFG generates VS twice the voltage set on the panel. Thus, Vef fective would be almost twice of the AFG panel setting, and the load circuits would receive unintended high input voltage. A solution to this problem is, setting AFG load impedance to HIGH-Z. With this setting AFG generates VS equal to the panel setting (as very low drop is expected across ZS). And thus Vef fective would be almost equal to the panel setting. If the input impedance of the load circuit is known, AFG load impedance can also be set to this specific value. With this setting, AFG precisely generates VS, such that Vef fective is equal to the panel setting.

Solutiion to the given problem: