Resolving a Factor of Two Error in Generator Amplitude

A function generator set to one volt may appear to deliver two volts when checked with a high impedance oscilloscope input. Before adjusting a calibration factor or replacing the instrument, examine the source resistance and the load assumed by its amplitude display. The voltage at the PCB connector is the quantity the circuit actually receives.

Draw the measurement circuit

Represent the generator as a source with its specified output impedance, followed by the cable and actual load. For a source with 50 ohms series resistance, a matched 50 ohm load forms a divider. A much higher resistance load produces a different terminal voltage. Tektronix explains this amplitude discrepancy for its AFG3000 generators: a high impedance measurement can show approximately twice the amplitude expected for a 50 ohm load.

Check whether the generator display is configured for 50 ohms, high impedance, or another load value. Treat that setting as an amplitude interpretation that must match the actual connection. Do not assume a menu selection physically installs the required termination.

Include every parallel path

A board termination plus an oscilloscope’s 50 ohm input can create a lower combined resistance when both are connected at the same node. A feedthrough terminator added to an already terminated path changes the load again. Sketch each connection rather than describing the setup only as a BNC cable.

Also identify AC coupling, series capacitors, protection components, and generator offset. A correct sine amplitude at one frequency does not establish the same pulse level or low frequency response. Record whether amplitude means peak, peak to peak, or RMS.

Verify at the receiving node

Measure the signal where it enters the relevant circuit using a probe or measurement port whose loading is understood. Begin with an amplitude and offset that remain within the board and instrument ratings for the actual load. Check termination power and input voltage limits before selecting a low impedance input.

For a hypothetical generator displaying 1 V peak to peak into 50 ohms, approximately 2 V peak to peak into a high impedance load is consistent with the divider model. It does not prove an exact voltage under every frequency, cable, or output range.

Capture the complete setup

Save generator amplitude, offset, load setting, waveform, frequency, cable arrangement, and receiver termination. Include the measured voltage at the board. When another bench obtains a different result, compare those details before blaming PCB gain or attenuation. A documented termination model turns an apparent calibration failure into a testable electrical explanation.