Signal generator amplitude and the load setting

Use the source-plus-resistor model

Represent the output by an open-circuit voltage Vₒc in series with resistance Rₛ. With a resistive load Rₗ, the expected voltage is Vₗ = Vₒc × Rₗ / (Rₛ + Rₗ). If Vset is specified into Rset, then Vₒc = Vset × (Rₛ + Rset) / Rset. For a high-impedance setting, the model uses Vₒc = Vset.

These equations describe an ideal linear resistive source, not a calibration. The generator-load calculator exposes both the selected load and the actual load so that a plausible factor-of-two error is easy to recognise.

Connect and record a controlled comparison

Use a low-amplitude, low-frequency sine wave and zero offset to start. Confirm that the scope and generator share a compatible reference. With output disabled, connect the documented cable/termination arrangement. Record whether the scope input is 1 MΩ or 50 Ω and whether an external terminator is present.

1. Read the generator’s source impedance and load-setting convention in its manual.
2. Set a modest amplitude, frequency and offset, then enable the output.
3. Record the measured Vpp and the generator’s amplitude units. Vpp, peak and RMS are not interchangeable.
4. If a second load is suitable and rated for the power, disable output, change the load and repeat.

For a sine wave without clipping, Vrms = Vpp / (2√2). Other waveform shapes need their own conversion. A DC offset adds to the peak voltage and can exhaust the output’s allowed range.

Why the real result may differ

The calculator omits output current limits, clipping, offset-dependent amplitude limits, cable reflections and frequency-dependent impedance. A scope’s 1 MΩ input has capacitance; it is not a perfect open circuit at high frequency. A 50 Ω terminator has a finite power and frequency rating. Its dissipation is P = Vrms² / R for the voltage actually across it.

A long cable carrying a fast edge is a transmission line. Matching and measurement location matter; a simple resistive divider cannot predict overshoot or reflections. Conversely, adding 50 Ω to a weak circuit node can heavily load it. A terminator is not a universal remedy for a messy trace.

Before calling the generator inaccurate

Check probe factor, amplitude units, selected versus actual load and whether the waveform is clipped. Reduce amplitude and frequency, remove unintended parallel loads, then repeat. Record the firmware and mode of a combination instrument: generator operation may change scope acquisition or share its reference.

Next review three-in-one trade-offs. Comparing a device’s generator with its own scope can find gross problems, but shared references and errors prevent that from being independent calibration.