Application Note: Understanding Measurements Using An Oscilloscope Versus A Lock In Amplifier

At Spectrum Detector, we calibrate our sensors for use with an oscilloscope. All our sensors have responsivity in Volts per Watt. When using a sensor with an oscilloscope, the power can be obtained by dividing the peak to peak voltage by the responsivity. When measuring very small light levels, an oscilloscope may not be able to provide a good measurement. The instrument of choice in these conditions is a Lock In Amplifier (LIA). When using an LIA, the readings it gives will not match those given by an oscilloscope. The reason for this is that the LIA measures the RMS value of the waveform, while the oscilloscope measures the peak to peak value. To make measurements with the LIA, a conversion factor must be found. Note that some digital scopes have an RMS measurement function, but the accuracy suffers along with the peak to peak measurement as the signal level drops. As will be shown, the RMS reading from a scope will not always match that measured by an LIA.

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