Electronic instruments like oscilloscopes respond to electrical potential difference measured in volts. Some applications require the measurement of other physical characteristics like light or sound. Many instruments provide ways to use other sensors or transducers to convert non-electrical parameters into voltage and measure them. This gives the ability to use existing sensors or transducers with your instrument.
For example, most mid-range and higher oscilloscopes include a rescale function that enables acquired data to be rescaled. This allows users to read the displays using non-voltage parameters and read in appropriate units directly. Rescale is a linear mapping from one set of units to another. In order to do this, you need to know the sensitivity of the sensor or transducer in terms of measurement units per volt. This is the multiplicative constant. There is also an input for an additive constant to deal with offset correction.
An optical to electrical (O/E) converter can be used to give an oscilloscope eyes. This lets it measure optical signals in applications like checking laser light sources or the outputs of fiber optic data links. Consider an O/E with a conversion gain of 600 V/W. Since the oscilloscope reads in volts, the data has to be multiplied by the reciprocal of this number or 1.667E-3 W/V. Figure 1 shows the rescaling operation employed to measure a fiber optic data link.
The rescale function takes both a multiplicative and an additive constant. The reciprocal of the conversion gain is used as the multiplicative constant. The optical input to the O/E is covered to measure the mean value of the O/E, the signal when the optical input is dark. This is defined as the dark power or current, which is basically an offset that has to be removed. This receiver had a measured dark power of -11.03 µW and required an addition of 11.03 µW to remove it. Note there is also a check box to override the units and specify the desired units, in this case it refers to watts.
The same technique can be used to give this instrument ears by connecting a wide-band microphone. Microphones measure sound pressure levels (SPL), expressed in Pascals (Pa). As in the previous case, the conversion multiplier is needed. In this case, the microphone is connected to a SPL calibrator, which generates 110 dB rms relative to 20 µPa at 1 kHz. The output of the microphone is read as 266.21 mV RMS. Converting the calibration level into Pascals, where 110 dB rms relative to 20 µPa works out to 6.32 Pa. The sensitivity of the microphone is 6.32/0.266 or 23.76 Pa/V. This is entered into the multiplier field in the rescale math function. There is no additive constant in this case. The Override units’ box is checked, and the output units are specified as PAL for Pascals. The readouts for the scope channel then reads in Pascals.
Most instruments have this ability to rescale measurements. In the case of the example above, the rescale function is a math operation. In many cases, the rescale operation occurs in setting up the instrument’s signal input.
The ability to rescale values and units adds a significant value to the data acquired by allowing the instrument to be applied to a broad range of non-electrical measurement applications.