The Rescale math function in Teledyne LeCroy oscilloscopes enables users to convert this measurement into the proper units.
Figure 1 shows the setup for using Rescale to calibrate the output of a third-party current probe so that it reads directly in Amperes (Teledyne LeCroy current probes automatically read in Amperes with no user intervention).
The upper trace, channel 1, is the voltage output of the current probe. The current probe’s manufacturer specifies that the probe sensitivity is 0.33 mV/mA when operating into 50 Ω. The dialog box below the trace summarizes the setup of the rescale function, showing the multiplicative and additive constants as well as the selected units. For this example, we must multiply the acquired voltage waveform by a factor of 3 in order to convert from mV to mA. This is the reciprocal of the probe sensitivity: 1/ (0.33 mV/mA) = 3 mA/mV.
Additionally, the Rescale function offers a choice commonly measured physical units. In this case, the unit Amperes is selected.
Once the units are defined, math operations performed using the rescaled waveform will result in correctly derived units. For example, multiplying voltages by current results in instantaneous power, measured in Watts. For a power measurement, you should also deskew the current and voltage waveforms using the resample function to align them correctly in time.
Another example of a unit conversion using the Rescale function is the measurement of sound pressure level (SPL) using an instrument microphone as the sensor. In Figure 2, we are measuring SPL in units of pressure (Pascals or Pa).
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 shown on channel 1. The rms voltage read by parameter P4 is 266.21 mV. First, we convert the calibration level into Pascals. The calibrated110 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. The Override units box is checked and the output units are specified as PAL for Pascals. The rms value of math trace F1 now reads 6.32 Pa. Likewise, the FFT of F2, shown in Figure 2, has an rms response of 6.32 Pa at 1 kHz.
The Rescale function extends the usefulness of Teledyne LeCroy oscilloscopes, allowing measurements of physical quantities other than voltage with correctly scaled and labeled axes.