Low Frequency Calibration and Digital Volt Meters

Recently, we have had a few reader questions regarding low frequency vibration calibration.  The questions mostly stem from the misconception that traditional digital volt meters cannot measure accurately below 20 Hz.  While this is true for inexpensive, entry-level digital volt meters, there are a number of other models which will work quite well in the low frequency range. 

There are several meters which can measure below 20 Hz, most notably the 8 ½ digit Agilent 3458A which can measure in RMS mode quite accurately to 1 Hz.  The 3458A also contains a digitizing mode, which can be utilized to permit measurement between DC and 1 Hz, or higher frequency.  The 3458A is a popular meter, in that any metrology laboratory can calibrate it with just two intrinsic standards.  However, the process of reading digitized data from a 3458A over general purpose interface bus (GPIB - IEEE488) is quite slow and RMS measurements, while fast, are subject to measurement errors related to distortion.  While this may be sufficient for the occasional accelerometer calibration, any metrology lab performing even a modest volume of vibration calibrations will see the benefit in improving calibration system throughput while improving signal processing.

For better labor efficiency and a more rapid cycle time on calibration, consider a system implementation as follows:  Use the 3458A as the “traceable meter” for calibration of a PC-based digital data acquisition system which can then be used for measurements as low as 0.1 Hz frequency and above.  The PC-based digital data acquisition system is then used for daily accelerometer calibrations.  The PC-based vibration calibration system digitized time sample stream can be digitally processed via the Discrete Fourier Transform (DFT) at precisely the frequency of excitation, eliminating signal processing errors like out of band distortion and windowing leakage.  This leads to faster calibrations, further reduction of measurement noise and ultimately enhances efficiency and uncertainties for metrology labs.