In making measurements of vibration characteristics of a machine having rotating components, the phase of a vibration signal is generally defined as the angular relationship between an amplitude peak in the vibration signal and a once-per-revolution reference signal, such as produced by a tachometer sensor. 
In most vibration measurement systems, the vibration signal from a vibration sensor is converted to a digital signal using an analog-to-digital converter (ADC).  Generally, to attain an accuracy of one degree at a rotational speed of 2 KHz, this time measurement must fix the tachometer leading edge relative to the ADC samples with a resolution of better than one microsecond. 
In situations in which data are collected by multiple data acquisition systems running asynchronously, the data acquisition processes are not synchronized, and there is no way to directly compare the phase of the data collected by the asynchronous systems.  In such a case, even if the tachometer edges are aligned, a comparison of data from the two systems would still be skewed, especially during non-steady-state operation. 
What is needed, therefore, is a system for accurately aligning tachometer pulses associated with vibration signal data collected by multiple data acquisition systems that are running asynchronously.