B&K Res. and Develop. Dept., 2850 Naerum, Denmark
Charge injection calibration is a new method for monitoring operation conditions of measurement chains consisting of a microphone with preamplifier, cables, and conditioning amplifier. The method replaces the insert-voltage calibration technique which is frequently used with large or unmanned systems. A great advantage of the new method is its ability to check the transducer which is practically ignored by the insert-voltage calibration. From a remote voltage source, a constant charge is ``injected'' into the microphone capacitance and preamplifier input circuit using a small capacitance integrated with the preamplifier. This leads to an output signal that is inversely proportional to the combined impedance of the transducer and input circuit. An accidental change in the microphone impedance---which is a good transducer condition indicator---would appear as a change in the preamplifier output. High stability and extremely high leakage resistance (typically 10[sup 14](Omega)) are required for the charge injection capacitor (typically 0.2 pF). This problem was solved and the (patented) method was found to work reliably over a wide frequency range (2 Hz--200 kHz). The new method is less costly and simpler to implement. It also preserves the systems resistance to extraneous electrical fields. The method and its ability to indicate malfunctioning are discussed.