Instrumentation for measuring aircraft noise and sonic boom

摘要

Improvements in instrumentation suitable for measuring aircraft noise and sonic booms. A converter produces an electric current proportional to the sound pressure level at a condenser microphone. The electric current is transmitted over a cable and amplified by a zero drive amplifier. The converter consists of a local oscillator, a dual- gate field-effect transistor (FET) mixer and a voltage regulator/impedance translator. The local oscillator generates a carrier voltage that is applied to one of the gates of the FET mixer. The FET mixer mixes the microphone signal with the carrier to produce an electrical current at the frequency of vibration of the microphone diaphragm. The voltage regulator/impedance translator regulates the voltage of the local oscillator and mixer stages, eliminates the carrier at the output and provides a low output impedance at the cable terminals. The improvements include automatic tuning compensation against changes in static microphone capacitance and means for providing a remote electrical calibration capability.