26 [ INSTRUMENTS USED FOR MEASURING AND TESTING PURPOSES-NAMELY, ] DIRECTIONAL COUPLERS [ , DISTORTION ANALYZERS, ELECTRONIC COUNTERS, FREQUENCY METERS, FREQUENCY AND MODULATION MONITORS, FREQUENCY CONVERTERS, OSCILLATORS, POWER METERS, PULSE GENERATORS, RATIOMETERS, SCALERS, SECONDARY FREQUENCY STANDARDS, SIGNAL GENERATORS, SQUARE WAVE GENERATORS, STANDING WAVE INDICATORS, TACHOMETER INDICATORS, VOLTMETERS, WAVE ANALYZERS, WAVE FUNCTION GENERATORS ]