The Type 603-A Standard-Signal Generator is a general- purpose instrument, designed to have the widest possible application in the measurement of receiver characteristics and transmitter field strengths. By means of easily accessible plug-in coils, the range extends from 100 kc to 25 megacycles, making the instrument particularly adaptable to the testing requirements of modern receivers operating in the high-frequency as well as in the broadcast spectra. The standard-signal generator, with no additional equipment, is suitable for determining the sensitivity and selectivity of a radio receiver and, when modulated by an external audiofrequency oscillator, it may be used in measuring over-all fidelity. When used in connection with a radio receiver, an output meter, and a antenna of known characteristics, the standard- signal generator may be used in making field-strength measurements. PRINCIPLE OF OPERATION — The Type 603-A Standard-Signal Generator consists, essentially, of a calibrated radio-frequency oscillator designed specifically for high stability and minimum frequency modulation. An additional oscillator, operating at 400 cycles is included to modulate the output of the radio-frequency oscillator. Provision is made for determining the percentage modulation and for adjusting accurately the radio-frequency output voltage.