A New Class of FM Subcarrier Oscillators

This paper describes a new class of millivolt-controlled subcarrier oscillators developed especially for operation in the rugged environments of orbital programs. The oscillators cover all the standard IRIG channels. The full IRIG signal-frequency response is achieved at a modulation index equal to five. A less-than-unity-gain, long-tailed pair, silicon-transistor, dc amplifier and a greater-than-unity-gain RC network are combined to form a sine-wave oscillator. The center frequency is deviated ± 15 per cent by a ± 10 millivolt control signal. Excellent linearity characteristics are achieved. A unique feature of the circuit is the inherent deviation limiting. Inductors are avoided in the oscillator design. The passive RC network synthesis procedure is developed to realize 1) a voltage gain greater than unity, 2) the proper percentage of deviation with excellent linearity characteristics, and 3) inherent deviation limiting to contain the FM signal spectra within their assigned channels. The center frequency is deviated by controlling the relative gain of the long-tailed pair type of differential dc amplifier. A pure sine wave is generated without additional filtering. The balanced nature of the circuit yields excellent center frequency stability from 0°C to + 85°C. Center frequency time drift provides an improvement of at least one order of magnitude over present state-of-the-art millivolt-controlled oscillators. Measured data is presented to support the procedures used in the design. The oscillator and associated circuitry are contained in a 4½-cubic-inch package that weighs 4 ounces.