Linear crystal controlled FM source for mobile radio application

A narrow deviation voltage controlled crystal oscillator (VCXO) circuit has been developed that exhibits near-linear tuning characteristic and, unlike previously reported circuitry, is extremely simple and straightforward in design and highly economical with regard to material and fabrication costs. The design principle makes use of a linear reactance modulator incorporating abrupt junction varactor diode developed by M. Driscoll, and previously reported by Driscoll and Healey in connection with the design of a wide deviation VCXO for radar application. Because the total oscillator turning requirements are only on the order of 1 part in 10⁵, however, much of the circuit complexity has been eliminated, and the problem of interdependence between linearity and center frequency adjustment has been solved. Network equivalence theorems have been utilized to reduce the total number of required circuit inductances to only one, and the composite resonator has been reconfigured to operate on the series resonance of its impedance characteristic allowing simple means for prevention of oscillation at spurious network resonances through the addition of a single resistor. Laboratory measurement of transmitter FM distortion using a number of FM oscillator modules reveals repeatable achievement of less than 0.5 percent distortion with linearity adjustment and less than 1½ percent without any adjustment. Also, the achievement of repeatable linear tuning characteristic has simplified the oscillator modules so that required temperature stability is obtained by addition of negative temperature coefficient (NTC) capacitors to the oscillator circuit and a single simple three-point thermistor compensation circuit that is easily designed and provides simultaneous temperature dependent compensating voltage for all radio oscillator modules. The oscillator design also allows for direct parallel connection of multiple modules (for multichannel operation) having commonly connec- ed tuning input and signal output ports.