A modulatable oscillator for radio frequencies is disclosed in which the single resonant tank includes a tightly coupled varactor diode for wideband modulation and frequency control. Nonlinearities introduced by the varactor diode are compensated by predistortion of the modulated signal yielding a predetermined modulation characteristic which is maintained over temperature.

Single resonant tank modulated oscillator

In a compensated VCO circuit arrangement for providing constant modulation level over a wide frequency band a VCO (55) is disclosed having a linear VCO gain factor vs. control voltage curve over the wide frequency band of interest. A compensation network (65) is coupled to the VCO to provide an audio output having linear output level vs. control voltage curve. By appropriately matching the two linear curves the variation in modulation level over the frequency band of interest is substantially reduced.

Linear gain voltage controlled oscillator with modulation compensation

In a frequency modulation system, the frequency modulation sensitivity of an oscillator is itself a function of the carrier frequency of the oscillator. The frequency modulation sensitivity is measured and stored for particular carrier frequencies, and subsequently used to control the amplitude of the modulation signal, so that when a frequency modulation signal is fed to the oscillator, a frequency deviation is obtained which is accurately related to the amplitude of the modulation signal.

Frequency modulators with compensation for variations in modulation sensitivity

A frequency modulator utilizing a frequency synthesizer is disclosed. The modulator is especially useful in a mobile radio communication apparatus. The synthesizer includes a phase locked loop made up of a voltage controlled oscillator (VCO), a programmable frequency divider, a phase comparator, a second frequency divider, a reference oscillator, and a low-pass filter. When a channel control signal is input to the programmable frequency divider, the modulator sensitivity of the voltage controlled oscillator is undesirably deviated according to the channel selection. An increase of the modulator sensitivity due to an increase of the oscillation frequency is compensated for by adding a simple FET to the VCO bias circuit. A control voltage from the phase locked loop is applied to both the voltage controlled oscillator and its bias circuit. The modulator can compensate for changes in sensitivity over a wider frequency bandwidth without expanding the circuit elements of the bias circuit therefore making the modulator especially useful for portable use.

Frequency modulator utilizing frequency synthesizer

A single input wideband modulation system for a phase locked loop is achieved by interposing the inverse analogue of the closed phase locked loop transfer function between the modulation source and the VCO modulation input. A first branch of the interposed circuit directly passes high frequency signals from the modulation source to a summing device tied to an input of the VCO. A second branch of the interposed circuit boosts low frequency signals and blocks high frequency signals recieved from the modulation source and passes such signal to the summing device where signals from the first and second branches are summed to permit wideband modulation of the VCO. All the modulation processing is performed outside the phase locked loop.

Phase locked frequency synthesizer with single input wideband modulation system

The nonlinear characteristics of a voltage controlled oscillator (VCO) in a phase-locked loop are approximately matched by a single diode in the audio circuitry to provide an essentially constant frequency deviation over the entire frequency range of the phase-locked loop during frequency modulation of the phase-locked loop. A compensating circuit which incorporates the diode samples the error voltage of the phase lock loop and biases the diode accordingly. The audio signal is converted to a current which in turn is used to modulate the bias current through the diode. The resulting AC voltage across the diode is thereby amplitude compensated by the bias current to approximately match the nonlinear characteristics of the VCO at each frequency setting of the phase-locked loop.

Albert Henry Hunt

Papers

Circuit for compensating vco nonlinearities in frequency modulated phase-locked loops and method thereof