A variable capacitance circuit includes a varactor having an anode side and a cathode side. A first variable bias voltage is applied to one of the sides and one of a plurality of voltages is applied as a second bias voltage to the other side for controlling the capacitance of the varactor. A voltage multiplier circuit connected to a voltage divider network is used for supplying the plurality of voltages. A decoder is responsive to input signals for selecting and applying one of the multiple voltage outputs. The variable capacitance circuit is used in a voltage controlled oscillator of a frequency synthesizer for providing extended frequency range.

Varactor tuning circuit having plural selectable bias voltages

There is described a signal generator capable of being tuned over a relatively high frequency range. The generator includes a VCO which is controlled in frequency by a phase locked loop. The loop operates to compare a reference frequency from an accurate crystal source with the divided oscillator frequency. The division is afforded by a first frequency counter which controls a programmable divider to cause the same to divide by the correct integer independent of the oscillator tuning. The phase locked loop includes a phase detector responsive to digital waveforms to provide a dual polarity error control signal by combining outputs of the phase detector in a differential operational amplifier. The operational amplifier is further controlled to cancel an error offset voltage so that the final output signal is free from spurious modulation products.

Signal generators employing digital phase locked loops and compensating circuits

The invention provides a method and apparatus for automatically controlling the local-oscillator frequency in a superheterodyne receiving section of a sound reproduction system through the use of a digital a.f.c. circuit. A frequency is derived from the local oscillator frequency and compared with a modulator frequency. The discriminator forms a dc voltage corresponding to the difference between the local oscillator frequency and the discriminator frequency. A varactor in the local oscillator is controlled with the dc voltage such that the local oscillator frequency varies opposite the direction of control of the dc voltage.

Digital automatic frequency control loop for a local oscillator

Apparatus for determining whether a movable body, such as a railway car traveling along a track, is present in a preselected zone of interest along its path of travel. In railway applications for detecting the presence of a car over a switch, for example, an inductive loop is laid along the track and forms a part of the tank circuit of an oscillator. The resulting signal is mixed with the output signal from a variable frequency reference oscillator to provide an intermediate frequency signal that is subject to frequency variations as the loop inductance changes. To correct for such variations, an automatic digital tuning arrangement is provided for the reference oscillator to maintain the IF signal at a normal frequency within a predetermined range within which the intermediate frequency is permitted to vary in response to changing environmental conditions. The response of the digital tuner is sufficiently rapid to correct gradual variations in frequency characteristic of changing environmental conditions, but is purposely limited in order to be incapable of making the correction in the event of a sudden frequency variation caused by the presence of a car. When this occurs, the intermediate frequency shifts outside of the normal range and the apparatus assumes an output condition indicative of the presence of the car. Once the car is detected, the automatic tuner is disabled to positively preclude frequency correction until the car is no longer sensed by the loop.

Presence detector having automatic digital tuning

Frequency control apparatus for an oscillator includes a first up-counter and a down-counter. A number representative of an oscillator''s frequency is stored in the first counter during a given interval and then transferred to said down-counter. The down-counter is then caused to count down said frequency during the same interval. At the termination of this interval, the downcounter has stored therein a count manifesting the variation of the oscillator''s frequency, which count is detected and processed to provide a control signal used to vary the oscillator''s frequency in a direction to tend to cause the down-counter to indicate all zeroes at the end of said interval.

Edmund I Schwartz

Papers

Oscillator with digital counter frequency control circuits