Electronic automatic frequency tuning system

Abstract

Apparatus and method for providing an automatic frequency tuning system capable of supplying a local oscillator signal with a preselected frequency. The frequency of an output signal of the local oscillator is electronically varied across a frequency region which includes the preselected frequency. The output signal of the local oscillator is combined with the output signal of a reference signal generator. After the combination of the local oscillator signal with the reference oscillator signal produces a preestablished condition which is the result of the local oscillator signal attaining the preselected frequency, the frequency of the local oscillator signal is held constant. The local oscillator signal is thereafter compared to the reference oscillation signal and a change in the local oscillator signal frequency is corrected by means of an automatic frequency control feedback loop containing a discriminator circuit. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to apparatus for automatically tuning an oscillator to a preselected frequency, and more particularly to an automatic tuning circuit for use with a plurality of information-bearing frequency channels systematically dispersed throughout a range of frequencies. Upon identification of a condition resulting for the attainment of the preselected frequency by the oscillator output signal, an automatic frequency control (AFC) circuit is used to stabilize the frequency of the oscillator signal. 2. Description of the Prior Art An automatic frequency control circuit typically is comprised of a local oscillator, a signal mixing circuit such as a heterodyne conversion transducer, an amplifier tuned to a predetermined intermediate frequency and a discriminator circuit designed to operate about the predetermined intermediate frequency. The output signal of the discriminator controls the frequency of the output signal of the local oscillator. A received signal, for which demodulation is desired, is typically applied to the mixing circuit. The combination of the received signal and the local oscillator signal produces an intermediate frequency signal which is applied to the tuned amplifier. The output signal of the local oscillator is used to convert the received signal and the relationship between the local oscillator signal frequency and the received signal frequency is stabilized by the circuit. It is known in the prior art to provide an AFC circuit with mechanical apparatus for providing the coarse adjustment of the local oscillator. The coarse adjustment typically places the intermediate frequency signal within the operating range of the tuned amplifier and the discriminator circuit. The AFC circuit thereafter provides the vernier tuning for the satisfactory demodulation of the received signal. Because of problems associated with deterioration of mechanical apparatus, it is desirable to provide coarse adjustment by elctronic means. The characteristics of the tuned amplifier and more importantly, the discriminator circuit, provide the limit for the range over which a received signal can be stabilized and demodulated. Providing an expanded frequency range for these characteristics, for example to provide greater signal tuning capability, would increase the possibility of identifying and demodulating an undesired signal falling within the range of the amplifier and discriminator characteristics. Furthermore, in conventional AFC circuits, the loss of the received signal, which contributes to the intermediate frequency signal, can produce undesirable drift in the frequency of the local oscillator output signal. There are many applications, such as commercial broadcast television, where the received signal (i.e. the signal transmitted by the television station) is rigidly controlled. The AFC circuit originally was provided to minimize the effects on the operation of a television receiver of frequency shifts in the local oscillator. Shifts of the broadcast signals are unimportant, so that the necessity for utilizing the received signal in the AFC loop is minimized. It is desirable, however, to provide for control of the local oscillator by the received signal, the reference signal controlling the local oscillator output signal frequency in the absence of the received signal. It is therefore an object of the present invention to provide an improved automatic frequency tuning circuit. It is another object of the present invention to provide an automatic frequency tuning circuit capable of electronic coarse adjustment over a preestablished frequency spectrum. It is a further object of the present invention to provide an automatic electronic frequency tuning circuit for information-bearing channels in a wide frequency range without compromising fine tuning capability. It is yet another object of the present invention to provide apparatus for stabilizing a local oscillator output signal by means of an AFC circuit during temporary loss of received signal. It is a more particular object of the present invention to provide a voltage controlled oscillator and a controllable voltage source in an automatic frequency tuning circuit for sweeping the frequency of the oscillator signal across a frequency spectrum in which at least one desired information-bearing frequency band is located. It is still another particular object of the present invention to provide apparatus for suspending scanning over a range of frequencies by an oscillator signal in an AFC circuit at a frequency determined by a reference frequency signal. It is yet another particular object of the present invention to provide apparatus for comparing a signal having a varying frequency with a reference frequency signal in order to identify a predetermined frequency of the varying signal. It is yet another object of the present invention to provide an electronic automatic frequency tuning circuit with a reference freqeuncy signal, which maintains a substantially constant oscillator frequency during loss of the received signal. It is yet a further object of the present invention to provide an electronic automatic frequency tuning circuit having a first feedback loop for identifying a predetermined oscillator signal frequency and a second feedback loop for maintaining the predetermined oscillator signal. It is a still further object of the present invention to provide an electronic automatic frequency tuning circuit which, upon entry of data, can identify and maintain an oscillator signal having a frequency determined by the entered data. It is yet another particular object of the present invention to provide an AFC circuit utilizing a received signal for control of the local oscillator, the local oscillator controlled by a reference signal in the absence of the received signal. It is another object of the present invention to provide for relatively rapid response for circuits controlling scanning of a frequency region by the output signal of a local oscillator, while providing a relatively slow response for stabilization of signals stabilizing the frequency of the local oscillator output signal. It is yet another object of the present invention to provide an amplifier permitting the same discriminator circuit to be used in an AFC circuit with a beat frequency signal above and with a beat frequency signal below the frequency of the reference signal generator being utilized by the AFC circuit. SUMMARY OF THE INVENTION The aforementioned and other objects are accomplished, according to the present invention, by an electronic automatic frequency tuning circuit comprising two sets of apparatus for controlling the signal of variable frequency local oscillator. The first set of apparatus varies the frequency of the local oscillator until a predetermined relationship between the local oscillator signal and a reference generator signal is identified. Upon identification of the predetermined relationship, the second set of apparatus stabilizes the frequency of the oscillator output signal at substantially constant value. To stabilize the frequency of the oscillator signal, the reference generator signal and the local oscillator signal are combined and a resulting difference frequency signal applied to a discriminator. The discriminator provides an output signal which applies a control signal to the oscillator resulting in oscillator signal frequency corrections compensating for changes in the oscillator signal frequency. The automatic frequency tuning circuit is not dependent on the received signal, the signal for which demodulation is typically desired, and therefore a loss of the received signal does not affect the frequency of the oscillator signal. However, in the presence of the received signal, an AFC circuit utilizing the received signal controls the frequency of the local oscillator output signal. The reference oscillator can be apparatus for producing a comb of harmonic frequencies. By varying the controllable oscillator signal frequency beginning at a known initial value, the number of signals resulting from a preselected relationship, e.g. predetermined beat frequencies, occurring between the reference oscillator signal and the variable oscillator can be used to determine the predetermined relationship. Apparatus is provided for increasing and for decreasing the frequency of the local oscillator. In the AFC circuit utilizing the oscillator and generator signals, an amplifier is utilized permitting a single discriminator circuit to be employed with either the beat frequency falling above or the beat frequency falling below the related reference generator component. These and other features of the invention will be understood upon reading of the following description along with the drawings.

Dependencies are inferred from the claim text.