This invention relates to the field of phase and frequency modulation of a carrier for the transmission of information by cable or radiation or other transmission media.
Phase and frequency modulation are related techniques which have been employed in the signal communications art for many years. Phase and frequency modulation are closely related concepts but are clearly distinguished in a theoretical or rigorous discussion of modulation systems. The textbook "Information Transmission, Modulation, and Noise" authored by Mischa Schwartz and published by McGraw Hill Book Company Incorporated, in 1959, classifies phase modulation and frequency modulation to each be special cases of angle modulation; see chapter 3, section 7, page 113. This relationship is further discussed in the book "Modulation Theory" authored by H. S. Black and published by D. Van Nostrand Co., Incorporated, 1953; see Chapters 3 and 12. The material of the Schwartz and Black books is hereby incorporated by reference into the present specification.
The prior patent art includes several examples of modulating apparatus which operate according to the principles of phase modulation, frequency modulation and phase shift keying modulation. These patents include the following examples from the U.S. patent files.
The patent of Eric B. C. Khu, U.S. Pat. No. 3,054,971 discloses a frequency shift oscillator which provides for the switching of resistive elements in an RC network to produce modified feedback and frequency determination in an oscillator circuit. The Khu patent comtemplates the use of differing components and differing phase shift in a RC network in order to achieve a pair of oscillator output frequencies.
The patent of M. Weinberg, U.S. Pat. No. 3,068,427 discloses an arrangement for achieving frequency modulation in a crystal controlled oscillator through the use of voltage controllable capacitance elements. In the Weinberg apparatus a modulating signal moves the operating point of a semiconductor diode along a characteristic curve in order that varying diode capacitance and reactance be achieved in a tuned circuit. The intent of the Weinberg patent is to produce a small frequency variation in the operating point of a piezoelectric crystal in order that frequency modulation be achieved with a crystal controlled oscillator.
The patent of B. Garland, U.S. Pat. No. 3,506,784 discloses a frequency shift keying arrangement wherein oscillating signals of two different frequencies are achieved by transistor switching of a capacitor in a tuned circuit. The Garland apparatus is principally intended for use in the telegraph art and is concerned with the components used to embody a two frequency oscillator circuit.
The patent of O. E. Rittenbach, U.S. Pat. No. 3,585,503, concerns a phase modulation arrangement for transmitting binary information by selecting between two possible output signals having a precise predetermined phase relationship. The Rittenbach '503 apparatus employs a balanced modulator circuit and phase reversal that occurs over a finite time in order that the harmonic frequencies generated during the phase reversal transient be limited. The Rittenbach '503 apparatus transmits a signal which is composed of switch selected portions of waveforms taken from four phase and frequency related sources in order to achieve smooth phase reversal in the output wave.
The additional patent of O. E. Rittenbach, U.S. Pat. No. 3,611,147, discloses a phase modulated binary data transmission system which uses a variable frequency oscillator shifted between two operating frequencies in what has recently been called a minimum shift keying arrangement to change the phase relationship between the oscillator output signal and a reference signal. The Rittenback '147 patent contemplates the use of a kylstron oscillator and also includes a frequency controlling feedback network for maintaining the frequency stability of the kylstron oscillator. The waveforms h and j in FIG. 2 of the Rittenbach '147 patent illustrate the cumulative changing of output signal phase which occurs in response to pulse controlled periods of operation at the second klystron operating frequency.
The Rittenback '147 patent discloses the use of only two phase relations, zero and .pi. radians, between the modulated carrier and the reference signal and contemplates the occurrence of carrier phase shift over the entire duration of the controlling digital waveform. In the Rittenback FIG. 2 drawing the changing of phase shown in line J occurs continuously during the control signal which is shown in line H. The Rittenback patent, therefore, stresses slowly altering the phase by dwelling for equal times in the two differing output frequencies.
The patent of Shui Yuan, U.S. Pat. No. 3,909,748, discloses the use of electrically variable semiconductor capacitance element such as metal-insulator-semiconductor (MIS) diodes for controlling the resonant frequency of an oscillator tank circuit in response to the output of a binary data register. The Yuan patent is concerned with improvement over the use of a varactor diode as a frequency control element. According to one aspect of the Yuan patent, four frequency control MIS diodes are arranged in weighting networks having the one, two, four, eight binary coded decimal capacitance values.
The patent of Itsuo Atkazawa, U.S. Pat. No. 4,303,894, discloses a frequency modulating arrangement which employs a transistor as a variable capacitance element. One aspect of the Akazawa patent is the attainment of increased junction capacitance in the modulating transistor by way of the particular circuit employed.