Abstract:
A microwave modulator especially adapted for use in a digital microwave transmitter and capable of operation over a broad range of data rates and carrier frequencies. The modulator is operative with a multiple level modulation signal having zero average amplitude and provides in response thereto a multiple phase coded signal having zero average phase. A plurality of microwave diodes are disposed in spaced relation along a transmission line and are operative in response to control inputs derived from the modulation signal as signal reflection means to phase modulate the microwave carrier signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to microwave transmission systems and more particularly to phase modulators for a digital microwave transmitter. 
     BACKGROUND OF THE INVENTION 
     Data communication systems are known for the conveyance of digital information over a communication path such as a microwave of cable path. Digital information is generally provided to the system at a predetermined data rate, and the system is synchronized to and operative only at the selected rate to transmit the digital signal over the communication path and to recover the digital data at the receiving end. In order to provide synchronization with a particular data rate, appropriate clock or timing apparatus must be provided. In addition, synchronous encoding apparatus must often be provided to translate an input signal to a form suitable for transmission, with similar decoding apparatus provided for translating a received data signal to an intended output format. In known systems operative only at a specified clock rate, a change in the data rate would necessitate physical modification of the apparatus to be operative at the different rate. 
     A digital communication system capable of operation over a wide range of data rates without system modification is the subject of a copending application Ser. No. 545,790 in the names of Harvey L. Pastan and Arthur H. Solomon, filed of even date herewith and the disclosure of which is incorporated herein by reference. The present invention relates to a phase modulator especially adapted for use in the system of this copending application, although the invention is not limited to such use. 
     Phase modulators operative at microwave and other frequencies are well known in the art but in general are limited to use at a single carrier frequency or frequency band. In order to accommodate different carrier frequencies, the modulator structure must usually be modified to account for changes in impedance and wavelength at the intended operating frequency. One known modulator is shown in U.S. Pat. No. 3,506,930 wherein a phase shifting network is employed having a plurality of diodes and capacitors spaced along a transmission line. The diodes are selectively rendered conductive and non-conductive in response to control signals derived from the conductive diodes providing short circuits for a modulation signal, reflection of the carrier wave and consequent phase modulation thereof. The disclosure of the aforementioned patent contemplates the use of ideal diode switches which are either substantially conductive or non-conductive and which are not readily realizable in practice, especially at microwave frequencies. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention a phase modulator is provided which is especially adapted for use with a digital microwave transmitter and which is capable of operation over a broad range of data rates and carrier frequencies within an intended system bandwidth. The novel modulator employs as a modulation input a multiple level signal having zero or substantially zero average amplitude. In preferred embodiment, the input signal can be a T-carrier signal which is itself of well-known signal format having three amplitude levels with a time average of zero amplitude. The presence of a pulse, which may be either positive or negative, represents one binary state, while the absence of a pulse represents the opposite binary signal state. The phase modulator is operative in response to a T-carrier or similar input signal to provide a multiple phase signal having phase states directly corresponding to the amplitude states of the input signal. 
     The modulator includes a plurality of microwave diodes such as Schottky diodes disposed at selected spaced positions along a transmission line and having predetermined electrical separation determined by a dielectric phase shifter disposed therebetween. A digital encoder is coupled to the diodes to provide in response to a multiple level input signal respective bias signals in accordance with the amplitude states of the modulating signal. The diodes function as bias controlled variable impedances to provide reflection of the carrier signal in accordance with the equivalent complex impedance and electrical separation of the diodes which determine the relative phase of the reflected wave. The reflected wave is coupled to the modulator output as the phase modulated output signal having phase states directly corresponding to the amplitude states of the modulation signal. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagrammatic representation of a digital microwave transmitter employing a modulator according to the invention; 
     FIG. 2 is a plot of a modulation signal of a type with which the invention is employed; 
     FIG. 3 is a schematic representation of a modulator according to the invention; and 
     FIG. 4 is a cross-sectional view of a microwave modulator constructed according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A digital microwave transmitter employing the invention is shown in FIG. 1. The transmitter includes a digital encoder 28 which receives the multiple level input modulation signal and which provides a correspondingly coded digital control signal to a modulator 30 embodying the invention, which also receives a carrier signal from a master oscillator 14. The phase-modulated output signal from modulator 30 is applied to a bandpass filter 32, the output of which may be applied to a circulator 34 through which it is conveyed to an injection-locked oscillator 36, such as a Gunn-effect or other suitable oscillator. The oscillator 36 functions as a limiting amplifier to provide an output signal relatively free of amplitude modulation, and locks the frequency of the output signal thereof to the frequency of the injected signal. The output of the circulator 34, which is the phase-modulated carrier signal is amplified by a microwave amplifier 38 and propagated by microwave antenna 40. Alternatively, the output of modulator 30 may be applied directly to antenna 40 or otherwise directly propagated over a transmission path. 
     A signal of the type with which the invention can be employed is shown in FIG. 2 and is seen to be a bipolar signal having a 50% duty cycle. The illustrated signal format is known as a T-format or T-carrier signal and is widely employed in digital communications systems and is itself of well known signal form. The T-carrier communication format is more fully discussed in Transmission Systems for Communications, Fourth Edition, pp. 553- 565, published by Bell Telephone Laboratories, Incorporated. The signal is seen to have three levels or amplitude states, a zero or reference level, a positive unit amplitude level and a negative unit amplitude level. Information is coded by the presence or absence of signal pulses. Typically, the presence of a pulse, either positive or negative, represents data of one binary value, while the absence of a signal pulse represents data of opposite binary value. Signal pulses alternate in polarity such that a time average of essentially zero amplitude is maintained. The multilevel signal is transmitted as a phase-coded representation thereof and can be directly translated back to multilevel form without need for recovering clock or timing information. The signal employed with the invention may be of other multilevel forms, such as a two level or square wave signal, and need not exhibit a 50% duty cycle. The modulation signal may have any number of discrete amplitude levels, so long as a reference signal having zero average amplitude can be derived from the resultant phase coded signal. 
     A preferred embodiment of the modulator 30 is shown schematically in FIG. 3. Referring to FIG. 3, there is shown a three-port circulator 60 having a first port coupled to receive the RF input signal and a second port coupled via a transmission line 62 to the anode of a first Schottky diode 64, and thence through a dielectric phase shifter 66 to the anode of a second Schottky diode 68. The cathodes of diodes 64 and 68 are coupled through respective low pass filter networks 70 and 72 to digital encoder 28 which provides biasing control signals for the respective diodes. The filters 70 and 72 provide a high frequency ground return for the phase-modulated signals and couple the direct current control signals to the diodes. A direct current return path to ground is provided by an inductor 74 or other conductive link coupled between the center conductor of transmission line 62 and ground. The dielectric phase shifter 66 provides a predetermined electrical separation and therefore phase delay between diodes 64 and 68 in accordance with the carrier frequency and phase coding employed. The third port of circulator 60 provides the phase-modulated RF output. 
     The Schottky diodes function as bias voltage responsive variable impedance signal reflection means for modulating the phase of the microwave carrier signal introduced by way of circulator 60 into the transmission line 62. The digital encoder 28 received the bipolar modulator signal, such as the T-carrier signal described above, and provides appropriate bias control signals to diodes 64 and 68 according to the amplitude state of the modulating signal. The RF carrier signal undergoes reflection in accordance with the equivalent complex impedance and electrical separation of the diodes, which determine the relative phase of the reflected wave. The reflected wave is received at the second port of circulator 60 and is conveyed to the third or output port thereof to provide the phase-modulated output signal. In the illustrated embodiment employing a T-carrier signal, a phase-modulated signal is provided having three discrete phase states directly corresponding to the three amplitude states of the input signal. 
     With diode 64 forward biased by a control signal from encoder 28, a zero phase reference state is defined. This reference state may be defined in association with a selected biased state of diode 68. A positive phase state is provided with diode 64 reverse biased and diode 68 forward biased, while a negative phase state is provided with both diodes being reverse biased. In the illustrated embodiment, the phase states are selected to be +90° and -90° relative to a reference phase and corresponding to the amplitude states of the T-carrier signal as shown in FIG. 2. The modulator thus provides a phase-modulated output signal directly corresponding to the modulating input signal. It is comtemplated that other phase states either greater than or less than ±90° can be selected with equal facility. 
     The specific combination of phase states provided at the output of the novel modulator is established by three variables; namely, the complex impedance of the modulating diodes, the characteristic impedance of the transmission line to which the diodes are coupled and the electrical separation between the modulating diodes. In the present invention, the electrical separation between the modulating diodes is determined by a dielectric phase shifter, and it will be appreciated that different phase shifters have requisite dielectric characteristics can be employed in the modulator to accommodate different carrier frequencies without material modification of the remaining modulator structure. 
     A microwave phase modulator embodying the invention is shown in FIG. 4. A coaxial transmission line composed of an inner conductor 76 and a surrounding outer conductor 78 is coupled to the second port of circulator 60 (FIG. 3) by a suitable coaxial connector (not shown). A first diode package 65 is disposed in the position shown with its anode terminal 62 in electrical connection with inner conductor 76 and its cathode terminal 69 in electrical connection with a conductive member 71. The member 71 is disposed in an opening provided in outer conductor 78 and is insulated therefrom by means of an annular sleeve 73. A second diode package 81 is disposed at the end of the modulator structure as shown with anode terminal 83 connected to the end of inner conductor 76 and cathode terminal 85 connected to conductive member 87 which is disposed within an opening in end plate 80 and insulated therefrom by an insulative sleeve 91. The conductive members 71 and 87 serve as input terminals for the digital control signals applied to the respective diodes. The members 71 and 87 include respective air chokes 93 and 95 which can be of any well-known form and which provide RF coupling of the diodes to the outer conductor 78. A suitable DC return path is provided between conductors 76 and 78 such as by conductive element 74. 
     An annular cylinder 82 of dielectric material forms a core disposed in the space between conductors 76 and 78 and between diode packages 65 and 81 and is operative as a dielectric phase shifter to provide predetermined electrical separation between the microwave diodes at the selected carrier frequency. In the illustrated embodiment, the cylinder 82 is shown in contact with the inner surfaces of outer conductor 78, but it will be appreciated that the dielectric cylinder can be disposed in any convenient manner within the modulator structure. The cross-sectional fill factor and length of the dielectric cylinder 82 in conjunction with its dielectric characteristics are selected in a well-known manner to provide an intended electrical separation between the diodes and desired phase separation between states. The modulator may be adapted to operate at different carrier frequencies or to have different phase separations between selected states simply by replacing the dielectric cylinder 82 with another dielectric member. For a given set of diodes, the modulator structure need not otherwise be changed. 
     The phase-modulated output signal provided at the third terminal of circulator 60 can be directly applied to a transmitting antenna or other transmitting means for direct propagation to a receiving station. However, the phase-modulated output signal can be subject to amplitude variations between the several phase states of the modulated output signal. Such amplitude variations can be minimized by appropriate modulator design. As a further embodiment, means may be provided for eliminating amplitude differences among states of the modulated output. One such embodiment is illustrated in FIG. 1 wherein the output of the phase modulator 30 is provided through bandpass filter 32 and circulator 34 to injection-locked oscillator 36. It is characteristic of an injection-locked oscillator that within a range of the bandwidth to which the oscillator is capable of locking to an injected signal, saturation amplification occurs without significantly changing the phase of the input signal. Therefore, a multiple phase output signal can be provided substantially free of amplitude variations. In particular embodiment, a phase-modulated signal is directed through circulator 34 to a Gunn-effect oscillator operating in an injection-locked mode. 
     Various modifications and alternative implementations will become apparent to those versed in the art without departing from the true scope of the invention. Accordingly, it is not intended to limit the invention by what has been particularly shown and described, except as indicated in the appended claims.