Variable phase shift apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to phase shift apparatus. More particularly, it is concerned with phase shift apparatus in which the amount of phase shift can be varied. 
     Variable phase shift apparatus of different types which are well-known are useful in many applications. One particular application requiring apparatus with variable phase shifting capability is in the display of graphical information on color television sets. In order to provide different colors throughout the display the color subcarrier frequency must be shifted in phase. It is desirable that the phase shift be variable in response to digital signals accompanying the information to be displayed. Although variable phase shift apparatus has been available for use in this particular application, because of the frequency involved (3.58 MHz is the standard color television subcarrier frequency), it has heretofore been necessary to employ high performance, expensive, components. 
     SUMMARY OF THE INVENTION 
     Variable phase shift apparatus in accordance with the present invention is relatively simple and inexpensive. The apparatus includes an input terminal for receiving an input signal. The apparatus also includes a first transistor means and a second transistor means. A phase control means is connected to the input terminal and produces a phase control signal of variable magnitude. The first transistor means is connected to the input terminal and to the phase control means and produces a first signal in phase with the difference between the input signal and the phase control signal. The first signal increases in magnitude as the magnitude of the phase control signal decreases, and decreases in magnitude as the magnitude of the phase control signal increases. The second transistor means is connected to the phase control means and produces a second signal at 180° phase shift with respect to the phase control signal. The second signal increases in magnitude as the magnitude of the phase control signal increases, and decreases in magnitude as the magnitude of the phase control signal decreases. An output means is connected to the first and to the second transistor means and combines the first and second signals to produce an output signal which is the resultant of the first and second signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing is a schematic circuit diagram of a variable phase shift apparatus in accordance with the present invention. 
     For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above described drawing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A variable phase shift apparatus in accordance with the present invention as illustrated in the single FIGURE of the drawing has input terminals 11 and 12 across which a continuous wave input signal is applied. Input terminal 12 is connected directly to ground. Input terminal 11 is connected through a coupling capacitance C2 and a resistance R1 to the emitter of an NPN bipolar transistor Q1. The collector of transistor Q1 is connected through a load resistance R2 to a positive voltage source +V. The output of the apparatus appears across an output terminal 13 which is connected to the collector of transistor Q1 and an output terminal 14 which is connected to ground. 
     The input terminal 11 is connected through a series arrangement of a capacitance C1 and a variable resistance Rx to ground. The juncture between the capacitance C1 and resistance Rx is connected directly to the base of transistor Q1. The base of a second NPN bipolar transistor Q2 is also connected directly to the juncture of capacitance C1 and resistance Rx. Its collector is connected directly to the collector of transistor Q1 and to the output terminal 13. The emitter is biased by a connection through two resistances R3 and R5 in series to a negative voltage source -V. The juncture between resistances R3 and R5 is connected to ground through an AC bypass capacitance C3. The emitter of transistor Q1 is connected through a resistance R4 to the negative voltage source -V to provide biasing. 
     An AC analysis of the apparatus as shown may be expressed by the following equations. ##EQU1## In the foregoing equations R is the resistance of resistance R2 which in this instance is equal to resistances R1 and R3 in order that the amplitude of the output signal e o  is equal to that of the input signal e in . C is the capacitance of capacitance C1, Rx is the resistance of resistance Rx, and ω is the frequency of the input signal. 
     Capacitance C1 and resistance Rx in series serve as a phase control arrangement. The input signal e in  produces a phase control signal e i  across the resistance Rx as shown by equation 2. The magnitude of voltage e i  varies with the resistance of resistance Rx in accordance with equation 2. 
     Transistor Q1 receives the input signal e in  at its emitter and the phase control voltage e i  at its base. The resulting output current i 1  as expressed by equation 3 is in phase with its magnitude related to the difference between the phase control voltage e i  and the input voltage e in . 
     The phase control voltage e i  is also applied to the base of transistor Q2. The resulting output current i 2  as expressed by equation 4 is of opposite polarity or at 180° phase shift with respect to the phase control voltage e i . Its magnitude is directly related to the phase control voltage e i . 
     As shown by equation 1 the two collector currents i 1  and i 2  both of which flow through the load resistance R2 combine to produce a resultant output voltage e o  across the output terminals 13 and 14. With resistances R1, R2, and R3 equal and the amplitude of e o  equal to e in  equations 6 and 7 express the relationship of e o  to e in . The phase angle between e o  and e in  is expressed in equation 7. As shown by equation 7 with a fixed value of capacitance C for capacitance C1, the phase is variable as a function of the variable resistance Rx. 
     In the circuit as shown and described the phase control arrangement of capacitance C1 and resistance Rx is connected between the input terminal 11 and ground with the resistance connected between the bases of transistors Q1 and Q2 and ground. By virtue of the connection of the resistance Rx to a point of fixed potential, that is ground, the variable resistance Rx may be provided by a set of resistances each in series with a solid state switching element connected in parallel between the bases of transistors Q1 and Q2 and ground. Digital signals appropriately decoded can be applied to the switching elements selectively to close and open individual switching elements, thus varying the value of resistance Rx and consequently the amount of phase shift. Phase shift angles greater than 180° may be obtained by employing phase shifting apparatus as shown together with an inverter to provide an additional 180° phase shift. 
     In a specific embodiment for providing 0° to 180° phase shift for a continuous wave input signal at the standard color television subcarrier frequency of 3.58 MHz, the following components were employed: 
     Q1: 2N3904 NPN Transistor 
     Q2: 2N3904 NPN Transistor 
     R1: 470Ω 
     R2: 470Ω 
     R3: 470Ω 
     R4: 2.7 kΩ 
     R5: 2.2 kΩ 
     C1: 50 pf 
     C2: 0.01 μf 
     C3: 0.01 μf 
     +V: 12 volts 
     -V: -12 volts 
     The phase angle of the output signal e o  with respect to the input signal e in  may be selectively varied from 0° to 180° by changing the value of resistance Rx over a range from 100Ω to 10 kΩ. 
     While there has been shown and described what is considered to be a preferred embodiment of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.