Abstract:
A differential oscillator is supplied with a constant total current. A signal-controlled single-ended shunt circuit bypasses current around the oscillator thus varying the tail current oppositely. This in turn varies the signal delay in the oscillator transistors and hence oscillator frequency.

Description:
BACKGROUND OF THE INVENTION 
     Voltage Controlled Oscillators (VCOs) are useful in Phase Locked Loop (PLL) circuits in which an oscillator responding to an error voltage is driven to reduce the error to minimum or zero. Typically, a phase detector is coupled to the oscillator which is driven to provide a particular signal phase. Such oscillators in the past have been rather complicated and required a large number of parts. Furthermore, their temperature drift is typically complex and uncompensated. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to simplify and reduce the parts count in a VCO. 
     It is a further object of the invention to provide single ended control of a VCO. 
     It is a still further object of the invention to temperature compensate a VCO. 
     It is a still further object of the invention to limit the signal response in a single ended control VCO. 
     These and other objects are achieved in a circuit that employs a differential oscillator in which the tail current is varied to change oscillator frequency. This can be done in an oscillator stage employing cross-connected transistors that are driven into saturation by the oscillation signal. In a saturated transistor the signal delay is a function of the operating current. The greater the current, the greater the delay and the lower the freqency of oscillation. Such an oscillator is operated where minority carrier storage amounts to an appreciable fraction of the oscillation cycle. Using the conventional NPN transistor in Integrated Circuit (IC) configurations, such oscillators are feasible in the 5 MHz to 200 MHz region. 
     According to the invention, a single ended control circuit shunts current around the oscillator transistors. Due to the constant circuit current the tail current varies in the opposite direction, thus changing oscillator frequency. A limiter circuit avoids excessive shunt current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a prior art VCO; and 
     FIG. 2 is a schematic diagram of the circuit of the invention. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     With reference to FIG. 1 a prior art VCO circuit is shown. A power supply connected between terminals 10 and 11 operates the circuit. The heart of the circuit is a differential oscillator 11. Transistors 11a and 11b have balanced load resistors 14 and 15. Inductor 13 and capacitor 12 comprise a frequency determining antiresonant tank. The transistor bases are cross coupled to complete the oscillator circuit. The transistors are operated in a frequency regime where they introduce appreciable signal delay. Thus, the oscillation frequency is a function of transistor delay which can be varied as a function of current. 
     Transistor 16 which is biased from a constant voltage source 26 supplies a fixed minimum current to oscillator 11. This current will be set by resistor 21. 
     Differential amplifier 17, which includes transistors 17a thru 17d, provides a variable oscillator current. Transistor 17a supplies a variable tail current to oscillator 11 and transistor 17b shunts current around oscillator 11. Differential amplifier 17 is supplied with tail current from transistor 18 at a level set by resistor 22. Resistors 19 and 20 degenerate differential amplifier 17. Transistors 17c and 17d are directly coupled emitter follower stages that drive transistors 17a and 17b respectively. Resistor 25 provides tail current to transistors 17c and 17d while resistors 23 and 24 degenerate them. 
     Transistor 27, as biased by resistors 28 and 29, provides a reference voltage at its emitter across resistor 31 for differential amplifier 17. Resistor 30 couples the reference voltage to the base of transistor 17d. Resistors 32 and 33 couple the reference voltage to the base of transistor 17c whereby the differential amplifier 17 is balanced at zero input. 
     Resistors 33 and 35 along with capacitor 34 provides low pass input filter action. The circuit input is operated by control circuits in block 36. 
     In operation, the input from block 36 produces current changes in tail current transistor 17a and shunting transistor 17b in opposite directions. As the tail current rises, the shunt current decreases and the oscillator frequency decreases due to increased delay in oscillator transistors 11a and 11b. 
     DESCRIPTION OF THE INVENTION 
     FIG. 2 shows the circuit of the invention which is intended for IC implementation. Where circuit elements are like those of FIG. 1, similar part numbers are employed. The oscillator 11 is the same as that of FIG. 1. However, transistor 39 sets the oscillator tail current while transistor 38 shunts current around the oscillator. 
     A constant voltage, typically about 1.7 volts, is applied to the base of transistor 39 and resistor 41 determines the total current. Resistor 40 determines the value of current shunted around oscillator 11. 
     Transistor 42 is an emitter follower with load resistor 43 and is directly coupled to the base of transistor 38. Control circuits 36 operate the input with zener diode 44 limiting the maximum positive excursion. Normally diode 44 is biased below its zener level by resistors 45 and 46, which act as a voltage divider providing a quiescent input voltage to said diode. The diode having a zener voltage greater than the quiescent input voltage and less than the supply voltage to the oscillator. Resistor 48 and capacitor 47 provide the low pass input filter action. Components 45 through 48 are normally exterior to the IC. 
     Input variations are directly coupled via emitter follower 42 to the base of transistor 38. This directly varies the shunt oscillator current and, due to common resistor 41, varies the oscillator tail current in the opposite direction. The current flowing in resistor 41 is relatively constant because the voltage across it is one V BE  below the value of source 26. 
     As the input voltage rises, the drop across resistor 40 increases. Thus, more current is shunted around the oscillator and less tail current flows. For this condition the oscillator frequency increases. As the input voltage falls, less current flows in transistor 38 and the shunt current decreases. This increases oscillator tail current and decreases its frequency. 
     Such an oscillator is subject to frequency drift with temperature. For example, in the basic oscillator as temperature increases the V BE  of transistor 39 decreases. This pulls the emitter toward the level set by source 26. This in turn produces more tail current and a frequency decrease. In addition, the signal delay in the oscillator tends to increase with temperature, which further decreases the oscillation frequency. In the circuit shown, the V BE  values of transistors 38 and 42 both decrease, thereby increasing the oscillator shunt current. This temperature drift is opposite to that of the oscillator, thus providing temperature compensation. 
     EXAMPLE 
     The circuit of FIG. 2 was constructed using typical IC NPN transistors. The following component values were used. 
     
         ______________________________________COMPONENT            VALUE______________________________________Resistors 14 and 15  750 ohmsResistor 40          1.6K ohmsResistor 41          285 ohmsSource 26            1.7 voltsResistor 43          10K ohmsDiode 44             5.6V zenerResistor 45          62K ohmsResistor 46          30K ohmsCapacitor 47         0.22 microfaradResistor 48          100 ohms______________________________________ 
    
     Parts 45 thru 48 were designed to be off-chip parts. The supply at terminal 10 was +6 volts with respect to ground. Resistor 45 was returned to a 12-volt external supply. Tank circuit components 12 and 13 were selected to resonate at 46 MHz. Varying the input voltage at the base of transistor 42 from a nominal 4 volts produced oscillator current variations at the emitters of transistors 11a and 11b from 1.5 milliamperes to 3.5 milliamperes. This produced a frequency change, ΔF, of about 4 MHz. 
     While the prior art implementation requires sixteen resistors and nine transistors, the circuit of the invention employs eight resistors and five transistors. Thus, the invention saves four transistors and eight resistors. This will be of substantial benefit in IC construction. 
     Additionally, the prior art circuit requires a higher operating voltage which is undesirable. 
     The invention has been described and a working example given. Clearly, when a person skilled in the art reads the foregoing, alternatives and equivalents within the spirit and intent of the invention will occur to him. Accordingly, it is intended that the scope of the invention be limited only by the following claims.