Wide Frequency Range Current-Controlled Oscillator

The present invention relates to current-controlled oscillators, and more particularly, to a wide frequency range programmable current-controlled oscillator.

Current-controlled oscillators (CCO's) of the prior art have typically utilized a Schmitt trigger to control the charging and discharging currents to the CCO timing capacitor. For example, the Signetics 565, illustrated in the Signetics Analog Data Manual, published by Signetics Corporation, 1977 at pages 832 and 833, is basically comprised of a programmable current source and a Schmitt trigger coupled to a timing capacitor. Another current-controlled oscillator described in U.S. Pat. No. 3,659,224, uses two comparators for comparing the CCO timing capacitor voltage to a pair of independent reference voltages. Both of the foregoing CCO's require a relatively large number of electrical components and are limited to operating frequencies less than 500 KHz. The frequency range of a CCO may be increased somewhat by utilizing the techniques described in the article, "A Wide Range Voltage-Controlled Oscillator", by Phillip E. Allen and William J. Parrish, published in the IEEE transactions on Instrumentation and Measurement, VOL. IM-24, No. 3, September 1975 at pages 253-261.

It is a general object of the present invention to provide an improved wide frequency range current-controlled oscillator that is operable at relatively high frequencies.

It is another object of the present invention to provide an improved current-controlled oscillator that is readily adaptable to integrated circuit fabrication.

It is a further object of the present invention to provide an improved current-controlled oscillator having a programmable operating frequency and readily controllable duty cycle.

In practicing the invention, a current-controlled oscillator includes a timing capacitor that is alternately charged and discharged to generate a digital output signal having a frequency controllable by an input control current. The current-controlled oscillator further includes a current source, a switchable current mirror, a differential voltage comparator and a switchable reference voltage circuit. The current source is coupled to the input control current for providing a first current having a predetermined first magnitude to the timing capacitor. The switchable current mirror is connected to the junction of the timing capacitor and current source for sinking a current having a magnitude related to the magnitude of the first current by a predetermined factor. The differential voltage comparator is differentially coupled to the timing capacitor and the switchable reference voltage circuit for differentially comparing the timing capacitor voltage to first and second reference voltages provided by the switchable reference voltage circuit. The differential comparator provides an output signal when the timing-capacitor voltage is greater than the first reference voltage from the switchable reference voltage circuit. In response to the differential-comparator output signal, the switchable reference voltage circuit couples the second reference voltage (less than the first reference voltage) to the differential voltage comparator, enables the switchable current mirror to discharge the timing capacitor and provides a predetermined first state of the digital output signal. The differential voltage comparator turns off and no longer provides the differential-comparator output signal when the timing-capacitor voltage discharges to substantially the same magnitude as the second reference voltage. In response to the absence of the differential-comparator output signal, the switchable voltage reference circuit couples the first reference voltage to the differential voltage comparator, disables the switchable current mirror to charge the timing capacitor and provides a predetermined second state of the digital output signal. The foregoing operation of the current-controlled oscillator is repeated to provide successive cycles of the digital output signal.

FIG. 1 illustrates a circuit diagram of a current-controlled oscillator embodying the present invention.

FIG. 2 illustrates waveforms of the digital output signal and the timing capacitor voltage of the current-controlled oscillator of FIG. 1.

In FIG. 1, there is illustrated a current-controlled oscillator (CCO) embodying the present invention. The CCO is operable over a relatively wide range of frequencies that may extend up to 1.5 MHz. The CCO is readily integrable onto an integrated circuit substrate using any suitable integration technique, such as, linear compatible I.sup.2 L techniques.

The CCO accepts an input control voltage at the base of transistor 120 and provides a digital output signal at the collector of transistor 114. The input control voltage is applied across resistor 121 by transistor 120 for establishing the magnitude of the control current I.sub.1. The control current I.sub.1 is applied to current mirror transistors 101 to 106 for providing dependent currents to the circuitry of the CCO. The geometry of the devices is scaled such that the collector currents from transistors 103-106 are twice the magnitude of control current I.sub.1. The CCO further includes a timing capacitor 107, a differential comparator 108-109, a switchable voltage reference circuit 110-117 and a switchable current mirror 118-119. The waveforms of the timing capacitor voltage V.sub.c and the digital output signal are illustrated in FIG. 2. In the preferred embodiment of the CCO, the timing capacitor 107 is symmetrically charged and discharged between voltage levels V.sub.1 and V.sub.2, respectively, by essentially equal magnitude currents 2I.sub.1, resulting in a 50% duty cycle on the digital output signal. Further more, the duty cycle of the digital output signal may be altered to any desired duty cycle by appropriately adjusting the magnitude of the discharging current provided by current mirror 118-119.

The CCO may be operated from any suitable DC supply voltage which may be as low as 3 VDC and as large as the rated breakdown voltage of the transistor devices. The timing capacitor 107 is charged by current 2I.sub.1 provided by transistor 103 until voltage level V1 is reached, at which time the voltage reference circuit 110-117 switches the voltage level from V.sub.1 to V.sub.2 and enables current mirror 118-119 to sink a current 4I.sub.1. The enabled current mirror 118-119 sinks the current 2I.sub.1 from transistor 103 and a current 2I.sub.1 from the timing capacitor 107. Thus, the timing capacitor 107 is charged and discharged by a current 2I.sub.1, resulting in the essentially symmetrical, saw-toothed voltage waveform and corresponding digital output signal illustrated in FIG. 2.

Transistors 108 and 109 form a differential voltage comparator that is fed by a current 3I.sub.1 from transistors 102 and 104. The timing capacitor voltage V.sub.c at the base of transistor 108 is referenced to the voltage at the base of transistor 109. When the timing capacitor 107 is charging, transistor 108 is on and transistor 109 is off. The timing capacitor 107 continues to charge until the timing capacitor voltage V.sub.c reaches the voltage level V.sub.1, formed by diode 110, the base-emitter junction of transistor 111 and the base-emitter junction of transistor 114. However, until the timing capacitor voltage V.sub.c reaches the voltage level V.sub.1, the current 2I.sub.1 provided by transistor 105 is coupled by way of diode 110 to transistors 111 and 114. Thus, transistor 111 is turned on to sink the current 2I.sub.1 provided by transistor 106 and to keep switchable current mirror 118-119 turned off. Transistor 114 is also turned on to provide a logic low state (low voltage) on the digital output signal.

When the timing capacitor voltage V.sub.c reaches the voltage level V.sub.1, transistor 108 turns off and transistor 109 turns on. The turning on of transistor 109 enables the current mirror 115-116. Transistor 116 of the enabled current mirror 115-116 sinks the current 2I.sub.1 from transistor 105 by way of diode 110, turning off transistors 111 and 114. Transistor 117 sinks the stored charge from the base of transistor 114 to insure that transistor 114 turns off rapidly. In the off state, transistor 114 provides a logic high state (high voltage +V) on the digital output signal. Furthermore, since transistor 111 is turned off, the current 2I.sub.1 from transistor 106 is applied to current mirror 118-119 via diode 112. Transistor 118 of the enabled current mirror 118-119 sinks a current 4I.sub.1 for discharging the timing capacitor 107.

When the timing capacitor voltage VC reaches the voltage level V2, formed by diode 110 and the collector to emitter saturation voltage of transistor 116, transistor 109 turns off and transistor 108 turns on again and the timing capacitor 107 begins charging. The foregoing operation is repeated during successive charging and discharging cycles of the CCO. Since the switching between charging and discharging cycles is controlled by the differential voltage comparator 108-109, the switching time is greatly decreased over the switching time of oscillators utilizing Schmitt triggers. Since the switching times are much faster than prior art oscillators, the CCO of the present invention may operate at much higher frequencies than prior art oscillators. In addition, all of the current mirror transistors 101 to 106 provide currents that are a function of the current I.sub.1, and therefore the CCO is more tolerant to transistor device variations and provides more symmetrical waveforms.

The frequency of operation of the CCO is varied in response to the input control voltage at the base of transistor 120. Variations of the input control voltage are applied across resistor 121 and directly vary the control current I.sub.1. The varying of the control current I.sub.1 varies the charge and discharge current to the timing capacitor 107, which provides a corresponding adjustment to the oscillating frequency of the CCO. The frequency F of operation of the CCO may be expressed by the following equation:

where C is the magnitude of the timing capacitor 107.

Although the CCO has been shown with an input control voltage at transistor 120, the CCO may also be configured to accept an input control current I.sub.1 directly at transistor 101, thus eliminating transistor 120 and resistor 121. The CCO of the present invention may be advantageously utilized in applications, such as portable radios, where it is desirable to minimize the number of circuit elements without degrading performance characteristics.