Voltage or current controlled multivibrator oscillator circuit

An oscillator circuit comprises two main transistors (Q1, Q2), between which is provided a positive feedback by connecting each transistor base to the collector of the other transistor via buffer transistors (Q3, Q4). A capacitor (C) is connected between the emitters of the main transistors. The frequency of the oscillator is controlled by two current sources (11, 12), which control the current (I2, I2) flowing through the capacitor (C). Additionally, a compensation current (Icom) is conducted via collector resistors (Rc1, Rc2) of the main transistors such that the current flowing through each resistor is essentially constant and independent of the control current (I1, I2), so the signal amplitude of the oscillator does not change during frequency control.

FIELD OF THE INVENTION
 The invention relates generally to oscillator circuits, i.e. oscillators,
 and specifically to controllable oscillators based on multivibrators.
 BACKGROUND OF THE INVENTION
 Current- and voltage-controlled oscillators (ICO and VCO) are important
 components in the structures of transmitters and receivers. When
 applications to portable wireless communications systems are concerned,
 the main requirements for VCO/ICOs are: an operational frequency range of
 1 to 20 GHz, a very low phase noise and the lowest possible operating
 voltage and power consumption. Depending on the structure, a
 communications device may comprise several VCO/ICOs needed for different
 purposes, e.g. frequency conversion, synthetization, modulation, etc.
 Their performance affects strongly the performance of the entire
 communications unit. On the other hand, the demand to implement these
 oscillators for silicon technologies faces several problems.
 During the last few years several research projects have focused on finding
 optimal solutions. Two types of oscillators are mainly used as the cores
 of VCO/ICOs: sinusoidal oscillators and relaxation oscillators. Sinusoidal
 oscillators usually produce the best parameters as far as high frequency
 and low phase noise are concerned, but they can be easily implemented
 mostly on GaAS technologies only. A transition to bipolar, CMOS or BiCMOS
 technologies causes several problems mainly due to the highly conductive
 substrate. On the other hand, the speed of such available technologies is
 a challenge to researchers, as at present transient frequencies of 10 to
 40 GHz are reached, which was previously considered to be a transient
 range possible to be covered only by materials based on GaAS. The speed of
 silicon-based technologies is sufficient enough for mobile communication
 in the frequency range of 1 to 20 GHz, used by most mobile stations and
 wireless LANs. An additional driving factor in the design of portable
 equipments has always been a high demand for as low an operating voltage
 as possible and a very low power consumption.
 In oscillators of LC type, the active circuit components are kept out of
 the non-linear operation range, whereas in relaxation oscillators, the
 sinusoidal signal is the result of the incapability of the pulse circuit
 to switch fast enough at very high frequencies.
 Oscillator circuits, i.e. oscillators, can be implemented by many different
 circuit structures. One of them is an astable (free-running)
 multivibrator. FIG. 1 shows a conventional emitter-coupled multivibrator
 circuit, which has been used for implementing voltage-controlled
 oscillators (VCO). The circuit comprises two transistors Q1 and Q2,
 between which is provided a positive feedback by connecting each
 transistor collector via a buffer transistor Q3, Q4 to control the base of
 the other transistor. The collectors of Q1 and Q2 are connected via
 resistors Rc1 and Rc2, respectively, to one potential of an operating
 voltage source 1 and the emitters are connected via current sources 3 and
 4, respectively, to the lower potential of the operating voltage source.
 Correspondingly, the emitters of the buffer transistors Q3 and Q4 are
 connected via current sources 5 and 6 to the lower potential.
 Additionally, a reference capacitor C is connected between the emitters of
 Ql and Q2. The positive feedback and series resonance circuits Rc1-C and
 Rc2-C constituted by the resistors RC1 and RC2 and the capacitance C lead
 to that the output of the multivibrator oscillates continuously
 between-two states, after the oscillation once has been trigged. The
 oscillation frequency is determined by the component values of the RC
 series resonance circuits. The oscillation frequency can be controlled by
 changing some of these component values, typically the capacitance C.
 In the following, the operation of the multivibrator will be examined
 closer. To begin with, it is assumed that Q1 and Q3 are off
 (non-conduction state). When Q1 is off, the collector of Q1 and the base
 of Q2 are generally at the operating voltage potential. Then Q2 is on
 (conducting state), and its emitter current is I1+I2. The buffer
 transistor Q4 is likewise on and feeds base current to Q2. When Q2 is
 conductive, the current I1 flows from the emitter of Q2 via the
 capacitance C to the emitter of Q1. Then the current I1 charges/discharges
 the charge of the capacitance C, whereby the emitter potential of Q1 falls
 at a predetermined speed until Q1 becomes conductive when the base emitter
 voltage of Q1 exceeds about 0.6 V. When Q1 becomes conductive, its
 collector voltage begins to fall, which leads to that the buffer
 transistor Q3 begins to close. On account of a positive feedback via Q4,
 the base voltage of Q2 falls as well and Q2 closes. Q2 closing makes the
 collector voltage of Q2 rise, which accelerates the opening of Q3. Q3
 opening increases the base current of Q1 via a positive feedback. A higher
 base current discharges parasitic capacitances of the base circuit of Q1
 faster and accelerates thus the opening of Q1. When Q2 is off and Q1 is
 on, the current I2 flows from the emitter of Q1 via the capacitance C to
 the emitter of Q2, where the emitter voltage begins to fall until it makes
 Q2 open again and Q1 close via Q3.
 The speed of such a multivibrator circuit (maximum resonance frequency)
 depends primarily on the properties of the transistors Q1 and Q2. The
 buffer transistors Q3 and Q4 increase the speed of the multivibrator
 circuit, because they make a higher base current possible, which again
 discharges the parasitic capacitances of the base circuit of the
 transistors Q1 and Q2 faster and accelerates thus the switching of the
 transistor from one state to another.
 The lowest possible operating voltage Vcc will be achieved when it is
 assumed that the current sources 3 and 4 are ideal, i.e. no voltage loss
 is provided in them. When the ideal current sources are replaced by some
 practical circuit structure, such as current mirrors, Vcc increases.
 Returning to the operating principle of the circuit, it can be stated that
 current paths are either Q1-C-current mirror4 or Q2-C-current mirror 3 and
 that the current mirrors produce a stable current through the reference
 capacitor C, which is the main reason for the typical low phase noise. In
 search of a new way of increasing the speed, the reference capacitor
 cannot be decreased much more, because it will be of the order of
 parasitic capacitances, which leads to the fact that a controlled planning
 of the circuit is not possible.
 Nowadays there is, however, a need of ever-increasing speeds while an
 operating voltage as low as possible is desired, especially in electronic
 equipments using battery power supplies.
 For an implementation of a voltage- or current-controlled oscillator by
 means of a multivibrator circuit, the circuit requires a suitable
 supplementary control. Such a control should be as simple as possible.
 In the circuit of FIG. 1, the pulse amplitude is determined by the sum of
 the currents I1+I2 multiplied by the value of the collector resistor Rc1
 or Rc2 of the corresponding cycle. The pulse width is determined by the
 value of the current which is supplied by I1 or I2 via the reference
 capacitor C during its recharge cycles. Accordingly, either the
 capacitance of the reference capacitor C or the current flowing through it
 has to be changed for the frequency control.
 The capacitance may be changed if a varactor is used as reference capacitor
 C. A problem is, however, that varactor technologies are not generally
 compatible with BiCMOS technologies, for instance. In the BICMOS
 technology, a PN junction can be used instead. But then the capacitor
 works in the circuit of FIG. 1 and changes continuously the polarity of
 the voltage. In this case, a serial connection of two varactors, opposite
 to each other, may be some sort of solution, but the operation of the
 forward voltage region of one diode shows certain non-linearities and the
 phase noise of the multivibrator could be so high that it is unacceptable.
 Another alternative is to change the current and, in consequence of that,
 the recharge speed of the capacitor. This a very effective way of
 controlling the frequency of the oscillations, but the main drawback is
 its direct influence on the amplitude of the pulses.
 SUMMARY OF THE INVENTION
 An object of the present invention is a novel voltage- or current
 controlled oscillator circuit provided with a simple frequency control, a
 higher speed and a lower operating voltage and power consumption than the
 circuits according to the prior art.
 The invention relates to an oscillator circuit, comprising
 an operating voltage source,
 a first non-linear amplifier component comprising a first and a second main
 electrode and a control electrode,
 a second non-linear amplifier component comprising a first and a second
 main electrode and a control electrode,
 a third amplifier component, the main electrodes of which are connected to
 the control electrode of the first amplifier component and to a first
 potential of the operating voltage source and the control electrode of
 which is functionally connected to the first main electrode of the second
 amplifier component in such a way that a positive feedback is provided,
 a fourth amplifier component, the main electrodes of which are connected to
 the control electrode of the second amplifier component and to the first
 potential of the operating voltage source and the control electrode of
 which is functionally connected to the first main electrode of the first
 amplifier component in such a way that a positive feedback isprovided,
 a capacitive component connected between the second main electrode of the
 first amplifier component and the second main electrode of the second
 amplifier component,
 a first and a second resistor, via which the first main electrode of the
 first amplifier component and the first main electrode of the second
 amplifier component, respectively, are connected to the first potential of
 the operating voltage source. The oscillator is characterized in that it
 comprises
 a first controllable current source connected in series between the second
 main electrode of the first amplifier component and a second potential of
 the operating voltage source,
 a second controllable current source connected in series between the second
 main electrode of the second amplifier component and the second potential
 of the operating voltage source, while the currents I1 and I2 of said
 first and second current source determine the frequency of the oscillator,
 means for conducting compensation current via the first resistor and the
 second resistor, respectively, in such a way that the current flowing
 through each resistor is essentially constant and independent of the
 currents I1 and I2.
 The relaxation oscillator according to the invention is based on a
 multivibrator structure comprising a first and a second amplifier
 component, which are cross-connected via a third and a fourth buffer
 amplifier component to provide a positive feedback. Frequency is
 controlled by controlling the current flowing through a reference
 capacitor. In order to make the amplitude of an output signal of the
 oscillator independent of the control current, an extra compensation
 current is conducted via the resistors connected between the first and the
 second amplifier component and a first potential of an operating voltage
 source. The compensation current is controlled preferably in the same way
 as the control current, but in a direction different from that of the
 control current in such a way that the current via the resistors is
 constant. In other words, the compensation current compensates for the
 changes in the control currents. This compensation current is generated by
 a fifth and a sixth amplifier component connected from the first main
 electrode of the first and the second amplifier component, respectively,
 via a compensation current source to ground. The fifth and sixth amplifier
 component are connected to follow the states of the first and the second
 amplifier component, respectively, by forced control.
 The third and the fourth buffer amplifier component may preferably have
 corresponding pull-down amplifier components, which are cross connected to
 follow the states of the second and the first amplifier component,
 respectively, by forced control. This increases significantly the speed
 and the effectiveness of emitter followers constituted by the third and
 the fourth amplifier component and provides a higher amplitude and a lower
 output resistance from the same low-voltage power supply in comparison
 with the prior art solutions.

PREFERRED EMBODIMENTS OF THE INVENTION
 The present invention is applicable to lowering operating voltage,
 increasing speed and implementing frequency control in oscillators based
 on so-called emitter-coupled multivibrator circuits. Although the
 oscillator in FIG. 2 uses bipolar transistors as amplifier means, the
 circuit solutions according to the invention may use any type of
 non-linear amplifier components, in principle, such as MOS, CMOS, SOI,
 HEMT and HBT transistors, microwave tubes and vacuum tubes. The names of
 the electrodes may vary in these components. The main electrodes of a
 bipolar transistor are a collector and an emitter and the control
 electrode constitutes a base. In FETs, the corresponding electrodes are a
 drain, a source and a gate. In vacuum tubes, these electrodes are usually
 called an anode, a cathode and a gate. Thus the term emitter-coupled
 multivibrator shall also be understood in this connection as a more
 general concept, covering e.g. the terms cathode-coupled or source-coupled
 multivibrator.
 FIG. 3 shows an oscillator according to a preferred embodiment of the
 invention, based on an emitter-coupled multivibrator circuit.
 The oscillator comprises six NPN bipolar transistors Q1, Q2, Q3, Q4, Q5,
 and Q6. The collector electrode of the transistor Q1 is connected via a
 resistor Rc1 to an operating voltage Vcc and the emitter via a current
 source 11 to an operating voltage potential OV. The collector of the
 transistor Q2 is connected via a resistor Rc2 to the operating voltage Vcc
 and the emitter via a current source 12 to the operating voltage potential
 OV. A capacitor C is connected between the emitters of the transistors Q1
 and Q2. A positive feedback is provided between the transistors Q1 and Q2
 by connecting the collector of Q2 via a buffer transistor Q3 to the base
 of Q1 and the collector of Q1 via a buffer transistor Q4 to the base of
 Q2.
 To be more exact, the base of Q3 is connected to the collector of Q2 and
 the collector to the operating voltage Vcc. The emitter of Q3 is connected
 to the base of the transistor Q1.
 Correspondingly, the base of Q4 is connected to the collector of Q1 and the
 collector to the operating voltage Vcc. The emitter of Q4 is connected to
 the base of the transistor Q1.
 Thanks to the buffer transistors Q3 and Q4, the base currents of the
 transistors Q1 and Q2 can be made higher, which accelerates the discharge
 of parasitic capacitances of the base electrodes and so the switching
 speed of the transistors.
 Additionally, a pull-down transistor M1, which is a MOS transistor, is
 connected in series between the emitter of Q3 and the operating voltage
 0V. Correspondingly, a pull-down transistor M2, which is a MOS transistor,
 is connected in series between the emitter of Q4 and the operating voltage
 0V. M1 and M2 are cross-connected to follow the states of the transistors
 Q2 and Q1, respectively, by mechanical control. To be more exact, the base
 of M1 is connected to the base of Q2 and the base of M2 is connected to
 the base of Q1. This increases the speed of the circuit and improves the
 shape of the output signal at symmetric outputs Vout1 and Vout2.
 The positive feedbacks and the series resonance circuits Rc1-C and Rc2-C
 constituted by the resistors Rc1, Rc2 and the capacitor C provide that the
 multivibrator output Vout1-Vout2 oscillates between two states, when the
 oscillation once has been triggered. The resonance frequency of the
 circuit is set by the values of Rc1, Rc2 and C.
 As was described earlier in connection with FIG. 1, the pulse amplitude is
 determined by the sum of the currents I1+I2 multiplied by the value of the
 collector resistor Rc1 or Rc2 of the corresponding cycle. The pulse width
 is determined by the value of the current supplied by I1 or I2 via the
 reference capacitor C during its recharge cycles. So the frequency can be
 controlled by controlling the current flowing through the reference
 capacitor C. In the preferred embodiment of the invention shown in FIG. 2,
 the frequency is controlled by controlling the currents I1 and I2 of the
 controllable current sources.
 This is an efficient way of controlling the frequency, but the main
 drawback is its direct influence on the pulse amplitude. In order to make
 the amplitude of the output signal of the oscillator independent of the
 control current, an extra compensation current Icom is conducted according
 to the invention via Rc1 and Rc2. The compensation current Icom is
 controlled preferably in the same way as the control currents I1 and I2,
 but in a direction different from that of the control currents in such a
 way that the current via the resistors Rc1 and Rc2 is constant, while the
 current via the capacitor C changes. In other words, the compensation
 current Icom compensates for changes in the control currents I1 and I2.
 In the embodiment of FIG. 2, this compensation current Icom is generated by
 the transistors Q5 and Q6, which are connected from the collector of Q1
 and Q2 via a compensation current source 22 to ground. Q5 and Q6 are
 connected to follow the states of Q1 and Q2, respectively, by mechanical
 control.
 The value of the currents I1 and I2 can be different, if a difference is
 required between the pulse width and a pause. Generally, I1=I2 is selected
 in such way that the current is within the area of the maximum transient
 speed of an avalanche process. This can be for instance a current by which
 the transient frequency f.sub.T of the transistors to be used is achieved.
 To be more exact, the collector of the transistor Q5 is connected to the
 collector of Q1 and the base to the base of Q1. The collector of the
 transistor Q6 is connected to the collector of Q2 and the base to the base
 of Q2. The emitters of the transistors Q5 and Q6 are interconnected and
 connected via a current source 22 to the operating voltage potential OV.
 In an ideal current source 11, 12 or 22, no voltage losses are provided.
 The real current source 11, 12 or 22 is, however, constituted e.g. by a
 current mirror, which is controlled by a voltage. A voltage loss is then
 provided across the current mirror, whereby a somewhat higher operating
 voltage is needed.
 Thus the circuit of FIG. 2 needs for operation an operating voltage of at
 least 2.2 V (=1.8V+0.4V), where the voltage across the current sources 11,
 12 and 22 is assumed to be about 0.4 V, when MOS transistors are used for
 the generation of the currents I1 and I2.
 If the current source 11, 12 or 22 is constituted by a current mirror
 controlled by a voltage, a voltage-controlled oscillator VCO is provided.
 FIG. 3 shows one way of implementing VCO from the circuit of FIG. 2,
 consisting in that the currents I1, I2 and Icom are supplied by current
 mirrors M6-M7 and M8, which are controlled by a differential amplifier
 M2-M3-M4-M5. The differential amplifier is controlled by a control voltage
 VCOcontrol. If the current source 11, 12 or 22 is implemented by a circuit
 solution controlled by a current, a current-controlled oscillator is
 provided. These different implementations of a current source are obvious
 to one skilled in the art.
 The circuit of FIG. 2 has been analyzed by using 0.8 .mu.m BiCMOS
 technology, in which bipolar NPN transistors have the transient frequency
 F.sub.T =14 GHz. The current flowing through the transistors is selected
 in such a way that it provides this transient frequency F.sub.TMAX, the
 current being about 800 .mu.A on this technology. The MOS transistors M1
 and M2 have W=1.2 .mu.m and W/L=100. The maximum oscillation frequency of
 about 1.1 GHz is achieved by the minimum value 0.5 pF of the capacitor C.
 The amplitude is about 0.4V and the power consumption only 5.6 mW from the
 2.2V operating voltage. The controllability of the circuit is 750 MHz/mA.
 The shape of the signal across the reference capacitor C remains as is
 typical of emitter-coupled multivibrators, which is the main reason for
 very low phase noise. The oscillator is capable of operating also at low
 frequencies, at which big external capacitors C can be used more easily.
 The invention can also be implemented exclusively by bipolar technique.
 The oscillator circuit according to the invention is particularly suitable
 for modern Phase-Locked Loops (PLL) in communications and microprocessor
 applications.
 The drawings and the related description are only intended to illustrate
 the invention. The details of the invention may vary within the scope and
 spirit of the attached claims.