CA1266895A - Voltage controlled oscillator - Google Patents

Abstract

ABSTRACT:

A voltage-controlled oscillator having an elec-trical means of frequency control which is able to operate from a supply voltage which is lower than is desirable to use with a varicap diode. The circuit is a modified Colpitt's oscillator. The oscillator comprises a pair of transistors whose base and collector electrodes are con-nected together, the base electrodes being connected to a supply rail (+Vcc) and the collector electrodes are con-nected to a tunable parallel LC resonant circuit. A fre-quency determining device e.g. a crystal is connected between a tapping of the resonant circuit and the emitter of one of the transistors to form a first feedback path.
The crystal together with an integratable small value capacitor connected between the emitters of the transis-tors form a second feedback path. The phase shifts due to the first and second feedback paths are summed vectorially at the collectors of the two transistors. By varying the gain of each of the two transistors then the resultant phase shift, and thereby the frequency of oscillation, are varied.

Description

l'l[1~ 3~

V~)~T~ CO~TROIr~ ')~IL~ATOR

rtle ~re3ent inventio-n relates to a voltage con-troLle(l oicilLator co1nprising two active elements having re.pective feedback path~s ~ith each pattl having a different p':lase -~hi-ft, -the phase shift in one of saicl t~o ~aths being sub-itantia11y constant, sigaal oltpUts o~`-the two feedback paths being summed v~ctorially.
A voltage controlled oscillator having ~wo feed bac1~ paths is known fron ~-ritish Patent Specification 1~62371. This knowll oscillator comprises a t`irst differerl-tial amplifier comprising emitter coi~pled first and secondtransistors, The collector elec-trode of -the -first transis-tor is collpled via a t`irst phase lag circuit to a regene-rative voLtage feed~ack path including a series arrange-ment ot an em-itter follo~er transistor, a piezo-electric crystal and a d.c. blocking capacitor, whicll arrangement is connected to the base el0ctrode of the 3econd transistor.
The feedback path including the first phase lag circuit has a substantially constant p'na3e sh-if`t. Another variable phase shif-t feedback path is formed by connecting ttle collector electrode of tha second transistor to the co~moned erl1itter electrodes of a second differential ampLifier ~ormed b~
third and fourthe transistors, via a second phase lag cir-cuit. A source of frequenc~- co.ntrol voltage is connected to the hase electrodes of the third and -fourth transistors.
The collec-tor of the third transistor is connected to the ennitter t`ollower transistor. In operation the emitter cur-rent supplied to the emitter electrodes of the third and four-th transistors is the phase shifted collector curren-t of the second transistor, W}liCh currerlt is split between the third ancl f`ourth transistors in response to the fre-quency control voltage. 13y varying che gain of the th-ird transistor a varia'~le phase s'lift component is added to the

2 i~ 3r g fixed phase shift component applied to the emitter follower transistor. Frequency control by varying the gain of one of the feedback paths, that is the one including the third transistor only, has the disadvantage that the frequency control range of the oscillator is limited and also the amplitude will vary considerably in response to the frequency control signalWhich is undesirable when connecting the oscillator to other circuitry. Furthermore the known oscillator circuit consumes a fairly high current especially in connection with the emitter follower transistor which is a disadvantage if the circuit is used in a battery powered receiver. Also by employing phase lag circuits which will suppress overtone frequencies the known oscillator operates around the fundamental crystal frequency which limits the useful frequency range of the oscillator circuit.
Another known method of controlling the frequency of an oscillator is to use a varicap diode. Whilst such a frequency control technique is used widely it does have some disadvantages.
Varicap diodes are relatively expensive and are preferably operated at high voltages to obtain an acceptable linearity.
Further as varicap diodes are fairly large physically then when integrating these into a monolithic circuit element a relatively large area has to be provided.
It is an object of the present invention to apply frequency control to a voltaged controll oscillator which avoids the disadvantages of the known techniques described above.
According to the present invention there is provided a voltage-controlled oscillator comprising two active elements having

3 1;~ 5 respective feedback paths with each path having a different phase shift, the phase shift in one of said two paths being substantially - constant, signal outputs of the two feedback paths being summed vectorially, and a tunable L.C. circuit for receiving said summed signal outputs, characterized in that means are provided for varying the gain distribution between the two feedback paths, said gain varying means comprising two control devices respectively coupled to the two active elements, and means for applying a fre~quency control voltage to each of the control devices to inversely vary the distribution of the current through their associated active elements to alter the gain in each path and thereby change the total phase shift and hence the frequency of the oscillator.
By varying the gain in both feedback paths a larger frequency control range is obtainable compared to controlling the gain in one feedback path and also the variation in amplitude in response to frequency control voltages is less. In implementing the oscillator circuit made in accordance with the present invention current feedback is used which leads to an overall reduction in current consumption compared with having voltage feedback.
Furthermore by coupling a tunable phase lead circuit to the combined output of the active elements then the oscillator can operate on the overtone mode frequencies of the frequency -~ determining device, for example a piezo electric crystal, as well as in the fundamental mode which increase the appllcability of the oscillator circuit.

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4 1~68'3'i The present invention also provides a voltage controlled oscillator characterized by two active elements having respective outputs, the signal outputs of which are summed vectorially and supplied to a tuned parallel L.C. circuit which includes two serially connected capacitors, a frequency determining device coupled between the junction of said two serially connected capacitors, and an input of one of the active elements, a phase shifting element coupled between the inputs of the two active elements, and control devices coupled respectively to the active elements for inversely controlling the gain of each of the two active elements and thereby the phase shift in feedback paths including the active elements.
The present invention further provides a voltage-controlled oscillator characterized by a semiconductor device having a collector, a base, and first and second emitter junctions, a tunable resonant circuit coupled to the collector and a frequency determining device coupled between resonant circuit and the first emitter junction, a capacitance coupled between the first and second emitter junctions, a differentially connected pair of transistors having collectors coupled respectively to the first and second emitter junctions, means for inversely varying the conductivity of the transistors of the differentially connected pair to vary thereby the conductivity of the transistors formed by the first emitter junction, the base and the collector and by the second emitter junction, the base and the collector and in so doing changing the total phase shift at the collector of the semiconductor device.

4a ~.26~;89~ 2ol04-7841 The oscillator made in accordance with the present invention comprises an electrical means of frequency control of the oscillator which does not require the use of a varicap diode and in consequence can be more readily integrated into a smaller area. Furthermore when the oscillator is used in low voltage equipment such as battery powered paging receivers one is able to obtain a more linear voltage to frequency curve than is possible from the simple use of varicap diodes ~ se are not linear devices. Consequently frequency modulation and demodulation are possible with the oscillator made in accordance with the present invention.
The present lnvention will now be explained and described, by way of example, with reference to the accompanying drawings, wherein:
. Figure 1 is schematic circuit diagram of a Colpitt's oscillator, Figure 2 shows the equivalent electrical circuit of a crystal which can be used in place of the impedance in the oscillator shown in Figure 1, Figure 3 is a schematic circuit diagram of a voltage-controlled crystal oscillator made in accordance with the present invention, and Figure 4 is a frequency/differential voltage transfer curve derived from the circuit shown in Figure 3.
Referring to the Colpitt's oscillator circuit shown in , . , : ~ Figure 1 it comprises a NPN transistor TRl whose base electrode ~ is connected to the positive voltage supply ~"."
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~2668~3.5 P~l r~ ; l7-o~-8!~

rai~ + Vcc. Its emitter is ~orlnected via a currenc source lO to a low ~oltage rail which itl ttle il1LIs`tra~ed elllbOdi-ment is at 0 vol-ts. A re;~ ant circuit comprising a varia-ble inductance l~ which in parallel wi-th two series con-inecled clpa~itors 14, l~ is co-nnec-ted `between the collector al~d the positive voltage supply tail + Vcc. An i~pedance - 13 is connected between the emitter of the transistor TR1 and che j-lnction of the capacitors 14, 16 of the resonant circuit. The impedarlce 18 must be slalL otllerwise tne re-sultant attentua-tion will not allow the loop gain of the circuit to e~ceed unity.
This circuit is well kno~n and need not be described in detail. However, in order for the circuit to function as an oscillator it is necessary for the loop gain of the oscillator to ~e greater than unity whilst the total phase shif`t around the same loop is zero o- a multiple of 3600. No-~ if the impedance l8 is zero then the oscillator will resonate at a frequency determined by the resonant circuit connected to the collec-tor of the transistor TR1.
; 20 HO~ever, if the impedance 18 is replaced by a crystal ha~-ing, at round the wanted oscillation frequancy, an electri~
al equivalent circuit of the type shown in Figure 2, the impedance of the crystal will only be low enough to allow oscillation at the desired series resonant mode of the crystal. If the resonant circuit of the oscillator is tuned t~ be the same as the series resonant frequency of the ~anted mode of crystal resonance then the frequency fl of oscillation is:
fl =
2~ ~L1.CI
where L1 and C1 are the series resonant conponents of the cry~tal in the wanted mode and ignoring the eftects of the j parasitic capacitance Co.
If a small capacitor of capacitance Ce is placed in series with the crystal in the oscillator loop, then if the circuit is oscillating, the phase shift around the loop must again be zero. However, the voltage phase shift between the emltter of the o~cillator and the tapping .

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1~66~95 P11fs 33~ 8 6 18~ 8!i point uf` the resc)n:-m~t circl1it -in t1le col`Lectc)r o~' t1le trallsistor 1`1~1 has not beeil sigt1:il`icalltly c11ang~d ~y tile addition r)L` tlle exl,ra secles capacitance. This n1eans that the new freque1lc~ ~f` o~;cillariol1 is deler1ni1lecl b~- the e~-l'ect of` the additional cap~cita-ce C~ ~eill~ in ~erie~ wi-th the resonant Cil~CUi t ~L` tlle crystal and in con,eq~1ence a ne~ f`requency r2 is:
f2 =
2Tr~L1,C'1.Ce/(C1+C~
f'rom t1lese two equations it can be shown that the available shirt in f`re(luency a f = f1(~(1+C1/Ce) -1) ), ignoring the effects of the parasitic capacitance Co and that this ap-proximates to ~ f = fl.C1/(2.Ce).
In the case of a third ove-rtone crystal with an equivalent circuit at 50 ~lz of '1.38 x 10 3 p~ (in se-ries with an inductance of 7.34 mH) a frequency shift of 3.L15 kH~ could be obtained by the insertion of 10 pF in series with the crystal.
The oscillator circuit shown in Figure 3 en-ables a series capacitance to be electronically switchedin and out of the circuit in a gradual manner -thereby changing the total pha~e shift. The circuit comprises two oscillator transistors TRl and TR2 whose base electrodes are connected together and a,re connected to the positive voltage supply line + Vcc. The collectors of the transis-tors TR1, TR2 are connected together and are coupled to a resonant circuit comprising an inductancs 12 which is con-nected in parallel with two series connected capacitors 14 and 16. A crystal 20 i9 connected in the feedback path between t~e junction of the capacitors 14, 16 and the , ernitter of the transistor TRl. A series capacitor Ce having ,~ a value oP for example 10 p~ is connected between the , emitters of the transistors TX1 and TR2 and in consequence the crystal 20 and the capacitor Ce may be considered as , 35 the feedback path of the transistor TR2. If desired the ¦ transistors TR1 and TR2 may be replaced by a single tran-sistor with two emitter diffusions when implementing the circuit on a monolithic semiconductor device. The circuit :, . ~
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.~ , ~2668~5 P}~B 3~ 7 l7-'0~_~4 describc?d ~o far tn.ly be regar(led as two active elements ha~in~ respecti~re f`eedback paths applying different phase s!lifts, i)llt eacll path has a _ommon element consti-tuted by the crystal 20. ~t -the collectors of the transistors TR1 and TR~ the output~ therel`orm are sulllmed vectorially. Vary-in~ tlle ~ains of the transistors TR1 and ~R2 varies the total gain applied by each path and thereby the resultant phase s.li~`t.
In order to -~ary the gains of the transistors TR1 and TR2 their emi~tters are connected respectively -to the collectors of dif-feren-tially connected transistors TR3 and TR4 whose emitters are coupled together and are co~-nected to ~ current source 22 which is connected to the lower ~oltage supply rail which in the present embodiment is zero volts. An automatic frequency control voltage, re-presented in ~igure 3 as a voltage source 24, is connected between the base electrodes of the transistors TR3 and TR4.
A voltage reference source 26 is coupled be-tween the base electrodes of the transistors TR3 and TR4 and the lower voltage supply line.
In operation the diffe~entially connected tran-sistors TR3 and TR4 deterrnine the distribution of current between the two oscillator transistors TRl, TR2. The con-trol of this current distribution is effected by the appli-cation of a differential voltage to the base electrodes ofthe transistors TR3 and TR4 which differential voltage is supplied by the source 24. It can be seen ~hen a large differential voltage is applied to these bases then one transistor of the differential pair will be fully switched on and the other ~ully off`. In such a situation only one of the two oscillator transistors T~1, TR2 is actively con-tributinh to the oscillator function. If one considers the two extreme cases when the differential pair of transistors TR3, TR4 is switched in one direction, the oscillator tran-sistor TR1 is fully on and the feedback path of the oscil-lator includes the crystaL 20 only and the frequency, fl, is determined by the phase shift due to the crystal 20 alone. When the differential pair of transistors TR3, TR4 ~" ..
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12~i~;89'i is switched the other way, TR2 is fully on and the oscillator feedback path includes both the crystal 20 and the 10 pF series capacitor Ce and the frequency f2 is determined by the phase shift die to these components being in series. Thus by the mechanism described, the frequency of oscillation will be modified by a predetermined amount.
Between these two extremes of control, the frequency shift will be a smooth function of the differential voltage applied to the transistors TR3, TR4 which are both active. This in turn means that the conductivity of the transistors TRl, TR2 is varied accordingly. The phase shifts due to each of the two feedback paths and the gain of their respective transistors TRl, TR2 are summed vectorially at the collectors of the transistors TRl, TR2. The final phase angle will thus be dependent on the current distribution between the two transistors because this determines the relative magnitude of the phase shifted and non-- phase shifted components. The remaining connection in the oscillator loop is formed by the crystal and therefore the final frequency of oscillation will be that at which the phase shift caused by the crystal 20 compensates the phase shift introduced by the active part of the circuit formed by the transistors TRl, TR2 and the capacitor Ce. Thus the final frequency of oscillation is determined by the current distribution between the transistors TR1, TR2 and is also a smooth continuous function of the differential voltage applied to the bases of the differential pair of transistors TR3, TR4. This smooth change is illustrated in Figure 4 wherein the abscissa is the differential control .;~ , , ` ` .

' 8a 1~i6895 voltage E in millivolts applied to the base electrodes of the transistors TR3, TR4 and the ordinate is the freguency change ~f in hertz.
In implementing the circuit shown in Figure 3, the transistors TRl to TR4 comprise small signal RF transistors to suit the frequency of oscillation, for example they comprise transistors type sFY90. The circuit may be implemented in other technologies such as MOS and gallium arsenide.

~26~i8~35 Pl-lB '~'30~ l7-08_84 .~ particular appLication Or the osci:Llator cir-cuit nlade irliaccor-lance witll the present invention is as a local osclllator in the direct modulation FM receiver dis-closed in British Patent Specification .~109~01A wherein the local oscillator freq-lency is o~fset fron the carrier fre-que;1cy by a predetermined amount. In order to maintain the tones received at their particular of`fset frequencies it is important that che oscillator is stable and by adjusting the voltage app:Lied to the base electrodes of the transis-tors TR~ and TR-I~ then the local oscillato~ frequency can be ~naintained fairly stable thereby e-nsuring that the centre frequencies of the two signalling tones are sufficiently d.ifferent ~o enable them to be distinguished from each other by !neans o.f bandpass filters.
Altho-ugh the present invention has been des-cribed ~ith reference to a crystal oscillator it is appli-cable to other 03cillators such as an LC oscillator.

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Claims (7)

PHB. 33.028 10 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A voltage-controlled oscillator comprising two active elements having respective feedback paths with each path having a different phase shift, the phase shift in one of said two paths being substantially constant, signal out-puts of the two feedback paths being summed vectorially, and a tunable L.C. circuit for receiving said summed sig-nal outputs, characterized in that means are provided for varying the gain distribution between the two feedback paths, said gain varying means comprising two control devices respectively coupled to the two active elements, and means for applying a frequency control voltage to each of the control devices to inversely vary the distribution of the current through their associated active elements to alter the gain in each path and thereby change the total phase shift and hence the frequency of the oscilla-tor.

2. An oscillator as claimed in Claim 1, charac-terized in that the respective feedback paths have at least one frequency determining device which is common to both the paths.

3. An oscillator as claimed in Claim 1 or 2, char-acterized in that the active devices comprise junction transistors connected in common base configurations, in that a capacitance is coupled between the emitter elec-trodes of said transistors, and in that the two control devices comprise a differential amplifier whose outputs are connected respectively to said emitter electrodes.

4. A voltage controlled oscillator characterized by two active elements having respective outputs, the signal outputs of which are summed vectorially and supplied to a tuned parallel L.C. circuit which includes two serially connected capacitors, a frequency determin-ing device coupled between the junction of said two serially connected capacitors, and an input of one of the PHB. 33.028 11 active elements, a phase shifting element coupled between the inputs of the two active elements, and control devices coupled respectively to the active elements for inversely controlling the gain of each of the two active elements and thereby the phase shift in feedback paths including the active elements.

5. An oscillator as claimed in Claim 1 or 4, charac-terized in that tunable L.C. circuit is provided to which the sum of said signal outputs is applied and the active elements and the control devices comprise semiconductor devices of the same conductivity type.

6. A voltage-controlled oscillator characterized by a semiconductor device having a collector, a base, and first and second emitter junctions, a tunable resonant circuit coupled to the collector and a frequency determin-ing device coupled between resonant circuit and the first emitter junction, a capacitance coupled between the first and second emitter junctions, a differentially connected pair of transistors having collectors coupled respectively to the first and second emitter junctions, means for inversely varying the conductivity of the transistors of the differentially connected pair to vary thereby the con-ductivity of the transistors formed by the first emitter junction, the base and the collector and by the second emitter junction, the base and the collector and in so doing changing the total phase shift at the collector of the semiconductor device.

7. An oscillator as claimed in Claim 6, wherein the semiconductor device comprises first and second junction transistors of the same conductivity type, the base elec-trodes of the first and second transistors being coupled together and their collector electrodes being coupled to the resonant circuit.