GB2039438A

GB2039438A - Tunable oscillator

Info

Publication number: GB2039438A
Application number: GB7901248A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1979-01-12
Filing date: 1979-01-12
Publication date: 1980-08-06
Also published as: GB2039438B

Abstract

An oscillator particularly suitable for integration including a series resonant circuit 6 connected at only one pin 5 with the rest of the circuit, includes a pair of emitter coupled transistors T1, T2, in common d.c. mode, having positive feedback from the collector 13 of T1 to the base 10 of T2 via an emitter follower transistor T3. The oscillator frequency may be varied by adding to the feedback current a variable component in quadrature thereto. This is done via a transistor T8, a variable current differential DELTA I between the outputs of sources 20 and 23 being subjected to a phase lag of 90 DEG by network 34,35. <IMAGE>

Description

SPECIFICATION Oscillator circuit This invention relates to an oscillator circuit and seeks to provide a circuit suitable in at least one embodiment for fabricating in integrated form.

According to this invention there is provided an oscillator circuit comprising a pair of transistors having emitters coupled to a shared current path and bases D.C. coupled and arranged to be driven in common D.C. mode; a series resonant circuit connected between the base of one transistor of the pair and a reference potential and feedback means connected to supply a feedback signal derived from a collector of the said one transistor of the pair to the base of the other transistor so as to produce self oscillation of the circuit.

The series resonant circuit may be provided in the form of a series mode crystal.

In carrying out the invention the feedback means may be provided by a feedback transistor having an input electrode coupled to the collector of the said one transistor and an output electrode connected to supply the feedback signal to the base of the said other transistor of the pair.

In a preferred embodiment the feedback transistor is connected as an emitter follower stage.

The oscillator circuit may be provided in the form of a controlled oscillator in which case means is provided for adjusting the phase of the feedback signal to control the frequency of oscillation of the oscillator.

The means for adjusting the phase of the feedback signal may comprise means for providing a signal in phase quadrature to oscillator signals provided by the collector of the said one transistor, the phase quadrature signal being added vectorially to the oscillator signals, the phase of the feedback signal being determined by the magnitude of the phase quadrature signal.

In an embodiment of the invention the means for providing a signal in phase quadrature to the oscillator signals comprises means for providing a control signal of adjustable magnitude and in phase with or anti-phase with the oscillator signals and means for imparting a quadrature phase shift to the control signal.

The means for imparting a quadrature phase shift to the control signal may be capacitor means connected between a signal path carrying the control signal and a reference potential.

The means for providing the control signal may comprise a further two pairs of transistors the transistors in each pair having emitters coupled to a respective further shared current path and bases coupled to respect bases of the transistor of the said pair, the transistors of each further pair which are coupled to different ones of the bases of a transistor of the said pair having collectors coupled together, one pair of coupled together collectors providing the control signal whilst the other pair of coupled together collectors is held at a reference potential, means being provided for relatively adjusting the currents in the shared current paths.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawing which shows an oscillator circuit in accordance with the invetion.

Referring to the drawing a pair of NPN transistor T1 and T2 have respective emitters 1 and 2 coupled together by a direct connection and to a shared current path 3. The transistor T1 has a base 4 connected to one terminal 5 of the series resonant circuit provided in the form a series mode crystal 6, a second terminal of which is connected to a reference potential 7. The base 4 of the transistor T1 is coupled via resistors 8 and 9 to base 10 of the transistor T2 which has a collector 11 connected to a reference potential 12.

Oscillation inducing feedback is provided by coupling signals from collector 13 of the transistor T1 in suitable phase to the base 10 of the transistor T2.

The feedback signals are coupled via a transistor T3 whose base 14 is connected to the collector 13 of the transistor T1 and which acts as an emitter follower, emitter 15 being connected to a point between the resistors 8 and 9 and collector 16 being connected to a reference potential 17.

The circuit as so far described provides an oscillator in accordance with the present invention. The transistors T1 and T2 are D.C. coupled together via the resistors 8 and 9 and are supplied with D.C.

biasing from a common D.C. source so that they operate in common D.C. mode. Oscillation sustaining feedback signals are taken from the collector 13 of the transistor T1 via the emitter vollower formed by the transistor T3 and via the resistor 9 to the base 10 of the transistor T2. The oscillator oscillates at the resonant frequency of the series mode crystal 6 which at resonance decouples the base 4 to the reference level 7 and the two transistors T1 and T2 receive no differential drive. The oscillator so formed has the advantage that connection to the circuit need only be made to one pin 5 of the crystal, the other pin being taken to reference potential. When the circuit is integrated this is advantageous since one less pin of the integrated circuit package need be used for connection to the oscillator.

The oscillator circuit so far described may be arranged to operate as a controlled oscillator. To effect control of the oscillator a further two pairs of transistors T4, T5, and T6 and T7 are provided. The transistors T4 and T5 have respective emitters 18 and 19 which are coupled together by a direct connection and to a controllable current path 20.

Likewise the transistors T6 and T7 have emitters 21 and 22 which are also coupled together by a direct connection and to a controllable current path 23. The two pairs of transistors formed by the transistors T4, T5 and T6 and T7 respectively are driven from the oscillator transistors in anti-phase to one another by connecting base 24 of the transistor T4 to the base 10 of the transistor T2 and also to base 25 of the transistor T7. Bases 26 and 27 of the transistor T5 and T6 are connected together and also to the base 4 of the transistor T1. The transistors T5 and T7 have collectors 28 and 29 respectively which are coupled together by direct connection and to a reference potential 30. The transistors T4 and T6 have respec tive collectors 31 and 32 which are also coupled together by a direct connection and to base 33 of an emitter follower transistor T8.The base 33 of the transistor T8 is also connected to one terminal of each of a resistor 34 and a capacitor 35, the other terminals of which are respectively connected to the reference potential 17 and a reference potential 36.

The transistor T8 has a collector 37 which is connected to the reference potential 17 and an emitter 38 which is connected via a resistor 39 to the base 14 of the transistor T3.

In operation current fed to the base 33 of the emitter follower T8 is passed via the emitter 38 and resistor 39 to the base 14 of the transistor T3 where it adds vectorially to current emanating from the collector 13 of the transistor T1 forming part of the oscillator circuit. The vectorially added currents are passed by the emitter follower T3 to the emitter 15 and then as a combined feedback signal to the base 10 of the transistor T2. The current passed to the base 33 of the transistor T8 is determined by the arrangement of the two pairs of transistors T4, T5 and T6, T7 as will now be described.

The current passed by the two emitter coupled pairs of transistors is determined by the relative values of the current provided by the controllable current paths 20 and 23 and for the purposes of explanation assume that the current path 20 is controlled to provide a current of value I + Al whilst the path 23 carries a current I - Al. In view of the anti-phase driving of the two pairs of transistors and the connection between the collectors 31 and 32 of the transistors T4 and T6 and the collectors 28 and 29 of the transistors T5 and T7, then when Al is zero no nett current will be passed to the base 33 of the transistor T8 and consequently no current will be added to the oscillator current provided by the collector 13 of the transistor TI so that the oscillator oscillates at a frequency determined by the natural resonance of the crystal 6. If the value of Al is positive then the transistors T4 and T5 will pass proportionally greater current than the transistors T6 and T7 and a nett signal will appear at the junction of the collectors 31 and 32. This net signal will be in phase with that appearing at the collector 13 of the transistor T1 and will increase in value with increasing Al up to a maximum value when Al = I at which point the transistors T4 and T5 pass a maximum current whilst the transistors T6 and T7 pass zero current.

The capacitor 35 and resistor 34 connected to the base 33 of the transistor T8 will produce a 90 phase lag between the signal appearing at the junction of the collectors 31 and 32 of the transistors T4 and T6 and that appearing at the base 33 of the transistor T8.For positive values of AI therefore the signal appearing at the base 33 of the transistor T8 and hence passed by emitter follower action for combination at the base 14 of the transistor T3, will be a signal 90 phase lagged from that provided by the collector 13 of the transistor T1 and whose magnitude varies in dependence upon the value of Al. The two signals appearing at the base 14 add vectorially and produce by the emitter follower action of the transistor T3 a signal fed back to the base 10 of the transistor T2 whose phase varies in dependence upon the value of Al. Since for oscillation to be maintained nett phase round the feedback loop must be zero then the imposition of the phase shift around the loop by the control of the value Al must be compensated internally by the crystal 6 whose internal phase properties change in order to compensate for the enforced phase round the loop and consequently the resonant frequency of the crystal 6 changes to produce a new oscillation frequency for the oscillator.

When Al is a negative value then current appearing at the junction of the collectors 31 and 32 of the transistors T4 and T6 will be in anti-phase to that appearing at the collector 13 of the transistor T1 and of value in dependence upon the magnitude of Al increasing to a maximum value when Al equals - I.

At this stage the transistors T6 and T7 share a maximum current of magnitude 21 whilst the transistors T4 and T5 pass zero current. The capacitor 35 and resistor 34 produce a similar 90 phase lag between the signal appearing at the junction of the collectors 31 and 32 and the base of the transistor T8 to produce a signal at the base 14 which is in phase quadrature to that emanating from the collector 13 of the transistor T1 but in anti-phase to that passed by the transistor T8 when LI was positive. Once again the crystal 6 is caused to change its frequency of oscillation in dependence upon the value of Al but this time the frequency moves in the opposite direction from its value when Al is zero to that in which it moves when Al was positive.

The arrangement thus described provides a crystal oscillator whose frequency of oscillation may be changed in dependence upon the value of a control current. Any convenient means may be used to produce the variable current paths 20 and 23 but a convenient method would be to provide a still further pair of emitter coupled transistors having controlled emitter current paths and collectors connected to provide a respective current path 20 and 23. An advantage of the use of the present circuit arrangement to provide a voltage controlled oscillator is that the loop gain around the system is independent of the frequency shift of the oscillator since variations in the feedback factor around the oscillator loop are compensated by equal and opposite variations in the crystal 6. As mentioned previously the oscillator requires only one input pin and also all components of both the oscillator and the control circuit are easily integrable.

Modifications may be made to the described arrangement without departing from the scope of the invention for example it is not essential that the oscillator be crystal controlled and an equivalent series resonant circuit could be used although of course this would be less advantageous where the circuit is to be integrated. Any convenient method may be used to control the currents in the paths 20 and 23 and also any other convenience means may be used to impart the 90 phase shift beteen the signals appearing at the junction of collectors 31 and 32 and the base 33 of the transistor T8.

Claims

1. An oscillator circuit comprising a pair of transistors having emitters coupled to share a current path and bases d.c. coupled and arranged to be driven in common d.c. mode- a series resonant circuit connected between the base of one transistor of the pair and a reference potential and feedback means connected to supply a feedback signal derived from a collector of the said one transistor of the pair to the base of the other transistor so as to produce self oscillation of the circuit.

2. An oscillator circuit as claimed in claim 1 in which the series resonant circuit is provided in the form of a series mode crystal.

3. An oscillator circuit as claimed in claim 1 or 2 in which the feedback means is provided by a feedback transistor having an electric input electrode coupled to the collector of the said one transistor and an output electrode connected to supply the feedback signal to the base of the said other transistor of the pair.

4. An oscillator circuit as claimed in claim 3 in which the feedback transistor is connected as an emitter follower stage.

5. An oscillator circuit as claimed in any preceding claim and provided in the form of a controlled oscillator means being provided for adjusting the phase of the feedback signal to control the frequency of oscillation of the oscillator.

6. An oscillator circuit as claimed in claim Sin which the means for adjusting the phase of the feedback signal comprises means for providing a signal in phase quadrature to oscillator signals provided by the collector of the said one transistor, the phase quadrature signal being added vectorially to the oscillator signals, the phase of the feedback signals being determined by the magnitude of the phase quadruture signal.

7. An oscillator circuit as claimed in claim 6 in which the means for providing a signal in phase quadrature to the oscillator signals comprises means for providing a control signal of adjustable magnitude and in phase with or anti phase with the oscillator signals and means for imparting a quadrature phase shift to the control signal.

8. An oscillator circuit as claimed in claim 7 in which the means for imparting a quadrature phase shift to the control signal is capacitor means connected between a signal path carrying the control signal and a reference potential.

9. An oscillator circuit as claimed in claim 7 or 8 in which the means for providing the control signal comprises a further two pairs of transistors the transistors in each pair having emitters coupled to a respective further shared current path and the bases coupled to respective bases of the transistors of the said pair, the transistors of each further pair which are coupled to different ones of the bases of a transistor of the said pair having collectors together, one pair of coupled together collectors providing the control signal whilst the other pair of coupled together collectors is sheld at a reference potential, means being provided for relatively adjusting the current in the shared current paths.

10. An oscillator circuit substantially as herein described with reference to and as illustrated in the drawing.