GB1603627A - Electrical oscillator circuits - Google Patents

Description

(54) IMPROVEMENTS RELATING TO ELECTRICAL OSCILLATOR CIRCUITS PUBLIC LIMITED COMPANY, a British Company organised under British Aerospace (Nominated Company) Order 1980 and British Aerospace (Appointed Day) Order 1980, of 160 Pall Mall, London, SW1Y 5HR, do heeby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to electrical oscillator circuit arrangements.

According to the invention there is provided apparatus for producing two synchro- nous output waveforms substantially in antiphase with one another, the apparatus comprising a symmetrical arrangement of two semiconductor amplifying members each having two main terminals and a control terminal for controlling current flow through the member between the two main terminals, and of tuning means connected to the two members for causing each of them to operate as an oscillator, the tuning means including capacitance and inductance and further including a common frequencydetermining, two-terminal, resonant circuit through which the control terminals of said members are interconnected.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing in which: Figure 1 is a circuit diagram of a two phase crystal oscillator, and Figure 2 is a sectional view of the differentially adjustable inductors used to differentially adjust the phases of the oscillator's outputs.

The oscillator shown comprises two transistors T1 and T2 and a symmetrical tuning arrangement including capacitors C1 and C2 connected between the collector and emitter terminals of the respective transistors, capacitor C4 between the emitters of the two transistors, a frequency determining, twoterminal resonant circuit constituted by quartz crystal X1 connected between the base terminals of the two transistors and a tuned load circuit comprising capacitor C3 connected between the collector terminals of the two transistors, and inductors constituted by the primary windings of two transformers L1 and L2 connected between the respective transistor collector terminals and the positive supply rail + HT. The transformer secondaries are connected via respective series combinations of an inductor L3 or L4 and a capacitor C5 or C6 to a load R3 or R4. Thus, the arrangement is such as to cause the two transistors to act as respective Colpitt's oscillators, the two oscillators being synchronised by the interconnection of the transistor base terminals via crystal X1. In this type of oscillator, the interelectrode capacitances of the transistors T1 and T2 are absorbed by the capacitors of the tuned circuits, the collector-emitter capacitances being absorbed into C1 and C2 respectively, the collector-base capacitances into C3, and the emitter-base capacitances into C4. The resistances R1 and R2 provide D.C. current paths for the transistors T1 and T2 respectively and provided their values are high compared with the reactance of capacitor C4, their effect on the A.C.

current paths of the circuit will be negligible.

The resonant frequency of the common load circuit shared by the antiphase oscillators is given by the expression: 1 f = 2irVt where L = the sum of the inductances of the primary windings of transformers L1 and L2, and C = the capacitance of capacitor C3.

The secondary windings of the transformers L1 and L2 supply respective anti-phase oscillating signals to the circuits comprising the loads R3 and R4 and the respective combinations of L3 and C5 and L4 and C6.

If the values of the capacitors C6 and C5 and inductors L3 and L4 are chosen so that each such circuit comprises a series resonant circuit having the same resonant frequency as the crystal frequency, then only the resistances of the loads will appear across the transformer secondaries. Thus, in operation, providing the values of the unshared components balance, the oscillating signals delivered to the loads are equal in amplitude and exactly opposite in phase.

The amplitudes of the two signals delivered to the loads may be differentially adjusted by means of appropriate differential adjustments to the inductances of the transfomers L1 and L2. For example, if the inductance of L1 is increased by a small amount and L2 is decreased by a corresponding amount, the voltage swing at the collector of transistor T1 is proportionally increased and the voltage swing at the collector of T2 is proportionally decreased.

However, the total inductance of the tank circuit is unchanged and therefore the resonant frequency of the oscillator remains unchanged.

The phases of the two signals may also be differentially adjusted, this time by differentially changing the inductances of inductors L3 and L4. As previously mentioned the loads circuits connected to the secondary windings of transformers L1 and L2 are series resonant at the oscillating frequency of the whole circuit so that the loads appear as pure resistances across the transformers. If the inductance of one of the inductors, say L3, is increased by a small amount the increase will appear as a small inductance in series with the resistance of the load and the signal in the load circuit will correspondingly lag the driving voltage of the oscillator. Conversely, if the inductance of L4 is decreased by a small amount this will appear as a correspondingly small series capacitance and the signal in the load circuit will lead the driving voltage.

For small angles of phase shift produced in this way the total circuit impedance will vary very little, and providing the changes are differential the two load currents will remain equal in magnitude to a first approximation.

Figure 2 shows a convenient mechanical arrangement for differentially varying the inductance of two inductors which may be used for both L1 and L2, and L3 and L4. In either case the coiled windings 10 and 11 of two inductors are formed on hollow coil formers 12 and 13 respectively. Where the inductors are transformers the windings 10 and 11 comprise primary and secondary transformer windings. The internal bores of the hollow formers 12, 13 are threaded throughout their length so that threaded inductor cores 14 and 15 may be screwed to any axial position within the respective formers.

The formers 12, 13 are mounted (by means not shown) co-axially and the cores 14, 15 are slidably mounted on a common shaft 16, formed of nylon material to a hexagon section. A thumbwheel 17 is keyed to the centre of the shaft between the formers to provide an easy means of rotating the shaft and changing the positions of the inductor cores.

WHAT WE CLAIM IS: 1. Apparatus for producing two synchronous output waveforms substantially in antiphase with one another, the apparatus comprising a symmetrical arrangement of two semiconductor amplifying members each having two main terminals and a control terminal for controlling current flow through the member between the two main terminals, and of tuning means connected to the two members for causing each of them to operate as an oscillator, the tuning means including capacitance and inductance and further including a common frequencydetermining, two-terminal, resonant circuit through which the control terminals of said members are interconnected.

2. Apparatus according to claim 1 wherein said tuning means includes a common tuned load circuit comprising inductors connected to each amplifying member and a shared capacitor connected between the two amplifying members.

3. Apparatus according to claim 2 including adjusting means for differentially adjusting the inductances of the load circuit inductors to correspondingly adjust the relative amplitudes of said two output waveforms.

4. Apparatus according to any preceding claim, including a secondary winding on each load circuit inductor for deriving proportional outputs from the two oscillators.

5. Apparatus according to claim 4 including two series inductor-capacitor combinations resonant at about the resonance frequency of said frequency determining two-terminal resonant circuit, the combinations being coupled to respective ones of said secondary windings and there being adjusting means for differentially adjusting the inductances of the respective inductors of the combinations to finely adjust the relative phase of the two output waveforms.

6. Apparatus according to claim 3 or 5, wherein the or each adjusting means comprises a movable adjusting member coupled to each of the corresponding two inductors for moving respective magnetic cores of the two inductors relative to the windings thereof in such manner that movement of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. oscillating signals to the circuits comprising the loads R3 and R4 and the respective combinations of L3 and C5 and L4 and C6. If the values of the capacitors C6 and C5 and inductors L3 and L4 are chosen so that each such circuit comprises a series resonant circuit having the same resonant frequency as the crystal frequency, then only the resistances of the loads will appear across the transformer secondaries. Thus, in operation, providing the values of the unshared components balance, the oscillating signals delivered to the loads are equal in amplitude and exactly opposite in phase. The amplitudes of the two signals delivered to the loads may be differentially adjusted by means of appropriate differential adjustments to the inductances of the transfomers L1 and L2. For example, if the inductance of L1 is increased by a small amount and L2 is decreased by a corresponding amount, the voltage swing at the collector of transistor T1 is proportionally increased and the voltage swing at the collector of T2 is proportionally decreased. However, the total inductance of the tank circuit is unchanged and therefore the resonant frequency of the oscillator remains unchanged. The phases of the two signals may also be differentially adjusted, this time by differentially changing the inductances of inductors L3 and L4. As previously mentioned the loads circuits connected to the secondary windings of transformers L1 and L2 are series resonant at the oscillating frequency of the whole circuit so that the loads appear as pure resistances across the transformers. If the inductance of one of the inductors, say L3, is increased by a small amount the increase will appear as a small inductance in series with the resistance of the load and the signal in the load circuit will correspondingly lag the driving voltage of the oscillator. Conversely, if the inductance of L4 is decreased by a small amount this will appear as a correspondingly small series capacitance and the signal in the load circuit will lead the driving voltage. For small angles of phase shift produced in this way the total circuit impedance will vary very little, and providing the changes are differential the two load currents will remain equal in magnitude to a first approximation. Figure 2 shows a convenient mechanical arrangement for differentially varying the inductance of two inductors which may be used for both L1 and L2, and L3 and L4. In either case the coiled windings 10 and 11 of two inductors are formed on hollow coil formers 12 and 13 respectively. Where the inductors are transformers the windings 10 and 11 comprise primary and secondary transformer windings. The internal bores of the hollow formers 12, 13 are threaded throughout their length so that threaded inductor cores 14 and 15 may be screwed to any axial position within the respective formers. The formers 12, 13 are mounted (by means not shown) co-axially and the cores 14, 15 are slidably mounted on a common shaft 16, formed of nylon material to a hexagon section. A thumbwheel 17 is keyed to the centre of the shaft between the formers to provide an easy means of rotating the shaft and changing the positions of the inductor cores. WHAT WE CLAIM IS:

1. Apparatus for producing two synchronous output waveforms substantially in antiphase with one another, the apparatus comprising a symmetrical arrangement of two semiconductor amplifying members each having two main terminals and a control terminal for controlling current flow through the member between the two main terminals, and of tuning means connected to the two members for causing each of them to operate as an oscillator, the tuning means including capacitance and inductance and further including a common frequencydetermining, two-terminal, resonant circuit through which the control terminals of said members are interconnected.

2. Apparatus according to claim 1 wherein said tuning means includes a common tuned load circuit comprising inductors connected to each amplifying member and a shared capacitor connected between the two amplifying members.

3. Apparatus according to claim 2 including adjusting means for differentially adjusting the inductances of the load circuit inductors to correspondingly adjust the relative amplitudes of said two output waveforms.

4. Apparatus according to any preceding claim, including a secondary winding on each load circuit inductor for deriving proportional outputs from the two oscillators.

5. Apparatus according to claim 4 including two series inductor-capacitor combinations resonant at about the resonance frequency of said frequency determining two-terminal resonant circuit, the combinations being coupled to respective ones of said secondary windings and there being adjusting means for differentially adjusting the inductances of the respective inductors of the combinations to finely adjust the relative phase of the two output waveforms.

6. Apparatus according to claim 3 or 5, wherein the or each adjusting means comprises a movable adjusting member coupled to each of the corresponding two inductors for moving respective magnetic cores of the two inductors relative to the windings thereof in such manner that movement of the

adjusting member in one direction increases the value of one of the two inductors and decreases the value of the other while movement of the adjusting member in an opposite direction decreases the value of said one inductor and increases the other.

7. Apparatus according to any preceding claim wherein said frequency determining, two-terminal resonant circuit comprises a quartz crystal.

8. Apparatus substantially described herein with reference to the accompanying drawings.