AN IMPROVED OSCILLATOR

Abstract: The invention relates to an electric oscillator comprising a resonant circuit. After switching on such oscillators it takes a minimum time to build up an oscillatory working ampli­ tude. To reduce the oscillation build-up time of the oscillator it comprises a current supply source, control elements and switching means for changing the operating mode of the oscillator from a standby mode to an oscillatory mode vice versa. During standby mode a charging current is supplied via the coil of the resonant circuit so that while switching on into the oscillatory mode the charging current is interrupted, and the control elements immediately can have the disposal over an oscillatory amplitude to work with.

Description: The present invention relates to an oscillator comprising a current source incorporating a control element (TR1) which the current magnitude may be controlled, a resonant circuit (1,2) and switching means (S) for changing the operating mode of the oscillatory from a standby mode to an oscillator mode and vice versa by switching the control element from one operating condition to another so that during the stand-by mode a charging current is supplied from the current source to the inductance of the resonant circuit and upon transition from the stand-by mode to the oscillatory mode the charging current supply is interrupted; which oscillator comprises postive feedback means. Such an oscillator is known from the US-A- 3 503 008. In oscillators which have to be switched off and on rapidly it is desired that transient phenomena at the switching moments are suppressed in order to obtain a high attainable switching speed. To acquire this, the known oscillator comprises a current source comprising a transistor which generates a DC current, which current is passed through the inductor of a resonant circuit when the oscillator is in the stand-by mode. To switch the oscillator from the stand-by mode to the oscillatory mode, the value of the current generated by the current source is decreased. This causes the current in the inductance to flow into a capacitor present in the resonant circuit and an oscillating voltage across the resonant circuit will develop without any transient phenomena. In the known oscillator the oscillator is sustained because the current source transistor acts as an amplyfying device with a positive feedback path from the output to the input. The positive feedback means consists substantially of the parasitic colector-base capacitance of the transistor. In the known oscillator the choise of the several element values is dependent on the value of the collector-base capacitance of the transistor. This capacitance value however is often not accurately known and can show considerable spreading among different transistors of one type. Due to this spreading a correct operation of the oscillator is not always guaranteed. An object of the invention is to obtain an oscillator of the kind set forth in the preamble whose operation is not affected by the problems of the prior art oscillators. The object of the invention is obtained by means of an oscillator as described in the preamble characterized in that the positive feedback means include a keying signal generator (9) whose output is connected to a control element (TR1) for keying the said control element (TR1) in order supply current pulses to the resonant circuit (1,2) synchronously with alternate half-cycles of the oscillatory voltage developed across the said resonant circuit (1,2). In the oscillator according to the invention the oscillation is sustained by applying current pulses by means of the keying generator to the resonant circuit which current pulses increase the energy stored in the resonant circuit, to compensate for the losses in the resonant circuit. So the operation of the oscillator according to the invention is not dependent on the values of parasitic capacitances. A further embodiment of the oscillator according to the invention is characterized in that amplitude stabilisation means (9) sensitive to the peak-to-peak amplitude of the oscillator voltage across the said tuned resonant circuit is provided to control the amplitude of the keying pulses supplied by the said keying signal generator (9) to key the said control element in such a manner that the amplitude of the current pulses is inversely dependent of the peak-to-peak amplitude of the oscillator voltage. When the amplitude of the current pulses is inversely dependent on the peak-to-peak amplitude of the oscillatory voltage, the amplitude of the oscillatory voltage will be stabilized at a certain value. If the amplitude of the oscillatory voltage is increased, the amplitude of the current pulses will decrease and so the increase of the oscillatory voltage will be counter acted. A decrease of the oscillatory voltage will be counter acted in a similar manner. A practical embodiment of the oscillator according to the invention is characterized in that a differential amplifier (TR2, TR3) is associated with the said control loop having a first input connected to the said keying signal generator (9) and a second input connected to the resonant circuit in order to monitor the oscillator voltage across the said resonant circuit (1,2), the output of the differential amplifier being connected to the control input of the control element in order to supply keying pulses to the said control element (TR1) with the differential amplifier operating to supply such keying pulses having a magnitude current dependent upon the difference between the peak-to-peak voltage of the oscillatory voltage across the resonant ciruit (1,2) and a reference voltage. When the oscillator described in the above mentioned United States Patent is switched from the oscillator mode to the stand-by mode, still a small transient phenomenon will occur. A further object of the invention is to acquire an oscillator which shows no transient phenomena when switched from the oscillator mode to the stand-by mode. This further object of the invention is obtained by means of an oscillator which is characterized in that the said switching means include a controlled delay means (FF) having a first input to sense the selected mode of operation and a second input coupled to the resonant circuit in order to monitor the voltage present across the resonant circuit, the controlled delay means (FF) when the stand-by mode is selected and the oscillator is in the oscillatory mode, delay the transition to the stand-by mode and the supply of the charging current to the inductance (1) of the resonant circuit, untill the flow of current in the said inductance (1) due to oscillator current flow in the resonant circuit (1,2) is substantially at a maximum in the direction opposite to that of the charging current. Now the controlled delay means cause the moment of switching the oscillator from the oscillatory mode to the stand-by mode to be postponed to a switching moment where no transient phenomena occur. The invention will now be described with reference to attached drawings in which:- Fig. 1 is a schematic diagram of the circuit of an oscillator in accordance with the present invention. Figs. 2a, 2b and 2c are diagrammatic representations of waveforms produced at different parts of the circuit of Fig. 1. Fig. 3 is a schematic diagram of the circuit of an oscillator in accordance with the invention incorporating stabilisation means in accordance with another aspect of the invention. Fig. 4 is a schematic diagram of the circuit of an oscillator in accordance with the invention and incorporating a controlled delay means in accordance with a further aspect of the invention. Figs. 5a, 5b, 5c, 5d and 5e are diagrammatic representations of wave forms related to the operation of the circuit of Fig. 4. In the circuit of Fig. 1, a tuned resonant circuit TC formed by an inductance 1 and a capacitance 2 is connected across the supply rails 3 and 4 of a direct current supply source (not shown) via a transistor TR1 having a resistance 5 connected between its emitter electrode and the positive rail 3. The base electrode of the transistor TR1 is connected to the junction of a bias network constituted by the resistances 6 and 7 with the end of the resistance 7 remote from the base of the transistor TR1 being connected to the switch S so that, depending upon whether the switch S is in the position marked "A" or the position marked "B", the end of the resistance 7 remote from the base of the transistor TR1 is either connected to the positive voltage rail 3 or to the negative voltage rail 4. The base of the transistor TR 1 is also connected via a resistance 8 to the output of a keying signal generator in the form of a voltage comparator 9 having its input terminals connected across the capacitance 2. The input terminal 11 of the comparator 9 is connected to the positive rail 3 via a resistance 12 and to the negative rail 4 via a resistance 13. The values of the resistances 5,6,7 and 8 are chosen so that the transistor TR1 in combination with the direct current supply source (not shown) functions as a supply source of substantially constant current for the resonant circuit TC. In addition, the transistor TR1 together with its associated circuitry, functions as a control element for controlling the magnitude of the current supplied to the resonant circuit TC. For the transistor TR1 to function so that a source of substantially constant current is provided in this manner, the values of the resistances 5,6,7 and 8 must be chosen such that, in operation, saturation of the transistor TR1 does not occur below the maximum peak-to-peak voltage developed across the resonant circuit TC throughout the control range exercised by the transistor TR1. With the switch S in the position marked "B", the oscillator of Fig. 1 is in the standby mode of operation with the voltage divider formed by the resistances 6 and 7 connected across the supply rails 3 and 4 so that current is fed via the resistance 5, the emitter-collector path of the transistor TR1 and through the inductance 1, magnetically charging the inductance 1. The magnitude of the charging current supplied through the inductance 1 is dependent upon the voltage present at the junction of the resistances 6 and 7. Upon switchover of the switch S to the position marked "A", the transistor TR1 is cut-off and the supply of current to the inductance 1 is sharply interrupted. Despite the cessation of current flow via the transistor TR1, the flow of current in the inductance continues as circulating current in the resonant circuit formed by the inductance 1 and the capacitance 2. Without the control loop provided by the voltage comparator 9 and the resistance 8, a damped train of oscillation would be produced in the resonant circuit TC. However, owing to the control of the transistor TR1 by the loop, additional energy is supplied to the resonant circuit TC via the emitter-collector path of the transistor TR1 at the correct times to ensure the losses of the resonant circuits TC are compensated so that oscillation is sustained. In this respect, the (positive) input 11 of the voltage comparator 9 is connected to that electrode of the capacitance 2 adjacent the negative supply rail 4 whereas the (negative) input 10 of the voltage comparator 9 is connected to the electrode of the capacitance 2 remote from the supply rail 4. When the (negative) input 10 of the comparator 9 is positive relative to the (positive) input 11, then the output of the comparator 9 will be "low", the "low" state at the output being substantially the potential of the negative rail 4 at approximately ground potential. Alternatively, when the (negative) input 10 is negative relative to the (positive) input 11, then the output of the comparator 9 will be "high". With the switch S in the position marked "A", when the output of the comparator 9 is "low", the end of the resistance 8 remote from the base electrode of the transistor TR1 is connected via the comparator 9 to the supply rail 4 so that the voltage divider formed by the resistances 6 and 8 is connected between the supply rails 3 and 4. Under these conditions, the voltage produced at the junction of the resistances 6 and 8 causes the transistor TR1 to conduct so that current flows to the resonant circuit TC via the emitter-collector path of the transistor TR1. With the switch S in the position marked "A", when the output of the comparator 9 is "high", the voltage level at the junction of the resistances 6 and 8 ensures that the transistor TR1 is "cut-off" and no current is supplied to the resonant circuit TC. Accordingly, with the switch S in the position marked "A", each time the potential of the point P swings positive relative to the supply rail 4, owing to the action of the loop formed by the comparator 9 and the resistance 8, current is supplied via the transistor TR1 to the tuned circuit TC. For explanation purposes, in time relationship with each other, Fig. 2a shows the level of current flow via the emitter-collector path of the transistor TR1, Fig. 2b shows the voltage level at the output of the comparator 9 and Fig. 2c shows the volage at the point P, respectively for the condition when the switch S is in the position marked "B" (shown in the figure as INT B) and the condition when the switch S is in the position marked "A"(shown in the figure as INT A). During the time that the switch S is in the position "B" the emitter collector current having the relatively high level depicted in Fig. 2a, flows also as charging current through the inductance 1. During this time the voltage at the point P is at a slightly more positive level than that of the supply rail 4. However, the bias network constituted by the resistances 12 and 13 positively biases the input 11 making that input slightly more positive than the input 10 thereby ensuring that the output of the comparator 9 is "high" when the switch S is in the position "B". Satisfactory transition from the standby mode (or the quiescent mode) to the oscillatory mode requires the changeover of the switch S from the position marked "B" to the position marked "A" to be a sharp changeover. In a practical embodiment of the invention, changeover of the switch S should be accomplished electronically thereby ensuring rapid changeover. During the time the switch S is in the position marked "A" continuous oscillation is produced in the resonant circuit TC and a continuous oscillatory voltage ideally of sinusoidal wave shape, is produced at the point P as indicated by Fig. 2c. As the voltage at point P is alternately positive and negative with respect to the rail 4, the output voltage of the voltage comparator 9 is alternately "low" and "high" and serves as a keying signal which is fed to repetitively switch the transistor TR1 between the conductive and the non-conductive states, the level of conduction in the conductive state being determined by the relative values of the resistances 6 and 8. As there is no stabilisation system associated with the circuit of Fig. 1, the peak-to-peak amplitude of the alternating voltage at the point P will become limited to a level determined by the magnitude of the supply voltage between the voltage rails 3 and 4. Pulses of current supplied via the emitter-collector path of the transistor TR1 are denoted in Fig. 2a by the numeral 21. The amplitude of the current flow is constant for the duration of the respective current pulses 21 and of the same amplitude for each pulse 21, being smaller than the amplitude of the constant amplitude flow of current through the transistor TR1 during the standby mode of the oscillator when the switch S is in the position B. An oscillator in accordance with the invention, wherein the amplitude of the oscillator output is stabilised, may be provided by means of the circuit arrangement of Fig. 3. In Fig. 3, similar parts to those of the circuit arrangement of Fig. 1 are denoted by like letters or numerals. In Fig. 3, an inverter 301 is employed in lieu of the switch S of the circuit of Fig. 1 for switching the oscillator from the standby mode to the oscillatory mode and vice versa. By means of a mode selection means (not shown) the input 302 of the inverter 301 is held "high" for the oscillator to be in the standby mode and "low" for the oscillator to be in the oscillatory mode. With the input 302 "high" the output of the inverter 301 is at the potential of the negative supply rail 4 and with the input 302 "low" the output of the inverter 301 is at the potential of the positive supply rail 3. The oscillator of Fig. 3 incorporates a differential amplifier constituted by the transistors TR2, TR3 and TR4 and their associated circuity. The collector electrode of the transistor TR2 is connected to the positive supply rail 3 and its base electrode is connected to the point P via a diode D1 and a resistance 303. The base of the transistor TR2 is also connected via a capacitance 304 to the negative supply rail 4 and via a resistance 305 to the collector electrode of the transistor TR4. The collector electrode of the transistor TR3 is connected to the base electrode of the transistor TR1 and its base electrode is connected, via, the diode D5, to the output of the voltage comparator 9. The base electrode of the transistor TR3 is also connected via a series combination of the diodes D2, D3 and D4 to the negative supply rail 4. The diode D5 is bridged by a series combination of the resistances 306 and 307, the junction of the combination being connected to the output of the inverter 301. The emitter electrodes of the transistors TR2 and TR3 are connected together and via the resistance 308 connected to the collector electrode of the transistor TR4, the emitter electrode of the latter being connected to the negative supply rail 4. The base electrode of the transistor TR4 is connected via a resistance 309 to the output of the inverter 301. In the oscillator of Fig. 3, the manner of connection of the voltage comparator across the capacitance 2 is different to that of the oscillator of Fig. 1. In the oscillator of Fig. 3, the (positive) input 11 of the comparator 9 is connected to the junction of the resistances 310 and 311 forming a voltage divider between the positive supply rail 3 and the point P whereas the (negative) input 10 is connected to the junction of the resistance 312 and 313 forming a voltage divider between the positive supply rail 3 and the negative supply rail 4. In this way with the oscillator of Fig. 3, the output of the voltage comparator 9 goes "high" when the voltage at the point P is positive with respect to the negative supply rail 4 and goes "low" when the voltage at point P is negative with respect to the supply rail 4. Owing to the presence of the voltage divider constituted by the resistances 312 and 313, the output of the comparator 9 will be "low" during the standby mode. The operation of the oscillator of Fig. 3 is basically similar to that of the oscillator of Fig. 1. However, during the oscillatory mode, by means of the differential amplifier incorporated in the control loop, the peak-to-peak amplitude of the oscillatory voltage developed across the resonant circuit TC is employed to control the level of energy supplied to the resonant circuit TC by the control element formed by the transistor TR1. In this respect, the oscillatory voltage present at the point P is rectified and compared with a reference voltage, keying pulses having a magnitude dependent upon the difference being employed to control the amplitude of the current pulses supplied via the transistor TR1 to the resonant circuit TC and this way stabilising the amplitude of the oscillatory voltage across the resonant circuit TC. The oscillatory voltage present across the resonant circuit TC, i.e. between the point P and the negative supply rail 4, is rectified by the diode D1 causing the capacitance 304 to become charged to a level determined by the peak-to-peak level of the oscillatory voltage. The time constant of the capacitance 304 and the resistance 303 is chosen to correspond approximately with the period of one cycle of the natural frequency of the resonant circuit TC thereby enabling the charge present across the capacitance 304 to rise rapidly with increase in the peak-to-peak level of the oscillatory voltage across the tuned circuit TC. The rate of decay of the charge of capacitance 304 is determined by the value of the resistance 305 and the time constant of the capacitance 304 and the resistance 305 is chosen to be equal to several cycles of the oscillation frequency. The positive going pulses produced at the output of the voltage comparator 9 when the oscillator is in the oscillatory mode have an amplitude approximately equal to the voltage difference between the supply rails 3 and 4 and, by means of the diode D5, the series combination of the diode D2, D3, D4 and the associated resistances 306 and 307, the amplitude level of each pulse supplied to the base of the transistor TR3 is clamped to a reference level determined by the characteristics of the respective diodes D2, D3 and D4. Preferably, the diodes D2, D3 and D4 are of the same diode type. The voltage at the base of the transistor TR3 will be zero when the oscillatory voltage at the point P is negative with respect to the supply rail 4 and the output of the voltage comparator 9 is "low". On the other hand, the voltage at the base of the transistor TR3 will be clamped to the reference level determined by the characteristics of the diodes D2, D3 and D4 when the oscillatory voltage at the point P is positive with respect to the supply rail 4 and the output of the comparator 9 is "high". If, for example, the junction voltage of each of the diodes D2, D3 and D4 is 0.6 volts then the reference level to which the voltage at the base of the transistor TR3 is clamped during the intervals that the output of the voltage comparator 9 is "high"will be 1.8 volts. The base electrode of the transistor TR4 is connected to the output of the inverter 301. Accordingly, when the oscillator is in the standby mode, the "low" voltage present at the output of the inverter 301 is fed to the base of the transistor TR4 so that the latter is cut-off and the transistors TR2 and TR3 are also cut-off. On the other hand, with the oscillator of Fig. 3 in the oscillatory mode, the "high" voltage fed from the output of the inverter 301 to the base of the transistor TR4 drives the transistor TR4 to saturation, the emitter-collector current of the transistor TR4 being shared between the respective collector-emitter paths of the transistors TR2 and TR3 in a manner depending upon the respective voltages supplied to the base electrodes. With the oscillator of Fig. 3 in the oscillatory mode, the base-emitter current of the transistor TR1 and hence the collector-emitter current of the transistor TR1 is determined by the magnitude of the collector-emitter current of the transistor TR3. Of course, the transistor TR3 is non-conductive when the output of the voltage comparator 9 is "low" and hence the base electrode of the transistor TR3 is also "low", However, each time the output of the voltage comparator 9 is "high" the transistor TR3 conducts, the magnitude of the collector-emitter current flow being determined by the voltage difference between the base voltages of the transistors TR2 and TR3. As the base voltage of the transistor TR3 is at the previously described reference level at the relevant period whereas the base voltage or the transistor TR2 is at a voltage corresponding with the instantaneous peak-to-peak level of the oscillatory voltage across the resonant circuit TC, the magnitude of the pulse of collector-emitter current produced by the transistor TR3 each time the output of the voltage comparator 9 is high and hence the magnitude of the pulse of current supplied via the emitter-collector path of the transistor TR1 is inversely related to the peak-to-peak level of the oscillatory voltage across the tuned circuit TC. As a rise in the peak-to-peak level of the oscillatory voltage at the point P causes a reduction of the energy supplied via the transistor TR1 to the resonant circuit TC and a fall in the peak-to-peak level causes an increase of such energy, the peak-to-peak level of the oscillatory voltage will become stabilised at a particular level dictated by the parameters of the circuit arrangement and at that particular level the pulses of current supplied via the transistor TR1 will be of magnitude to precisely compensate for circuit losses. Since the transistor TR4 is cut-off when the oscillator of Fig. 3 is in the standby mode, a charge present across the capacitance 304 at the instant of transition from the oscillatory mode to the standby mode will be retained for a significant period of time so that upon switch-over to the oscillatory mode at a later time, the charge present across the capacitance 304 ensures stabilisation at the same level as before. Such an arrangement is advantageous with vehicle detection apparatus wherein a plurality of oscillators operating in a standby mode are each repeatedly and in sequence switched to an oscillatory mode for a short period of time. In the oscillator of Fig. 4, similar parts to those of the oscillator of Fig. 1 are again denoted by a similar letter or numeral. The oscillator of Fig. 4 is substantially the same as that of Fig. 1 except that instead of the switch S of Fig. 1 there is provided the combination of a mode selector switch MS, a D-type flip-flop FF and an inverter 401 interconnected as shown in the figure. The mode selector switch MS is a two-position switch. With the switch MS in the position P1, the standby mode is selected and the common terminal 402 is connected to the negative supply line 4 whereas with the switch MS in the position P2, the oscillatory mode is selected and the common terminal 402 is connected to the positive supply rail 3. The data input terminal D of the D-type flip-flop FF is connected to the common terminal 402 of the switch MS and also via the inverter 301 to the set-direct input terminal S of the flip-flop FF. The clear-direct input terminal C of the flip-flop FF is connected to the junction of a resistance 403 and a capacitance 404 which are connected in series between the voltage rails 3 and 4. The clock input terminal CLK of the flip-flop FF is connected to the output of the voltage comparator 9 and the Q output terminal terminal Q of the flip-flop FF is connected to the end of the resistance 7 remote from the base electrode of the transistor TR1. It will be appreciated that when the output Q of the flip-flop FF is "low", the oscillator will operate in the standby mode so that the transistor TR1 will be conductive and charging current will flow through the inductance 1 via the emitter-collector path of the transistor TR1 and when the output Q is "high", the oscillator will operate in the oscillatory mode so that the output of the voltage comparator 9 will be alternately "low" and "high" as the voltage present at the point P repeatedly swings positive and negative relative to the negative supply rail 4, rendering the transistor TR1 conductive each time the output of the voltage comparator 9 is "low" so that pulses of current are fed via the emitter-collector path of the transistor TR1 thereby supplying energy to the resonant circuit TC and sustaining oscillations. The operation of the D-type flip-flop FF is such that data present at the data-input D is transferred to the output Q on the positive-going edge of a pulse applied to the clock-input terminal CLK. In addition, each time the set-direct terminal S becomes "low", the output at the output terminal Q is set to "high" irrespective of the state of the clock input CLK or the input D, whereas each time the clear-direct terminal becomes "low", the output at the output terminal Q is cleared to "low". By way of example, the flip-flop FF may be one of the flip-flops incorporated in a signetics type 74LS74. The operation of the switching arrangements incorporated in the oscillator of Fig. 4 are as follows: when the supply voltage is applied to the circuit, owing to the time taken for the capacitance 404 to become sufficiently charged, the terminal C is kept in a "low" state for a sufficient length of time to "clear" the flip-flop FF so that the output Q is "low". Thereafter, the capacitance 4 is rapidly charged via the resistance 403 to the supply potential and the capacitance 404 and the resistance 403 serve no further function until the supply voltage is removed whereupon the capacitance 404 is discharged in readiness for the next application of the supply voltage. The mode selection switch MS must initially be in the position P1 and in this position the terminal 402 is "low" so that the output of the inverter 401 is "high" and does not affect the state of the flip-flop FF. Since the output Q is "low", the oscillator operates in the standby mode as previously described. Changeover of the selector switch MS to the position P2, causes the terminal 402 to go "high" and for the output of the inverter 401 and the input S of the flip-flop FF to become "low" thereby setting the output Q to the "high" state. With the output Q in the "high" state, the oscillator operates in the oscillatory mode as previously described. The oscillator of Fig. 4 should be so proportioned that the magnitude of the circulating current of the resonant circuit TC whilst the oscillator is in the oscillatory mode is equal to the magnitude of the charging current supplied to the inductance 1 whilst the oscillator is in the standby mode. With the oscillator of Fig. 4 in the oscillatory mode and with the switch MS in the position P2, the positive-going edge of a "clocking pulse" is supplied to the input terminal CLK from the output of the voltage comparator 9 for every negative half cycle of oscillation and with the edge of each such pulse the data present at the input D of the flip-flop FF is transferred to the output Q. Since the switch MS is in the position P2, the terminal 402 and the input D is "high". Accordingly, the output Q remains "high" and the oscillator continues to operate in the oscillatory mode so long as the switch MS remains in the position P2. When the mode selection switch MS is returned to the position P1 so as to terminate the oscillatory mode of the oscillator, the terminal 402 is made "low" so that, via the inverter 401, the terminal S becomes "high". Such removal of the set signal from the terminal S does not change the state of the output Q and, since the output Q is "high", the oscillator remains in the oscillatory mode until the next-occurring positive going pulse produced by the voltage comparator 9 whereupon the output Q of the flip-flop FF goes "low". The occurrence of each positive-going edge in the waveform supplied by the voltage comparator 9 to the clocking terminal CLK occurs at the instants of cross-over from positive to negative of the oscillatory voltage present at the point P. At such instants, the amplitude of the oscillatory circulating current flowing in the resonant circuit TC is at a maximum, that is to say, flowing at a maximum through the inductance 1. Since the voltage at the output terminal Q changes from "high" to "low" at the first such instant subsequent to return of the mode selector switch MS to the position 1, the transistor TR 1 is switched to a conductive state so that current having a magnitude substantially equal and opposite to that of the circulating current is supplied to the inductance 1 causing the oscillation of the resonant circuit to substantially cease and for the charging current to thereafter be supplied via the emitter collector path of the transistor TR 1 until the next changeover of the mode selector switch MS. The transition of the oscillator of Fig. 4 from the standby mode to the oscillatory mode and again to the standby mode is shown graphically by Figs. 5a to 5e. Fig. 5a shows the voltage level of the common terminal 402, Fig. 5b shows the voltage present at the point P, Fig. 5c shows the voltage at the output of the voltage comparator 9, Fig. 5d shows the voltage at the output of the terminal Q of the flip-flop FF and Fig. 5e shows the level of current supplied via the emitter-collector path of the transistor TR1 to the inductance 1. Prior to the constant T1, the oscillator is in the standby mode so that a charging current having a relatively large magnitude is supplied via the transistor TR1 to the inductance 1 as depicted by the current level shown in Fig. 5e. At the instant T1, switchover of the mode selection switch MS from the standby position P1 to the oscillatory mode position P2 occurs so that a "high" voltage is produced at the terminal 402 causing the output terminal 2 of the flip-flop FF to go "high" as previously described and as depicted by the wave shape of Fig. 5d, cutting the flow of current via the emitter collector path of the transistor TR1 so that oscillation commences. Between the instance T1 and T3, whilst the oscillator is in the oscillatory mode the voltage at the point P follows a substantially sinusoidal wave shape as illustrated by the wave in Fig. 5b causing the output of the voltage comparator 9 to alternate between a "high" and a "low" level in accordance with the wave shape of Fig. 5c with the result that pulses of current are supplied via the transistor TR1 supplying energy to the tuned circuit TC, the magnitude of the current pulses being considerably lower than the magnitude of the charging current, as depicted by the shape of the wave form of Fig. 5e. At the instant T2, as shown by Fig. 5a, the mode selector switch MS is returned to the standby position P1 so that the voltage at the terminal 402 becomes "low". However, the output terminal Q of flip-flop FF remains "high" until the instant T3 when, simultaneously with the leading edge of the next occurring positive-going pulse at the output of the voltage comparator 9, as depicted by the wave form of Figs. 5c and 5d, the voltage of the output terminal Q becomes "low" switching the transistor TR1 to the conductive state so that the flow of charging current is resumed via the emitter-collector path of the transistor TR1 simultaneously inhibiting oscillation so that the oscillatory mode of the oscillator is terminated and the standby mode is resumed.

Claims: An oscillator comprising a current source incorporating a control element (TR1) which the current magnitude may be controlled, a resonant circuit (1,2) and switching means (S) for changing the operating mode of the oscillator from a standby mode to an oscillatory mode and vice versa by switching the control element from one operating condition to another so that during the stand-by mode a charging current is supplied from the current source to the inductance of the resonant circuit and upon transition from the stand-by mode to the oscillatory mode the charging current supply is interrupted; which oscillator comprises postive feedback means, characterized in that the positive feedback means include a keying signal generator (9) whose output is connected to said control element (TR1) for keying the said control element (TR1) in order supply current pulses to the resonant circuit (1,2) synchronously with alternate half-cycles of the oscillatory voltage developed across the said resonant circuit (1,2). An oscillator as claimed in Claim 1, characterized in that amplitude stabilisation means (9) sensitive to the peak-to-peak amplitude of the oscillator voltage across the said tuned resonant circuit is provided to control the amplitude of the keying pulses supplied by the said keying signal generator (9) to key the said control element in such a manner that the amplitude of the current pulses is inversely dependent of the peak-to-peak amplitude of the oscillator voltage. An oscillator as claimed in Claim 2, characterized in that a differential amplifier (TR2, TR3) is associated with the said control loop having a first input connected to the said keying signal generator (9) and a second input connected to the resonant circuit in order to monitor the oscillator voltage across the said resonant circuit (1,2), the output of the differential amplifier being connected to the control input of the control element (TR1) in order to supply keying pulses to the said control element (TR1) with the differential amplifier operating to supply such keying pulses having a magnitude current dependent upon the difference between the peak-to-peak voltage of the oscillatory voltage across the resonant cirut (1,2) and a reference voltage. An oscillator in accordance with either Claim 1, 2 or 3, characterized in that the said switchng means include a controlled delay means (FF) having a first input to sense the selected mode of operation and a second input coupled to the resonant circuit in order to monitor the voltage present across the resonant circuit, the controlled delay means (FF) when the stand-by mode is selected and the oscillator is in the oscillatory mode, delay the transition to the stand-by mode and the supply of the charging current to the inductance (1) of the resonant circuit, until the flow of current in the said inductance (1 due to oscillator current flow in the resonant circuit (1,2) is substantially at a maximum in the direction opposite to that of the charging current.

Inventors: LILL, GRAHAM DAVID
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: H03L3
IPC codes: H03B11, H03L3, H03B5