GB1566095A - Modulation circuits - Google Patents

Abstract

The modulation circuit comprises a tetrode (1) as switching element which is arranged in order to prevent any excessive dissipation in the grid-screen circuit. The grid-screen is supplied by a constant current source (23, 24) so as to prevent any excess current being diverted from the cathode to the grid-screen when the impedance of the anode circuit rises to a large value. This circuit can be used in the output stages of radio transmitters. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO MODULATION CIRCUITS (71) We, THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex, CMl 1PL, do hereby 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: This invention relates to modulation circuits and more specifically to amplitude modulation circuits utilising a pulse width modulator. In order to provide a high efficinecy amplitude modulation circuit it has been proposed to use a pulse width modulator which, itself, absorbs relatively little power to produce a digital pulse train having a variable mark-space ratio and to follow it by a low pass filter which derives a variable amplitude waveform from the pulse train.

A gridded valve such as a tetrode valve can be used as an efficient pulse width modulation switch and, whilst it is rendered conductive, it can pass a large current from its cathode to its anode whilst absorbing relatively little power itself. If the impedance of the circuitry is such as to prevent this large current, which is emitted from the cathode, from passing to the anode a large proportion of the emitted current is diverted to the screen electrode. This results in an excessive screen current flowing and a consequential excessive power dissipation in the tetrode valve which is unacceptably large.

The present invention seeks to provide an improved modulation circuit in which the screen electrode is driven in a relatively simple and economical manner which avoids excessive dissipation and power in the tetrode valve itself.

According to this invention, a modulation circuit includes a pulse width modulation switch in the form of a switchable valve having a grid electrode to which, in operation, is applied a pulse width modulation control signal, and a screen electrode; means for applying a substantially constant bias periods whilst the valve is rendered conductive by the application of a suitbale control signal to the grid electrode; and a low pass filter arranged to attenuate frequencies at the switching frequency of said control signal. Preferably, the switchable valve is a tetrode. Preferably, a further switch able device is connected in parallel with a pair of input terminals of said low pass filter, and which is arranged to provide a shunt path for current in the low pass filter whilst said pulse width modulation switch is non-conductive.

The further switchable device may be a transistor or triode valve, but, preferably, it is also a tetrode valve having a grid electrode to which a signal is applied in antiphase to the pulse width modulation control signal.

Preferably, the two tetrode valves have common current source means for providing a bias current for their respective screen electrodes. Preferably, again, said common current source means comprises an inductor in series with a source of potential. As each tetrode valve is rendered conductive, alternately, under the action of the respective anti-phase grid control signals, the screen current supplied by the inductance and source of potential (which together act as a substantially constant current generator) is diverted to the screen electrode of whichever tetrode valve is at that instant conductive.

In some cases it may be desirable to provide a resistive impedance in series with said inductance to limit excessive current flow under adverse operating conditions, but normally the intrinsic resistance of the screen electrode circuit and the internal impedance of the source of potential will provide sufficient impedance.

In embodiments of the pulse width modulation system in which the cathode of the switchable valve which comprises the pulse width modulation switch is arranged to be switched between earth potential and an H.T. potential under the action of the pulse width modulation control signal which is applied to the grid electrode of said switchable valve, the grid drive circuit which applies the control signal to the grid electrode is, preferably, electrically isolated from the earth potential by means of an optical coupler.

Preferably again, the optical coupler comprises an electro-optical transducer and an opto-electrical transducer coupled by an electrically insulating optical fibre.

Where, as specified above, two tetrode valves are provided, preferably the respective grid electrode control signals are derived using a common optical coupler using a common optical fibre path.

The invention is further described, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates a known amplitude modulation circuit utilising a pulse width modulator, Figure 2 illustrates an amplitude modulation circuit utilising a pulse width modulator in accordance with the present invention, and Figures 3, 4, 5 and 6 show possible modifications of part of the circuit shown in Figure 2.

Referring to Figure 1, a switchable tetrode valve 1 is connected to a terminal 2 at which is applied a source of H.T. potential, typically 25kV. The cathode of the tetrode 1 is connected to the cathode of a diode 3 and also via an inductor 4 to a low pass filter 5. The output of the low pass filter 5 is fed to an r.f. amplifier stage 6 consisting of a further tetrode valve 7 in series with an amplitude modulation of a carrier frequency which is fed at a very high power level to a radiating antenna 9. The carrier frequency is applied to the grid electrode of the tetrode valve 7. The incoming audio frequency signal is suitably encoded in a pulse width modulation form (by means not shown) using a samnlinp nr chnnpin freauency which is much higher than the highest audio frequency. The coded signal is amplified and applied to the control grid electrode 10 of the first tetrode 1. This causes a high level pulse width modulation waveform of amplitude substantially equal to the H.T. supply voltage present at terminal 2 to appear across the inductor 4 which re-constitutes the pulse width modulation waveform into a high level audio signal. The low pass filter 5 further removes undesirable signal components from the audio signal which is then presented to the r.f. amplifier 6 whose maximum amplitude swim is from zero volts (earth) to the level of the H.T. supply volt- age less any losses nroduced within the tetrode valve 1 and other series components.

A capacitor 11 is connected acorss the output terminals of the H.T. supply source.

The diode 3 may be a conventional diode having only an anode and a cathode or, alternatively, it may be a valve having a control grid which is permanently biassed into the conductive state so that it conducts whenever a forward voltage is applied to it.

This state normally occurs whenever the tetrode valve 1 is non-conductive. The reconstituted audio waveform flows as a current through the inductor 4 continuously and this current flows through the tetrode valve 1 whenever it is conductive and through the diode 3 whenever the tetrode valve 1 is non-conductive. AIthough this audio frequency current is determined by the pulse width modulation switching action of the tetrode valve 1, the instantaneous amplitude and phase of the current is controlled by the inductor 4 and the low pass filter 5. Whilst the tetrode valve 1 and the diode 3 are in their conductive state they must be biassed in readiness to pass the peak value of the audio frequency current flowing through the inductor 4 plus any ripple and transient signals associated with the high frequency pulse width modulation chopping waveform.

It is convenient to use a tetrode valve in place of the diode 3 so that valves having similar characteristics can be used throughout the circuit and if the valves have been economically chosen for this application they will have little reserve of peak current capability and will, therefore, require positive screen electrode and control grid electrode voltages close to their maximum permissible ratings in order to pass the peak current at low anode-cathode voltage which is necessarv for high efficiency and low distortion. Whilst the peak anode current is flowing, the currents drawn by the screen electrode 12 and control grid electrode 10 will be fairly low. If the screen and control grid electrodes are maintained at full positive voltage levels when the anode current is substantially less than its maximum peak value, the excess cathode emission drawn by the two electrodes, but not now required by the anode circuit, will flow, principally, to the screen electrode. As the valves operate for most of the time at substantially less than peak anode current, this condition gives rise to serious over-dissipation in the screen circuit and is unacceptable.

Referring to Figure 2, there is shown therein a circuit in accordance with the present invention and it will be seen that the basic circuit layout is very similar to that shown in Figure 1, but the low pass filter 5 and the amplifier 6 have for convenience, been combined in a single block 21. The tetrode valve 1 is connected via the inductor 4 as previously to the low pass filter, but ths diode 3 is now provided by an additional tetrode valve 22 which is connected in parallel with the input terminals 14 and 15 of the low pass filter. The tetrode valves 1 and 22 share a common screen electrode bias circuit which is constituted by an induc tor 23 in series with a voltage source 24, typically about 500 volts. The grid control waveform ideally consists of a pulse width modulation signal having approximately 100nS edges and because the markspace ratio of the pulse width modulation waveform can vary from 0 to 100 per cent under varying conditions of modulation, the grid drive system requires a bandwidth extending down to the lowest audio frequencies to be used, typically 30Hz. The grid drive signal must be coupled from the low voltage generating circuits which generate the pulse width modulation waveform to the valve deck which is being switched between earth and the H.T. level applied at terminal 2. The valve deck is that part of the circuitrv associated with the tetrode valves 1 and 22 which operate at substantially cathode potential. The pulse width modulation grid drive signal is applied to an electro-optical converter 26 which includes a device such as a light emitting diode which produces modulated optical signal in accordance with the pulse width modulation signal applied to the converter. An optical fibre 27 transfers the ontically modulated signal to an optical recevier 28 which is at the cathode potential of the two tetrode valves 1 and 22. Bv this means the grid drive circuits 29 and 30 are electricallv isolated from earth. The optical receiver 28 contains circuitry which provides signals in mutual anti-phase to the grid drive circuits 29 and 30 so that one tetrode valve is conductive whilst the other is nonconductive and vice-versa. The grid drive circuits 29 and 30. themselves, merely provide the required degree of voltage drive amplification. The mark-space ratio of the signal applied to the grids from the electrooDtical converter 26 can be set to determine the overall power level of the signal trans emitted bv the outDut r.f. amnlifier bv controlling the average mark-space ratio of the nulse width modulation signal.

The use of the optical isolation enables the cathode-to-earth canacitance to be kent to an absolute minimum. The presence of undeeirable cathode-to-earth capacitance significantly distorts the re-constituted audio signal provided bv the inductor 4 and the low pass filter and it is important to minimise this as far as is possible.

The screen drive circuit constituted by the inductor 23 and the voltage source 24 provides a particularly economical circuit since, as the two tetrode valves 1 and 22 are driven in anti-phase by the two grid drive circuits 29 and 30, the screen bias current is automaticallv diverted to whichever of the two tetrode valves is, for the time being, in conduction.

Figures 3 and 4 show in greater detail alternative forms of the screen drive circuit which produces a substantially constant bias current for screen electrodes 38 and 39. In Figure 3 an inductor 31 is connected to a rectifying diode bridge network 32, and an additional inductor 34 is provided between the network 32 and a transformer 35. The transformer 35 could conveniently be connected via terminals 36, 37 to the filament transformer required for the tetrode valves.

The inductor 34 provides additional current stabilisation which may be desirable whilst the anode current of the conductive tetrode valve is small.

In Figure 4 a voltage doubling circuit is shown, and this is capable of providing a substantially lossless controlled current output. In both Figures 3 and 4, a capacitor 40 can be provided, and tappings on the transformer can be used to adjust the initial current level. Additionally, in both cases, a small series resistor can be connected directly at each screen electrode, to prevent parasitic oscillation or reduce the effects of valve arc.

A modified screen electrode bias circuit is shown in Figure 5 which could be used if tetrode valve 1 were used alone. The tetrode valve has a screen grid 41 connected to an inductor 23 and a voltage source 24, as in the circuit shown in Figure 2. In this case, an additional shunt circuit consisting of a switch 43 and an impedance 42 is connected between cathode potential and the screen electrode 41. When the tetrode valve 1 is rendered conductive by means of a suitable grid drive signal applied to the grid electrode 44 the switch 43 is closed, thereby providing a low impedance shunt path for the excessive voltage which would otherwise develop across the inductor 22 under these conditions.

If the other tetrode valve 22 were used alone to perform the duty of diode 3, the arrangement of Figure 6 may be employed to apply suitable and substantially constant bias current to both screen and grid electrodes, to provide adequate forward conductivity between anode and cathode with- out over-dissipation at either screen or grid electrodes. The voltage sources and inductors at screen and grid electrodes, shown in Figure 6, may be replaced by bias supplies as in Figures 3 or 4 to give improved control of bias currents if desired. Additional series resistance may also be employed. Since, for diode duty, this tetrode need not be controlled at the grid to render it non-conductive, no additional switch corresponding to switch 43 of Figure 5 is necessary.

WHAT WE CLAIM IS: 1. A modulation circuit including a pulse width modulation switch in the form of a switchable valve having a grid electrode to which, in operation, is applied a pulse width modulation control signal and a screen

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. tor 23 in series with a voltage source 24, typically about 500 volts. The grid control waveform ideally consists of a pulse width modulation signal having approximately 100nS edges and because the markspace ratio of the pulse width modulation waveform can vary from 0 to 100 per cent under varying conditions of modulation, the grid drive system requires a bandwidth extending down to the lowest audio frequencies to be used, typically 30Hz. The grid drive signal must be coupled from the low voltage generating circuits which generate the pulse width modulation waveform to the valve deck which is being switched between earth and the H.T. level applied at terminal 2. The valve deck is that part of the circuitrv associated with the tetrode valves 1 and 22 which operate at substantially cathode potential. The pulse width modulation grid drive signal is applied to an electro-optical converter 26 which includes a device such as a light emitting diode which produces modulated optical signal in accordance with the pulse width modulation signal applied to the converter. An optical fibre 27 transfers the ontically modulated signal to an optical recevier 28 which is at the cathode potential of the two tetrode valves 1 and 22. Bv this means the grid drive circuits 29 and 30 are electricallv isolated from earth. The optical receiver 28 contains circuitry which provides signals in mutual anti-phase to the grid drive circuits 29 and 30 so that one tetrode valve is conductive whilst the other is nonconductive and vice-versa. The grid drive circuits 29 and 30. themselves, merely provide the required degree of voltage drive amplification. The mark-space ratio of the signal applied to the grids from the electrooDtical converter 26 can be set to determine the overall power level of the signal trans emitted bv the outDut r.f. amnlifier bv controlling the average mark-space ratio of the nulse width modulation signal. The use of the optical isolation enables the cathode-to-earth canacitance to be kent to an absolute minimum. The presence of undeeirable cathode-to-earth capacitance significantly distorts the re-constituted audio signal provided bv the inductor 4 and the low pass filter and it is important to minimise this as far as is possible. The screen drive circuit constituted by the inductor 23 and the voltage source 24 provides a particularly economical circuit since, as the two tetrode valves 1 and 22 are driven in anti-phase by the two grid drive circuits 29 and 30, the screen bias current is automaticallv diverted to whichever of the two tetrode valves is, for the time being, in conduction. Figures 3 and 4 show in greater detail alternative forms of the screen drive circuit which produces a substantially constant bias current for screen electrodes 38 and 39. In Figure 3 an inductor 31 is connected to a rectifying diode bridge network 32, and an additional inductor 34 is provided between the network 32 and a transformer 35. The transformer 35 could conveniently be connected via terminals 36, 37 to the filament transformer required for the tetrode valves. The inductor 34 provides additional current stabilisation which may be desirable whilst the anode current of the conductive tetrode valve is small. In Figure 4 a voltage doubling circuit is shown, and this is capable of providing a substantially lossless controlled current output. In both Figures 3 and 4, a capacitor 40 can be provided, and tappings on the transformer can be used to adjust the initial current level. Additionally, in both cases, a small series resistor can be connected directly at each screen electrode, to prevent parasitic oscillation or reduce the effects of valve arc. A modified screen electrode bias circuit is shown in Figure 5 which could be used if tetrode valve 1 were used alone. The tetrode valve has a screen grid 41 connected to an inductor 23 and a voltage source 24, as in the circuit shown in Figure 2. In this case, an additional shunt circuit consisting of a switch 43 and an impedance 42 is connected between cathode potential and the screen electrode 41. When the tetrode valve 1 is rendered conductive by means of a suitable grid drive signal applied to the grid electrode 44 the switch 43 is closed, thereby providing a low impedance shunt path for the excessive voltage which would otherwise develop across the inductor 22 under these conditions. If the other tetrode valve 22 were used alone to perform the duty of diode 3, the arrangement of Figure 6 may be employed to apply suitable and substantially constant bias current to both screen and grid electrodes, to provide adequate forward conductivity between anode and cathode with- out over-dissipation at either screen or grid electrodes. The voltage sources and inductors at screen and grid electrodes, shown in Figure 6, may be replaced by bias supplies as in Figures 3 or 4 to give improved control of bias currents if desired. Additional series resistance may also be employed. Since, for diode duty, this tetrode need not be controlled at the grid to render it non-conductive, no additional switch corresponding to switch 43 of Figure 5 is necessary. WHAT WE CLAIM IS:

1. A modulation circuit including a pulse width modulation switch in the form of a switchable valve having a grid electrode to which, in operation, is applied a pulse width modulation control signal and a screen

electrode: means for applying a substantially constant bias current to said screen electrode for those periods whilst the valve is rendered conductive by the application of a suitable control signal to the grid electrode; and a low pass filter arranged to attenuate frequencies at the switching frequency of said control signal.

2. A modulation circuit as claimed in claim 1 and wherein the switchable valve is a tetrode.

3. A modulation circuit as claimed in any of the preceding claims and wherein a further switchable device is connected in parallel with a pair of input terminals of said low pass filter, and which is arranged to provide a shunt path for current in low pass filter whilst said pulse width modulation switch is non-conductive.

4. A modulation circuit as claimed in claim 3 and wherein the further switchable device is a tetrode valve having a grid electrode to which, in operation, a signal is applied in anti-phase to the pulse width modulation control signal.

5. A modulation circuit as claimed in claim 4 and wherein the two tetrode valves have a common current source means for providing a bias current for their respective screen electrodes.

6. A modulation circuit as claimed in claim 5 and wherein said common current source means comprises an inductor in series with a source of potential.

7. A modulation circuit as claimed in any one of claims 2 to 6 and wherein the cathode of the switchable valve which comprises the pulse width modulation switch is arranged to be switched between earth potential and an H.T. potential under the action of the pulse width modulation control signal which is applied to the grid electrode of said switchable valve, the grid drive circuit which applies the control signal to the grid electrode being electrically isolated from the earth potential by means of an optical coupler.

8. A modulation circuit as claimed in claim 7 and wherein the optical coupler comprises an electro-optical transducer and an opto-electrical transducer coupled by an electrically insulating optical fibre.

9. A modulation circuit as claimed in claim 7 or 8 and wherein the respective grid electrode control signals for the two tetrode valves are derived using a common optical coupler using a common optical fibre path.

10. A modulation circuit substantially as illustrated in and described with reference to Figure 2 of the accompanying drawing.