587,562

PATENT SPECIFICATION

Application Date: March 31, 1941. No. 4321/41.

Complete Specification Left: March 31, 1942.

Complete Accepted: April 30, 1947.

-------------------------

PROVISIONAL SPECIFICATION

Improvements in or relating to Apparatus for the Reception of Modulated Oscillations

I, ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention to be as follows:-

This invention relates to apparatus for the reception of modulated oscillations.

It is often necessary, as for example, in the detection of reflecting objects such as aircraft, to receive oscillations modulated by a pulse waveform of short duration. The reception of such signals does not normally present much difficulty, since they are of short duration and are unlikely to arrive simultaneously with other pulse like signals of a similar character. It is, however, found that the reception of such signals may be seriously jammed by continuous oscillations.

Providing that overloading of the receiver up to the output circuit of the detector can be avoided, this jamming can be largely overcome by providing a high pass circuit in said output circuit so that only signals having a relatively high frequency modulation are transmitted. This method, however, requires the use of large valves in the later stages of the amplifier feeding the detector and is therefore uneconomical. Alternatively, very low level detection may be used in order to avoid the necessity for such high power amplifier stages and post detector amplification provided to bring the pulses up to the desired amplitude. This arrangement has the disadvantage that the detection will not be linear and the pulses will be modulated by the interfering signals. The provision of an automatic volume control arrangement for the amplifier preceding the detector will prevent overloading of the amplifier, but the relative amplitudes of the pulse modulated and unmodulated oscillations will remain unchanged so that, as a result of the volume control, the amplitude of the pulse modulated oscillations will be greatly reduced in the presence of the unmodulated oscillations.

It is the object of the present invention to provide apparatus by which the above mentioned jamming difficulty may be overcome without the disadvantages referred to.

Apparatus for carrying out the invention may comprise a thermionic valve amplifier having an input circuit to which said oscillations can be applied and an output circuit arranged to feed a receiver, the output circuit of said rectifier being arranged to apply the rectified signal as a negative bias to the control electrode of said valve over a circuit responsive only to signals having a rate of change of amplitude less than a predetermined amount, the apparatus being such that in operation, said unwanted oscillations are limited by being at least partly biassed beyond the anode current cut-off of said valve without substantially affecting the amplification of said desired signals.

In order that my invention may be more fully understood, it will now be described in grater detail with reference to the accompanying drawings, in which

Figure 1 shows a general schematic arrangement of apparatus according to the invention,

Figure 2 shows practical details of part of the circuit of the arrangement of Figure 1, and

Figures 3 and 4 show simplified alternative circuit arrangements of the same general kind as those of Figure 1.

Referring to Figure 1, the receiver will be recognised as a superheterodyne of know type, comprising the carrier frequency amplifier 1, the mixer 2 and local oscillator 3, the intermediate frequency amplifier 4, of which the last two stages are shown as 5 and 6 respectively, and the detector 7. The output for the detector 7 is fed on the one hand via the series condenser 8 and shunt resistance 9 to additional amplifiers (not shown) and on the other hand via series resistance 10 and shunt condenser 11 to a D.C. amplifier 12, the output of which is applied as a bias to the control electrode of the valve of the final intermediate frequency stage 6 and potentiometer 13 is similarly applied to the control electrode of the valve of the penultimate intermediate frequency stage 5.

The time constant of the condenser 8 and resistance 9 is arranged to be short, so as only to pass the rectified currents due to modulated oscillations having a modulation wave form with a relatively high rate of change, such as pulse modulation. In the case of pulse modulation, this time constant may be made equal to a few time the duration of the pulse.

The time constant of the condenser 11 and the resistance 10 is also arranged to be of the order of a few times the pulse length, so that rectified currents derived from the pulse modulated oscillations do not give rise to any appreciable voltage across condenser 11 and do not control the D.C. amplifier 12.

The arrangement operates as follows. In the absence of interference, the amplifier 12 is inoperative, since the rectified currents resulting from the pulse modulated oscillations are bypassed by the condenser 11. If, however, unmodulated oscillations are fed to the receiver, the amplifier 12 will be controlled, since the rectified currents do give rise to a voltage across condenser 11. Thus, if the amplitude of said unmodulated oscillations exceeds a given value, the negative bias upon the control electrode of the valves of the intermediate frequency amplifiers 5, 6 is increased, and this change of bias is arranged to be sufficient to bias said valves beyond anode current cut-off so as to limit the amplitude of the unmodulated oscillations without at the same time limited the amplitude of, or substantially reducing the amplification of, the pulse modulated oscillations superimposed on said unmodulated oscillations. By operating said valves in this manner, the amplitude of the unmodulated oscillations are limited and reduced in amplitude with respect to the pulse modulated oscillations.

It will be appreciated that if the amplitude of the unmodulated oscillation is very large, similar control may also have to be applied to earlier stages of the intermediate frequency amplifier 4.

Referring to Figure 2, in which practical details o the arrangement of Figure 1 are shown, it will be seen that the final intermediate frequency amplifier 6 is coupled to the detector 7 via the transformer 14. The output circuit of the detector 7 comprises the resistance 15 and intermediate frequency bypass condenser 16, and the end of resistance 15 remote form the cathode of detector 7 is connected to a negative bias such that the valve of the D.C. amplifier 12 is normally biased beyond anode current cut-off. Output resistance 16a, decoupling resistance 17 and decoupling condenser 18 are provided in the anode circuit of the valve of amplifier 12, and the anode of said valve is also connected via resistance 19 and 20 to a negative bias such that the junction of said last-mentioned resistances is given a potential suitable for application to the control electrode of the valve of the amplifier 6 via the secondary winding of the input transformer 20a. A decoupling condenser 21 for the grid circuit of said amplifier 6 is provided, and a condenser 22 is connected in parallel with resistance 19 to compensate for the rise in a amplification at low frequencies due to said decoupling condenser 21.

Suppose now that the amplifier 6 is capable of feeding to the detector 7 a signal sufficient to provide a maximum rectified output of 15 volts without overload, and that the desired rectified output of the pulse modulation is 5 volts. If the negative bias applied to the detector 7 is 10 volts and the grid base of the valve of the D.C. amplifier 12 is 5 volts, then said valve will not conduct until unmodulated oscillations are received providing a rectified output exceeding 5 volts. As the amplitude of the unmodulated oscillations increased beyond this value, the current passed by the valve of amplifier 12 increases, thus applying increasing negative bias on the second grid of the valve of the amplifier 6. Even when the bias on the control grid of the valve of the D.C. amplifier 12 is reduced to zero by unmodulated oscillations of very large amplitude providing a rectified output of 10 volts in spite of the limitation of said oscillations by the full negative bias applied to amplifier 6 by D.C. amplifier 12, amplifier 6 is still capable of supplying without overloading sufficient amplitude of the pulse modulated oscillations to provide the desired 5 volts rectified output.

As the arrangement described is in principle a form of negative feedback circuit, excessive phase shits, round the feedback loop should be avoided. For this reason the time constant of resistance 16a and the capacity effectively in shunt therewith should be short compared with the time constant of condenser 11 and resistance 10.

As an alternative to the use of time constant circuit comprising condenser 10 and resistance 11 for rendering the D.C. amplifier 12 inoperative for pulse like modulation, a terminated time delay network and a hexode valve may be employed. The hexode is used as the valve of the D.C. amplifier 12, and the two control electrodes are connected to spaced tapping points on the delay network and so biased that the hexode cannot pass current unless a voltage exceeding a given value is a applied to both of said control electrodes simultaneously. The delay network is terminated to avoid reflections, and its unterminated end is connected across resistance 15. It will be seen that the D.C. amplifier 12 can then only operate upon signals having a duration sufficient to bias both control electrodes of the hexode simultaneously and short pulses will not control the hexode.

If desired, the D.C. amplifier 12 may be omitted, and each amplifier such as amplifiers 5 and 6 may be arranged to provide its own bias. Arrangements of this kind are shown in Figures 3 and 4. In the arrangement of Figure 3, the output circuit of the amplifier 6 is provided with a resistance 23 in series with the output transformer 14, and the lower end of said resistance is connected to resistance 24 which is in series with the grid decoupling resistance 20 common to the input and output circuits of said amplifier. A bypass condenser 25 if provided in shunt with resistance 24. The valve of the amplifier 6 is normally biased near to cut off, and if said valve passes an increased current for a time greatly exceeding the duration of a pulse, additional negative bias is applied to its grid via resistance 24 and the incoming signal is limited without limiting signals having the pulse modulation in the manner hereinbefore described. The time constant of condensers 25 and 21 in series and resistance 23 should be of the order of a few times the pulse length so as to prevent change of grid bias on the pulse modulated oscillations. Suitable values may be as follows:-

Resistance 23

30,000 ohms

Resistance 24

1 megohm

Resistance 20

0.5 megohm

Condenser 25

0.001 ¼F

Condenser 21

0.002 ¼F

The effective time constant with the above component values is approximately 20 microseconds so that pulse modulation having a pulse duration of a few microseconds will not cause any change of bias.

The arrangement shown in Figure 4 is similar in principle, the different being that the bias is developed in the cathode circuit across the cathode resistance 26, which is bypassed by condenser 27 to prevent change of bias by pulse modulation, the time constant of resistance 26 and 27 being chosen to be several times longer than the duration of the pulse as before.

It will be appreciated that the invention may be applied to a unilaterally conducting device, such as a diode, the valve of the amplifier 6, for example, being replaced by a diode having an output circuit tuned to the intermediate frequency and being biased by the D.C. amplifier 12 so as to limit the amplitude of the unwanted oscillations without limiting the amplitude of the wanted oscillations.

Although the invention has been described with reference to the reception of oscillations modulated by a pulse like modulation waveform, it will be appreciated that it is generally applicable to the reception of oscillations modulated at a frequency, which is high relative to the frequency which may be zero of the modulation of the interfering oscillations.

Dated this 31st day of March, 1941.

F. W. CACKETT.

Chartered Patent Agent.

PROVISIONAL SPECIFICATION

Improvements in or relating to Apparatus for the Reception of Modulated Oscillations

I, ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

This invention relates to apparatus for the reception of modulated oscillations.

It is often necessary, as, for example, in the detection of reflecting objects such as aircraft, to receive short pulses of oscillations. The reception of such signals is not normally subject to much interference by other pulse-like signals since the desired and undesired signals are unlikely to occur simultaneously. It is, however, found that the reception of such signals may be seriously jammed by continuos oscillations.

Providing that overloading of the receiver up to the output circuit of the signal rectifier can be avoided and the continuous oscillations are either unmodulated or modulated at a frequency low compared with the pulse recurrence frequency, this jamming can be largely overcome by providing a high pass circuit in said output circuit so that only rectified signals derived from the pulses of oscillations are passed on. This method, however, requires the use of large valves in the later stages of the amplifier feeding the rectifier, and is therefore uneconomical. The provision of an automatic volume control arrangement for the amplifier preceding the detector will prevent overloading of the amplifier, but the amplification of the pulses of oscillations and the continuous unmodulated oscillations will be equally controlled so that, as a result of the volume control, the amplitude of the pulse oscillations will be greatly reduced in the presence of the continuous oscillations.

Alternatively, detection at a very low level may be used in order to avoid the necessity for such high power amplifier stages and post detector amplification may be provide to bring the pulses up to the desired amplitude. This arrangement has the disadvantage that the detection will not be linear and the pulses of oscillations will be modulated by the continuous oscillations.

It is the object of the present invention to provide apparatus by which the jamming of desired modulated oscillations by undesired oscillations modulated at a lower frequency than said desired oscillations may be overcome without the disadvantages referred to.

According to the present invention there is provided apparatus for the reception of desired modulated oscillations in which means are provided for reducing interference by undesired oscillations which are either unmodulated or modulated at a lower frequency than said desired oscillations, said means comprising a limiting device and biassing means arranged to adjust in accordance with the amplitude of said undesired oscillations the limiting level of said limiting device so as to cause said undesired oscillations to be substantially limited, whereby in operation the relative amplitude of the desired and undesired oscillations in the output of said limiting device is grater than the relative amplitude of the desired and undesired oscillations applied to said limiting device.

Said means may comprise a rectifier for rectifying said superimposed desired and undesired oscillations and a frequency selective circuit between said rectifier and said limiting device, said frequency selective circuit, which may conveniently comprise series resistance and shunt capacity, being adapted to transmit substantially only currents having frequencies lower than the modulation frequency of said desired oscillation.

If desired, said rectifier may comprise a thermionic valve arranged to operate as an anode bend rectifier and having its output circuit so coupled to its input circuit that in operation rectified signals developed in said output circuit are fed to said input circuit so as to cause said valve to operate as said limiting device.

In order that my invention may be more fully understood, it will now be described with reference to the drawing accompanying the provisional specification, in which:-

Figure 1 shows a general schematic arrangement of apparatus according to the invention,

Figure 2 shows practical details of part of the circuit of the arrangement of Figure 1, and

Figures 3 and 4 show simplified alternative circuit arrangements of the same general kind as those of Figure 1.

The invention will be described as applied to apparatus for the reception of pulses of oscillations which may be subject to interference by undesired oscillations which are either unmodulated or modulated at a lower frequency than the frequency of recurrence of said pulses.

Referring to Figure 1, the apparatus diagrammatically illustrated comprises a radio receiver of the superheterodyne type, including the carrier frequency amplifier 1, the mixer 2 and local oscillator 3, the intermediate frequency amplifier 4, of which the last two stages are shown as 5 and 6 respectively, and the rectifier 7. The output from the rectifier 7 is fed on the one hand via the series condenser 8 and shunt resistance 9 to additional amplifiers (not shown) and on the other hand via series resistance 10 and shunt condenser 11 to a D.C. amplifier 12, the output of which is applied as a bias to the control electrode of the valve of the final intermediate frequency stage 6 and a fraction of said output taken from potentiometer 13 is similarly applied to the control electrode f the valve of the penultimate intermediate frequency stage 5.

The time constant of the condenser 8 and resistance 9 is arranged to be short, preferably equal to a few times the duration of the pulse, so as only to pass the rectified currents due to the pulse oscillations.

The time constant of the condenser 11 and the resistance 10 is also arranged to be of the order of a few times the pulse duration, so that rectified currents derived form the pulse oscillations do not give rise to any appreciable voltage across condenser 11 and do not control the D.C. amplifier 12.

The arrangement operates as follows. Assume that undesired oscillations, either unmodulated or modulated at a relatively low frequency, are fed to the receiver. Rectified currents flowing in the output circuit of the rectifier 7 will be of sufficiently low frequency to set up appreciable voltage across condenser 11 and the D.C. amplifier 12 will therefore be controlled and will increase the negative bias on the control grids of the control electrodes of the valves of the intermediate frequency amplifier 5, 6. This increase of bias is arranged to be sufficient to bias said valves beyond anode current cut-off and to adjust the limiting level of said amplifier so that only the positive peaks of the undesired oscillations control the anode current of said valves. Thus, the amplitude of said undesired oscillations is greatly limited by the bias applied to said valves by the D.C. amplifier 12.

Now assume that the pulses of oscillations which it is desired to receive are applied to said receiver simultaneously with said undesired oscillations above referred to. To simplify explanation it will also be assumed that the frequency of these pulse oscillations is large compared with the frequency of the undesired oscillations. The relative phase of the two oscillations will vary from moment to moment and the pulses of oscillations will be superimposed upon the positive peak amplitudes, the negative peak amplitudes and intermediate amplitudes of the undesired oscillations at different instants. Due to the fact that the positive peaks of the undesired oscillations are passed by the limiting amplifiers 5 and 6, only those pulses of oscillations will be transmitted by the amplifiers 5 and 6. The fact that these pulse oscillations are transmitted does not cause any increase in the negative bias of amplifiers 5 and 6 which would change the limiting level thereof and introduce further limitation, since due to the relatively high frequency of the rectified currents derived from said transmitted oscillations they will not set up any appreciable voltage across condenser 11. Thus, the relative amplitude of the pulse and undesired oscillations in the output circuit of amplifier 6 will be greater than the relative amplitude of said oscillations applied to amplifier 5.

If the oscillation frequency of the pulse oscillations and the undesired oscillations are not greatly different, the two oscillations will sometimes add in phase so that the amplitude of the superimposed oscillations will be increased and will sometimes add in anti-phase so that the amplitude will be decreased. As in the case previously discussed, those portions of the pulse oscillations which exceed the limiting level to which amplifiers 5 and 6 are adjusted by said gain control circuit will be transmitted and will appear in the output, so that the same advantage results as in the case previously discussed. It should be noted, however, that in this case, the rectified current resulting from the pulse oscillations will not necessarily be unidirectional, as, according to the phase relationship between the pulse and the undesired oscillations, the rectified signal derived from the pulse oscillations may be positive or negative.

It will be appreciated that if the amplitude of the undesired oscillations I very large, similar control may also have to be applied to earlier stages of intermediate frequency amplifier 4.

Referring to Figure 2, in which practical details of the arrangement of Figure 1 are shown, it will be seen that the final intermediate frequency amplifier 6 is coupled to the rectifier 7 via the transformer 14. The output circuit of the rectifier 7 comprises the resistance 15 and intermediate frequency bypass condenser 16, and the end of resistance 15 remote from the cathode of rectifier 7 is connected to a negative bias such that the valve of the D.C. amplifier 12 is normally biased beyond anode current cut-off. Output resistance 16a, decoupling resistance 17 and decoupling condenser 18 are provided in the anode circuit of the valve of amplifier 12, and the anode of said valve is also connected via resistances 19 and 20 to a negative bias such that the junction of said last-mentioned resistances is given a potential suitable for application to the control electrode of the valve of the amplifier 6 via the secondary winding of the input transformer 20a. A decoupling condense 21 for the grid circuit of said amplifier 6 is provided, and a condenser 22 is connected in parallel with resistance 19 to compensate for the rise in amplification at low frequencies due to said decoupling condenser 21.

Suppose now that the amplifier 6 is capable of feeding to the rectifier 7 a signal sufficient to provide a maximum rectified output of volts without overload, and that the desired rectified output from the pulse oscillations is 5 volts. If the negative bias applied to the rectifier 7 is 10 volts and the grid base of the valve of the D.C. amplifier 12 is 5 volts, then said valve will not conduct until undesired oscillations are received providing a rectified output exceeding 5 volts. As the amplitude of the undesired oscillations increased beyond this value, the current passed by the valve of amplifier 12 increased, thus applying increasing negative bias on the control grid of the valve of the amplifier 6. Even when the bias on the control grid of the valve of the D.C. amplifier 1 is reduced to zero by undesired oscillations o very large amplitude providing a rectified output of 10 volts in spite of the limitation of said oscillations by the full negative bias applied to amplifier 6 by D.C. amplifier 12, amplifier 6 is still capable of supplying without overloading sufficient amplitude of the desired pulse oscillations to provide the desired 5 volts rectified output.

As the arrangement described is in principle a form of negative feedback circuit, excessive phase shifts round the feedback loop should be avoided. For this reason the time constant of resistance 16a and the capacity effectively in shunt therewith should be short compared with the time constant of condenser 11 and resistance 10.

As an alternative to the use of time constant circuit comprising condenser 10 and resistance 11 for rendering the D.C. amplifier 12 inoperative for pulse modulated oscillations a terminated time delay network and a hexode valve may be employed. The hexode is used as the valve of the D.C. amplifier 12 , and the two control electrodes are connected to spaced tapping points on the delay network and so biased that the hexode cannot pass current unless a voltage exceeding a given value is applied to both of said control electrodes simultaneously. The delay network is terminate at one end to avoid reflections, and its unterminated end is connected across resistance 15. It will be appreciated that the D.C. amplifier 12 can then only operate upon signals having a duration sufficient to bias both control electrodes o the hexodes simultaneously and the rectified pulse oscillations will be of too short duration to control the hexode.

If desired, the rectified current providing the bias control, instead of being derived from the diode 7, may be derived from existing amplifiers such as amplifiers 5 and 6 by arranging said amplifiers to effect partial rectification of the signals. Arrangements of this kind are shown in Figures 3 and 4. In the arrangement of Figure 3, the output circuit of the amplifier 6 is provided with a resistance 23 in series with the output transformer 14, and the lower end of said resistance is connected to resistance 24 which is in series with the grid decoupling resistance 20 common to the input and output circuits of said amplifier. Bypass condenser 25 is provide in shunt with resistance 24. The valve of the amplifier 6 is normally biased near to cut-off, and when, due to the arrival of undesired oscillations said valve passes an increased current for a time greatly exceeding the duration of a pulse, additional negative bias is applied to its grid via resistance 24 and the undesired oscillations are limited in the manner hereinbefore described. The time constant of condensers 25 and 21 in series and resistance 23 should be of the order of a few times the pulse duration so as to prevent any change of the grid bias by the pulse oscillations. Suitable values may be as follows:-

Resistance 23

30,000 ohms

Resistance 24

1 megohm

Resistance 20

0.5 megohm

Condenser 25

0.001 ¼F

Condenser 21

0.002 ¼F

The effective time constant with the above component values is approximately 20 microseconds so that pulse modulation having a pulse duration of a few microseconds will not cause any change of bias.

The arrangement shown in Figure 4 is similar in principle, the difference being that the bias is developed in the cathode circuit across the cathode resistance 26, which is bypassed by condenser 27 to prevent change of bias by the rectified pulse modulate oscillations, the time constant of resistance 26 and condenser 27 to prevent change of bias by the rectified pulse modulated oscillations, the time constant of resistance 26 and condenser 27 being chosen to be several times longer than the duration of the pulse as before.

In all the arrangements hereinbefore described it is advantageous to apply high voltages to the screening grids of valves used in the amplifiers, such as 5 and 6 which are biased so as to limit the undesired signals, so as to enable high anode currents to be passed during the peaks of the undesired oscillations. Further, the flow of grid current should as far as possible be avoided, because if permitted, it will tend to crush the pulse modulated oscillations which are superimposed on the positive peaks of the undesired oscillations. In addition, in the arrangements described with reference to Figures 3 and 4, a high anode voltage supply and also a high negative grid bias supply is desirable, as such supplies enable high freed resistances to be used. Thus, in the arrangement described in Figure 3, for example, the use of a high voltage anode supply enables the resistance 23 to be increased in value, and the high voltage negative grid bias supply also enables the resistance 20 to be increased, so that the condenser 21 may be decreased in value without shortening the time constant of said condenser with said resistance. Thus, the control voltage developed from the rectified signals and applied to the grid of the amplifier 6 is increased.

It will be appreciated that instead of using an amplifying valve to effect limitation of the undesired oscillations, any unilaterally conducting device, such as a diode, may be used. Thus, the valve of the amplifier 6, for example, may be replaced by a diode having an output circuit tuned to the intermediate frequency and being biassed by the D.C. amplifier 12 so as to control its limiting level in such a manner as to limit the undesired oscillations without limiting the pulse oscillations superimposed on the peaks of the undesired oscillations.

It has been explained that the pulse oscillations may sometimes appear in the output circuit of the limiter in the positive sense and sometimes in the negative sense due to the changing phase relationship between the two oscillations. Thus, the pulses may appear in the rectified signals in both positive and negative senses. This may be undesirable if, for example, the pulses are to be integrated, for example, in the manner described in the specifications of Applications Nos. 31157/39 (Serial No. 577,275) and 31241/39 (Serial No. 585,907). A unidirectional output signal may, however, be obtained if desired by including in series with the output resistance of the rectifier of the receiver a high resistance having a relatively large capacity in parallel therewith. For example, if the detector output resistance has a value of 5000 ohms, the resistance and capacity referred to may have values of 1 megohm and 0.004 microfarads respectively. Although such a circuit is able to response to increases of signal amplitude, it is unable to respond to decreases of signal amplitude which are rapid compared with 20 microseconds and consequently will not give rise to any output when the pulse oscillations are in anti-phase with the undesired oscillations, and the pulse output will therefore be unidirectional.

Although the invention has been described with reference to the reception of pulses of oscillations, it will be appreciated that it is generally applicable to the reduction of interference in the reception of desired modulated oscillations by undesired oscillations which may either be unmodulated or modulated at a lower frequency than said desired oscillations.

Having now particularly described and ascertained the nature of may said invention and in what manner the same is to be performed, I declare that what I claim is:-

  1. Apparatus for the reception of desired unmodulated oscillations in which means are provided for reducing interference by undesired oscillations which are either unmodulated or modulated at a lower frequency than said desired oscillations, said means comprising a limiting device and biasing means arranged to adjust in accordance with the amplitude of said undesired oscillations the limiting level of said limiting device so as to cause said undesired oscillations to be substantially limited whereby in operation the relative amplitude of the desired and undesired oscillations in the output of said limiting device is greater than the relative amplitude of the desired and undesired oscillations applied to said limiting device.
  2. Apparatus according to Claim 1 in which said biassing means comprises a rectifier for rectifying said desired and undesired oscillations, the output of said rectifier being arranged to adjust the limiting level of said limiting device and a frequency selective circuit is provided between said rectifier and said limiting device adapted to feed to said limiting device substantially only currents having frequencies lower than the modulation frequency of said desired oscillations.
  3. Apparatus according to Claim 2 in which said frequency selective circuit comprises series resistance and shunt capacity.
  4. Apparatus according to Claim 2 or 3 in which an amplifier for said rectified signals is included between said rectifier and said limiting device.
  5. Apparatus according to Claim 2 or 3 in which said rectifier comprises a thermionic valve arranged to operate as an anode bend rectifier and in which the output circuit of said valve is so coupled to the input circuit thereof that in operation rectified signals developed in said output circuit are fed to said input circuit so as to cause said valve to operate as said limiting device.
  6. Apparatus substantially as described with reference to any of the Figures of the drawing accompanying the provisional specification.

Dated this 30th day of March, 1942.

F. W. CACKETT.

Chartered Patent Agent

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. - 1947.

Published at The Patent Office, 25, Southampton Buildings, London, W.C.2, from which copies, price 1s. 0d. each (inland) 1s. 1d (abroad) may be obtained