554,715

PATENT SPECIFICATION

Application Date: Nov. 16, 1939. No. 30182/39

Complete Specification Left: Nov. 14, 1940.

Complete Specification Accepted: July 16, 1943.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Apparatus for Modulating the Frequency of Electrical 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 modulating the frequency of electrical oscillations, such apparatus being frequently employed in radio transmitting apparatus where it is desired to transmit a frequency modulated carrier wave; the invention is, however, also applicable for any purpose where it is desired to modulate the frequency of electrical oscillations.

When employing a radio transmitter for transmitting frequency modulated waves it is of course necessary to employ a circuit the frequency of which can be readily varied by the signals to be transmitted and since the circuit must be capable of readily changing its frequency it is usually found that such transmitters are not stable and it is therefore considered desirable to employ a correcting circuit which observes the mean frequency of the transmitted carrier wave so as to provide a correcting signal which is employed for stabilising purposes. Furthermore, when it is desired to transmit frequency modulated oscillations at very short wavelengths, the modulating circuits are often complex and difficult to handle.

It is the object of the present invention to provide improved apparatus for frequency modulating electrical oscillations with a view to providing apparatus which is more stable in operation and an apparatus which is particularly suitable for operating at very short wavelengths.

According to the present invention apparatus for modulating the frequency of electrical oscillations is provided comprising a resonant circuit constituted by a concentric line and capable of being tuned to a predetermined wavelength and wherein means are provided for varying the length of the inner member of the concentric line under the control of electric signals so as to modulate the frequency of oscillations injected into said circuit.

The concentric line comprises, in known manner, an inner rod or tubular member electrically connected to, and concentric with, an outer hollow cylindrical shielding member of effective length of the inner member being capable of being adjusted to a quarter of a predetermined wavelength so as to form a quarter wave resonant line. The length of the inner member is preferably adjusted for modulation purposes by providing at the end of said member a diaphragm which is caused to vibrate in a plane transverse to the axis of the member under the control of modulating signals. The diaphragm may be caused to vibrate by connecting the diaphragm to a moving coil arranged in the air-gap of a permanent or electromagnet the modulating signals being fed to said coil so as to cause the diaphragm to vibrate to known manner.

For the purpose of describing the invention more in detail reference will now be made to the accompanying drawings in which:

Figure 1 is a cross-sectional view of apparatus according to the invention, and

Figure 2 is a cross-sectional view on an enlarged scale of the end of the inner member shown in Figure 1.

As shown in Figure 1, there is provided a cylindrical shielding member 3 provided at one end with a rigid metal plate 4 which serves to support a tubular rod 5, the rod 5 and shielding member 3 constituting a resonant circuit of very low damping and being capable of being tuned by adjustment of the axial position of the rod 5 with respect to the shielding member 3 to resonate at a frequency lying within a predetermined range. The length of the rod 5 is preferably a quarter of the operating wavelength, although of course, multiples of this length may be employed if desired. The rod 5 at the end projecting beyond the plate 4 is screw –threaded, as shown at 6, into the nut 7 attached to the plate 4, the rod 5 being provided with a micrometer head 8 by which the rod 5 can be rotated and the length projecting into the cylindrical shielding member 3 adjusted for tuning purposes. The length of the rod 5 projecting into the shield 3 may be read from the micrometer head 8 and a device, not shown, may be incorporated to count or indicate the number of turns of the micrometer head made during adjustment so as to afford an indication of the frequency to which the resonant line is tuned. The rod 5 passes freely through an aperture in the end plate and for the major part of its length the rod 5 is shielded by a further tubular member 9 electrically connected at one end to the plate 4 and at its other end is provided with rubbing contacts 10 contacting with the rod 5. The provision o the tubular member 9 avoids the difficulty of attempting to make a good electrical contact at the point where the rod 5 passes through the end plate 4 and, moreover, the electrical contact is moved to a point on the rod 5 where the current density is much smaller than it is at the point where the rod 5 passes through the end plate 4. The apparatus so far described is a typical quarter-wavelength resonator with a central member of adjustable length and such a resonator may be coupled to an oscillator valve so as to serve in known manner to control the frequency of the resultant oscillations. The dotted line 11 represents a loop coupling an oscillator valve, not shown, with the resonator, and the dotted line 12 represents a loop feeding a concentric feeder coupling, for example, an aerial with the resonator. Several loops may be employed to couple several oscillator valves to the resonator. The elements of the resonator are preferably made of copper or other high conductive substance, or are plated on their inner surfaces with copper or silver. The rod 5 is preferably mainly composed of a metal known by the Registered Trade Mark "Invar", or its length may be controlled by the use of such a metal so that its length is substantially constant despite variations in temperature or, alternatively, temperature correction may be effected by forming part of the rod 5 of temperature compensating members.

For the purpose of varying the length of the rod 5 for modulating purposes the end of the rod 5 which projects into the shield 3 is provided, as shown in Figure 2, with a diaphragm 13, preferably made of aluminium, to which is attached by rivets 14 a cylindrical coil former 15 carrying a coil 16 arranged to move in the annular air-gap of a small permanent or electromagnet 17. The diaphragm 13 is, as shown, mounted so as to vibrate in a plane normal to the axis of the rod 5, so that when the diaphragm 13 vibrates the length of the rod 5 is effectively varied.

The diaphragm 13 is provided with a crinkled periphery as shown at 18, so as to define the region about which the diaphragm vibrates, the diaphragm being supported at its periphery by clamping it between the metal supporting sleeve 19 attached to the end of the rod 5 and a clamping ring 20. The movement of the diaphragm should preferably be stiffness controlled over the working frequency range and added stiffness may be provided by the air cavity between the diaphragm and the magnet 17. To prevent temperature changes distorted the diaphragm an exhaust duct may be provided through the magnet 17 as indicated at 21. The duct 21 may also serve to carry the leads, not shown, for conveying the modulating signals to the coil 16. The said leads may pass through the rod 5 and may be connected to a suitable slip ring arrangement, not shown, beyond the micrometer head 18 so that adjustment of the micrometer head does not affect the electrical connection between the leads and the coil 16. On the application of modulating signals to the coil 16 the diaphragm 13 will vibrate so as to cause the effective length of the rod 5 to vary and so change the resonant frequency of the quarter-wavelength line. Thus assuming that the resonator is tuned to a predetermined frequency and oscillations are injected into the resonator by the loop 11 and modulating currents are fed to the coil 16, the output to the loop 12 will be in the form of a frequency modulated wave.

The diaphragm is preferably formed so that, as far as possible, the major part of its area moves as a piston. Provision must be made to afford good electrical contact for high frequency currents flowing from the end of the diaphragm through the clamping ring 20 and the support 19 to the rod 5. If desired, the diaphragm may be provided with an annular extension 22, as shown, so as to make the effective moving area of the diaphragm substantially equal to the area of the rod 5 so as to increase the modulation effect of the diaphragm. Charging currents flowing on the outer surface of the extension 22 are conducted to the rod 5 via the inner surface of the extension 22 and via the joint between the extension and the diaphragm and consequently this joint must be of good electrical character to enable the damping currents to flow freely without producing losses.

Dated this 15th day of November 1939

F. W. Cackett

Chartered Patent Agent

COMPLETE SPECIFICATION

Improvements in or relating to Apparatus for Modulating the Frequency of Electrical 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 modulating the frequency of electrical oscillations, such apparatus being frequently employed in radio transmitting apparatus where it is desired to transmit a frequency modulated carrier wave; the invention is, however, also applicable for any purpose where it is desired to modulate the frequency of electrical oscillations.

When employing a radio transmitter for transmitting frequency modulated waves it is of course necessary to employ a circuit the frequency of which can be readily varied by the signals to be transmitted and since the circuit must be capable of readily changing its frequency it is usually found that such transmitters are not stable and it is therefore considered desirable to employ a correcting circuit which observes the mean frequency of the transmitted carrier wave so as to provide a correcting signal which is employed for stabilising purposes. Furthermore, when it is desired to transmit frequency modulated oscillations at very short wavelengths, the modulating circuits are often complex and difficult to handle.

Various proposals have heretofore been made with a view to providing a stable frequency modulating circuit in the Specification of Patent No. 504,238 and in the Specification of the Application for that Patent as laid open to public inspection under Section 91 of the Patents Acts. In the latter Specification one such proposal comprised a transmission line having inner and outer members, one end of the outer member being closed by an acoustic metallic diaphragm adapted to vibrate, when sound waves are impressed thereon, relatively to a disc attached to the inner member so as to vary the capacity of the line. It is stated in the Specification that the diaphragm may be driven by a loudspeaker movements or the like.

It is, however, the object of the present invention to provide improved apparatus for frequency modulating electrical oscillations with a view to providing apparatus which is more stable in operation and an apparatus which is particularly suitable for operating at very short wavelengths.

According to the present invention apparatus for modulating the frequency of electrical oscillations is provided in which the frequency determining the circuit of an oscillator comprises a resonant transmission line having an inner member and a surrounding shielding member, wherein a diaphragm is provided associated with the inner member of the line within the shielding member, said diaphragm being connected to a moving coil arranged in the air-gap of a permanent or electromagnet so that on the application of electric signals to said coil the diaphragm is caused to vibrate and so vary the effective length of the inner member of said line so as to modulate the frequency of oscillations set up in said circuit.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the drawings accompanying the Provisional Specification, in which:

Figure 1 is a cross-sectional view of apparatus according to the invention, and

Figure 3 is a cross-sectional view on an enlarged scale of the end of the inner member shown in Figure 1.

As shown in Figure 1, there is provided a cylindrical shielding member 3 provided at one end with a rigid metal plate 4 which serves to support a tubular rod 5, the rod 5 constituting a resonant circuit of very low damping forming the frequency determining circuit of an oscillator and being capable of being tuned by adjustment of the axial position of the rod 5 with respect of the shielding member 3 to resonate at a frequency lying within a predetermined range. The length of the rod 5 is preferably a quarter of the operating wavelength, although of course, multiples of this length may be employed if desired. The rod 5 at the end projecting beyond the plate 4 is screw –threaded, as shown at 6, into the nut 7 attached to the plate 4, the rod 5 being provided with a micrometer head 8 by which the rod 5 can be rotated and the length projecting into the cylindrical shielding member 3 adjusted for tuning purposes. The length of the rod 5 projecting into the shield 3 may be read from the micrometer head 8 and a device, not shown, may be incorporated to count or indicate the number of turns of the micrometer head made during adjustment so as to afford an indication of the frequency to which the resonant line is tuned. The rod 5 passes freely through an aperture in the end plate and for the major part of its length the rod 5 is shielded by a further tubular member 9 electrically connected at one end to the plate 4 and at its other end is provided with rubbing contacts 10 contacting with the rod 5. The provision o the tubular member 9 avoids the difficulty of attempting to make a good electrical contact at the point where the rod 5 passes through the end plate 4 and, moreover, the electrical contact is moved to a point on the rod 5 where the current density is much smaller than it is at the point where the rod 5 passes through the end plate 4. The apparatus so far described is a typical quarter-wavelength resonator with a central member of adjustable length and such a resonator may be coupled to an oscillator valve so as to serve in known manner to control the frequency of the resultant oscillations. The dotted line 11 represents a loop coupling an oscillator valve, not shown, with the resonator, and the dotted line 12 represents a loop feeding a concentric feeder coupling, for example, an aerial with the resonator. Several loops may be employed to couple several oscillator valves to the resonator. The elements of the resonator are preferably made of copper or other high conductive substance, or are plated on their inner surfaces with copper or silver. The rod 5 is preferably mainly composed of a metal known by the Registered Trade Mark "Invar", or its length may be controlled by the use of such a metal so that its length is substantially constant despite variations in temperature or, alternatively, temperature correction may be effected by forming part of the rod 5 of temperature compensating members.

For the purpose of varying the length of the rod 5 for modulating purposes the end of the rod 5 which projects into the shield 3 is provided, as shown in Figure 2, with a diaphragm 13, preferably made of aluminium, to which is attached by rivets 14 a cylindrical coil former 15 carrying a coil 16 arranged to move in the annular air-gap of a small permanent or electromagnet 17. The diaphragm 13 is, as shown, mounted in a plane normal to the axis of the rod 5 and vibrates in a direction parallel to said axis, so that when the diaphragm 13 vibrates the effective length of the rod 5 is varied. The diaphragm 13 is provided with a crinkled periphery as shown at 18, so as to define the region about which the diaphragm vibrates, the diaphragm being supported at its periphery by clamping it between the metal supporting sleeve 19 attached to the end of the rod 5 and a clamping ring 20. The movement of the diaphragm should preferably be stiffness controlled over the working frequency range and added stiffness may be provided by the air cavity between the diaphragm and the magnet 17. To prevent temperature changes distorted the diaphragm an exhaust duct may be provided through the magnet 17 as indicated at 21. The duct 21 may also serve to carry the leads, not shown, for conveying the modulating signals to the coil 16. The said leads may pass through the rod 5 and may be connected to a suitable slip ring arrangement, not shown, beyond the micrometer head 18 so that adjustment of the micrometer head does not affect the electrical connection between the leads and the coil 16. On the application of modulating signals to the coil 16 the diaphragm 13 will vibrate so as to cause the effective length of the rod 5 to vary and so change the resonant frequency of the quarter-wavelength line. Thus assuming that the resonator is tuned to a predetermined frequency and oscillations are injected into the resonator by the loop 11 and modulating currents are fed to the coil 16, the output to the loop 12 will be in the form of a frequency modulated wave.

The diaphragm is preferably formed so that, as far as possible, the major part of its area moves as a piston. Provision must be made to afford good electrical contact for high frequency currents flowing from the end of the diaphragm through the clamping ring 20 and the support 19 to the rod 5. If desired, the diaphragm may be provided with an annular extension 22, as shown, so as to make the effective moving area of the diaphragm substantially equal to the area of the rod 5 so as to increase the modulation effect of the diaphragm. The periphery of the extension 22 does not engage the clamping ring 20 so that the diaphragm is still free to vibrate about the crinkled periphery. Charging currents flowing on the outer surface of the extension 22 are conducted to the rod 5 via the inner surface of the extension 22 and via the joint between the extension and the diaphragm and consequently this joint must be of good electrical character to enable the damping currents to flow freely without producing losses.

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

  1. Apparatus for modulating the frequency of electrical oscillations in which the frequency determining circuit of an oscillator comprises a resonant transmission line having an inner member and a surrounding shielding member, wherein a diaphragm is provided associated with the inner member of the line within the shielding member, said diaphragm being connected to a moving coil arranged in the air-gap of a permanent or electro-magnet so that on the application of electric signals to said coil the diaphragm is caused to vibrate and so vary the effective length of the inner member of said line so as to modulate the frequency of oscillations set up in said circuit.
  2. Apparatus according to claim 1, wherein the diaphragm is provided with an annular extension so as to make the effective movable area of the diaphragm substantially equal to the cross-sectional area of the inner member.
  3. Apparatus according to claim 1 or 2, wherein said inner member is supported from and passes through a conducting plate secured to the shielding member and wherein electrical contact between said plate and the inner member is afforded by a tubular member attached to said plate and through which the inner member passes, said tubular member being electrically connected to the inner member at the end of the latter remote from the portion of the inner member which passes through said plate.
  4. Apparatus for modulating the frequency of electrical oscillations substantially as described with reference to the Figures of the drawings accompanying the Provisional Specification.

Dated this 14th day of November 1940

F. W. Cackett

Chartered Patent Agent