US2283793A - High frequency modulated radio signal transmitter - Google Patents

Description

y 19, 1942 I E. c. CORK ETAL 2,283,793

HIGH FREQUENCY MODULATED RADIO SIGNAL TRANSMITTER Filed.Feb 9, 1940 2 Sheets-Sheet l CONDENSER MICROPHONE CONDENSER MICROPHONE CONDENSER I MICROPHONE INVENTOR EDWARD CECIL cok k JOgP/l L405 PAWSEY m ATTORNEY May 19, 1942. E. c. coRK ET AL HIGH FREQUENCY MODULATED RADIO SIGNAL TRANSMITTER Filed Feb. 9, 1940 2 Sheets-Sheet 2 CONDENSER MICROPHONE CONDENSER MICROPHONE lNV EDWARD CEC ENTOR S /L CORK wag/l LADE PAWSE Y ATTORNEY Patented May 19, W42

HIGH FREQUENCY MODULATED RADIO SIGNAL TRANSMITTER Edward Cecil Cork, Ealing, London, and Joseph Lade Pawscy, Ickenham, England, assignors to Electric 8; Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application February 9, 1940, Serial No. 318,044

. In Great Britain June 7, 1939 Claims.

The present invention relates to frequency modulated radio signal transmitters in which modulation of the output of the transmitter is effected by operation of a transmitting device as a capacity microphone, reed or like device, of which the capacity varies in response to the signals to be transmitted, and has particular but not exclusive reference to transmitters operating at the so-called ultra-short wave frequencies, above about 50 m0. sec.

In order to obtain adequate output it isdesir- -able to use in a transmitter of the kind set forth an oscillator including a balanced pair or pairs of electron discharge valves. In this case the frequency controlling circuit, usually the anode circuit, of the oscillator, is a balanced circuit and the transmitter device is required to operate to produce balanced capacity variations in the control circuit. Thus, in cases whe're the transmitter device is connected directly in the frequency control circuit both plates of. the device must, in most cases, be subjected to high frequency and high tension voltages, which is a disadvantage.

The object of the present invention is to provide an arrangement in which an unbalanced conence by way of example to the accompanying drawings of which the various figures show circuit diagrams of suitable oscillator arrangements for use in transmitters according to the invention. Like reference numerals in difierent figures of the drawings indicate like parts.

In the drawings Figs. 1 to 3, 3a and 4, inclusive, each illustrate a wave generator of the balanced type, a modulating means of the unbalanced type with circuits and means for coupling the modu lating means to the balanced generator without upsetting the balance thereof. In Fig. 1, the generator is of the push-pull tuned circuit type with the microphone coupled thereto by a transformer.

In Fig. 2, the tuned circuits of the generator.

comprise lines shunted by tuning straps and the modulator is coupled to the generator by concentric lines. In Figs. 3, 3a and 4, the generators are in general as in Fig. 2. In the latter figures, however, the modulating means is coupled by concentric lines to one side ofthe generating circuits and the effect thereof is counteracted by additional means connected to the other side of the generating circuit.

Figure 1 of the drawings shows a simple form of the invention. :In this case the arrangement includes a pair of valves i connected in push-pull relation, the valves having atuned grid circuit 2 including the inductance. capacity combination 3 and an output circuit 5 including the secondary winding of transformer 5 to the mid-point of which the positive pole of the high tension source 6 is connected in the usual way. The cathode be modulated by actuation of said microphone in accordance with the signals to be transmitted. Preferably said tuned circuit is a balanced line circuit and is excited by a pair of valves connected in push-pull relation. The microphone or other signal responsive device is preferably connected in the tuned circuit by means of a concentric line arrangement which is such that balanced capacity variations are developed in the circuit includes suitable biasing arrangements for valves I, these arrangements being indicated conventionally at 2a and 2bof the grid circuit.

The output circuit 4 may be tuned by the stray capacity of the circuit indicated by the condenser 5, shown dotted and the inductance of the secondary winding of transformer 5 and the regenercarrier wave signals are produced. The output of the transmitter is connected to the output circuit 4 in any suitable manner.

The arrangement illustrated in Figure 1 is not suitable for ultra short wave operation, for example at frequencies above about 50 mc./sec., as in-this case the stray capacities of the circuits render satisfactory operation very difflcult. It is thus preferred to adopt, for ultra short wave working, arrangements such as those shown in Figures 2, 3 and 4.

Thus, referring to Figure 2 the microphone is shown connected to a concentric line So surrounded by a further cylinder 5b a quarter of a wave length long extending from the end of the line So remote from microphone I towards the microphone and earthed at the end remote from the microphone and connected to the outer con-' ductor of line So at the end nearest the microphone. The conductors of the line 50. are connected through blocking condensers 8 and 9 respectively one to each of a pair of anode conductors I and II associated one with each of the valves I and tuned by the shorting bar I2 to which the high tension source 6 for the anodes of valves I is connected.

The cylinder b serves two purposes. In the first place, it substantially eliminates unbalance in the output circuit between the plates of the tubes, and in the second place it substantially removes radio frequency currents from the surface of line 5a between the condenser microphone and the point where line So is connected to the cylinder 5b. These two functions may be explained best separately. The preservation of balance may most easily be shown by considering the outer surface of 5a and the inner surface of 5b as forming a quarter wave tank shorted at one end. It is well known that such a tank presents an extremely high impedance, the high impedance in this case occurring between ground and the point where condenser 8 is connected to line So.

The impedance between the ground and the point where condenser 9 is connected to the inner conductor of 5a is the sum of the impedance between the conductors of 5a and the very high impedance between the outer conductor of 5a and ground. Hence the effective impedance between condensers 8 and 9 is substantially solely the impedance between inner and outer conductors of 5a and the system is balanced because there is no admittance to ground in the circuit between condensers 8 and 9 of appreciable magnitude.

On the other hand, to explain why the cylinder 5b substantially eliminates currents from the outer surface of 50, it is only necessary to note that since the currents flowing on the inner and outer conductors of the quarter wave tank formed by the outer surface of 5a. and the inner surface of 5b, respectively, are necessarily substantially equal, the current flowing along the outside of 511 from left to right is substantially neutralized at the point of contact with 5b by the equal and opposite current flowing from the contact point to the inside of 511. Thus of the current originally assumed to be flowing along the outside of So on the left of this point of contact, there is substantially nothing remaining to the right hand side of the point of contact.

Thus the cylinder 5b may be considered either as a by-pass path of the effectively zero impedance for currents flowing along 50 or it may be considered as a neutralizing path so energized as to feed into the line So an equal and opposite current to that flowing along 5a. In any case. the currents on the outer surface of 5a are substantially eliminated and this result is important because if any current were to flow on this outside surface the capacity between the line So and other objects would enter into the determination of frequency of oscillation, and this is, of course, highly undesirable.

For ultra short wave operation, the grid circult preferably includes the lecher wire arrangement 3a rather than the capacity inductance combination 3 shown in Figure 1.

A further arrangement for ultra-short wave operation is shown in Figure 3. In this case unbalanced to balanced working from the microphone I is obtained by the use of the concentric line I5 across one end of which the microphone I is connected and the other end of which extends inside the tubular anode conductor I0 connected to the upper valve I as shown in the drawing. A cross connection is taken from the inner conductor of line I5 through the D. C. blocking condenser 8 to some point on the outside of anode conductor I I associated with the lower valve I as shown in the drawing. Also the control grid and anodes of the valves I may be cross-connected through condensers I3 and I4 as shown, the leads to these condensers passing through suitable apertures or slots in conductors III and II. The line I5 tends to cause unbalance and loss of power, but the balance is restored by the provision of a similar concentric feeder I6 in association with the lower valve I. The feeder I6 can be'used to afford a power output feeder of the transmitter by means of the adjustable cross connection between the inner conductor of feeder I6 and the outside of conductor I!) through the blocking condenser 9. The-lines I5 and I6 are preferably interrupted as shown at I8, the gap being bridged by condenser I9 which serves in conjunction with the condensers 8 and 9 to prevent high tension voltage from source 6 from being applied to either electrode of microphone I.

In Figure 3 the feeder I5 is shown as being tapped down the anode tuned conductor I0. This arrangement is adopted in order that the losses introduced by the line I5 will have less effect, but the use of the arrangement is not restricted to the case in which the feeder is tapped down the conductor I0. In the arrangement shown in Figures 2 and 3 the frequency of the transmitter is readily adjustable by sliding the shorting bar I2 along conductors I0 and II.

If desired, in order to improve the stability of the transmitter the anode of the valves I may be connected to tappings on the conductors III and II instead-of to the ends of the conductors as shown.

The line I5 is preferably between a quarter and a half wave length long, such that the end of the line I5 remote from the microphone presents an open circuit.

Figure 3a shows a modification of the arrangement shown in Figure 3. In this case the lines I5 and I6 extend parallel to the conductors I0 and II, the inner conductors of lines I5 and I6 extending into the conductors I0 and II and being supported in the associated outer conductors and in conductors I0 and II' by means of insulating discs 2'5, which give a low loss arrangement.

The shorting bar I2 is adjustable to provide a coarse frequency control and a fine control of frequency is afforded by providing slidable rods 26 which are supported in sleeves on the outside of conductors I0 and II. Rods 26 can be actuated through a suitable device such as a rack or pinion to provide a ready means for close adjustment of the frequency.

Capacity coupling between conductors l and H and the outer conductors of lines and I6 respectively is afforded by arranging an insulating sheet between the tubes Ill and II and the metal chassis diagrammatically represented at 21, on which the arrangement is mounted, the outer conductors of lines l5 and 56 being in direct contact with the chassis. The chassis is preferably suitably recessed under conductors l0 and H to accommodate the insulating sheet;

Variable condenser 28 may be connected to either line ID or II as desired to balance the capacities of lines and II to earth.

A further possible arrangement according to the invention is shown in Figure 4. In this case the transformation of the microphone output from an unbalanced to a balanced condition is effected by means of a half wave length concentric conductor 26 which is connected between two open wire sections 26 and 22 each approximately half a wave length long which are connected to the anodes of the valves l. The high tension source 6 is connected to the said anodes through the choke 23 which are connected to voltage nodes 24 on the conductors 2| and 22. The conductors of concentric lines l5 and it are connected one to each end of the concentric line 20 and the inner conductors thereof are connected respectively through blocking condensers 8 and 9 to the adjacent conductors 2| and 22.

We claim:

1. In a frequency modulation system an oscillation generator including a pair of electron discharge devices each having input and output electrodes, oscillatory circuits connected with said input and output electrodes including lines symmetrically coupled with said output electrodes and balanced with respect to ground, an unsymmetrical modulating reactance, and means for symmetrical modulating capacity, and means for coupling said unsymmetrical modulating capacity to said balanced circuits including a tubular conductor having a length approximateLv equal to M4, a concentric line including inner and outer members within said tubular conductor each member being connected to said modulatin capacity at one end and coupled to said balanced circuits at the other end, and means connecting the outer member of the concentric line to said tubular conductor to prevent transmission of currents from the tuned circuits to the modulating capacity along the outer surface of the outer member of said concentric line.

3. In a frequency modulation system an oscillation generator including. a pair of electron discharge devices having electrodes connected in a high frequency tuned oscillatory circuit includeach of said tubes in push-pull relation, said circuit being balanced with respect to ground, and means for modulating the frequency of the oscillations generated by means of an unbalanced condenser microphone without upsetting said balance of said tuned circuit'including, a condenser microphone, a concentric line having its inner and outer members connected to-the elements of said condenser microphone at one end, a tubular member, a connection between a point on the outer member of said concentric line and said tubular member, said point being spaced from the end of said tubular member a distance approximately equal to M4, and a coupling between the members of said concentric line and said first named lines.

4. In a frequency modulation system an oscil lation generator including, a pair of electron discharge devices having electrodes connected in a high frequency tuned oscillatory circuit including lines connecting corresponding electrodes in each of said tubes in push-pull relation, said circuit being balanced with respect to ground, and means for modulating the frequency of the oscillations generated by means of an unbalanced condenser microphone without upsetting said balance of said tuned circuit including, a condenser microphone, a concentric line having its inner and outer members connected to the elements of said condenser microphone at oneend and coupled to at least one of the lines of said oscillatory circuit at the other end. and a second concentric line coupled at least to the other of said lines of said oscillatory circuit to restore the balance of said balcuit including lines connecting corresponding electrodes in each of said tubes in push-pull relation, said lines being balanced with respect to ground, and means for modulating the frequency of the oscillations generated by means of an unbalanced condenser microphone without upsetting said balance of said tuned circuit including,

a condenser microphone, a concentric line having its inner and outer members connected to the elements of said condenser microphone at one end and coupled to one of the lines of said oscilcondenser microphone and to provide an output circuit.

6. In a frequency modulation system an oscillation generator including, a pair of electron discharge devices having electrodes connected in high frequency tuned oscillatory circuits including conductors connecting corresponding electrodes in each of said tubes in push-pull re-' edto the elements of said condenser microphone at one end, means coupling the other end -of the members of said concentric line to one side of said oscillatory circuit. a second concentric ing lines connecting corresponding electrodes in 7'5 line coupled to the other side ofsaid oscillatorycircuit to provide an output circuit for said system, and a means connecting the outer members of said concentric lines together and to ground at a point intermediate the ends thereof.

7. In a modulation system, a pair of electron discharge tubes having electrodes coupled in a balanced oscillation generator system including a frequency determining tank circuit, a signal controlled unbalanced reactance element, a concentric line coupling said reactance element to said oscillation generator tank circuit for modulating the oscillations generated in accordance with signals, a radio frequency voltage being impressed from the frequency determining oscillation generator tank upon the signal controlled reactance element by way of said concentric line over the surface of the outer member of which generated current may reach the reactance element, and

' means for maintaining balance in said oscillation generator by eliminating generated radio frequency currents from the outer surface of said line between a point on said line and said reactance element including a conductor connected to the outer member of said line at said point between said element and said tank circuit, and means to supply to said line at said point a current substantially equal and opposite to the generated current flowing at said point on said line toward said reactance element including a coupling between said conductor and said tank circult.

ated current flowing at said point on said cable toward said reactance.

9. In a wave length modulation system a pair 'of electron discharge devices each having a 8. In a balanced oscillator having two tubes I with lines connecting corresponding electrodes thereof in a push-pull tank circuit, and frequency modulating means including an unbalanced reactance variable in accordance with signals coupled to the said lines by a concentric cable over the outer member of which generated voltages from said first lines may reach said signal controlled reactance, means for maintaining balance in said oscilaltor and for eliminating generated radio frequency currents from the surface of the outer member of said cable between a point on said cable and said reactance including a conductor the length of which is substantially v4 connected to the outer member of said cable at a point between said reactance and said tank circuit and cooperating with said outer member to supply to said cable at said point a current substantially equal and opposite to the generplurality of electrodes including a cathode connected in a high frequency oscillatory circuit including a tank circuit coupled to corresponding electrodes in said devices and balancedwith respect to said cathodes. an unbalanced reactance variable at signal frequency, and means for coupling said variable reactance to said tank circuit including, a concentric line having its inner and outer conductors coupled at one end to said variable reactance and at the other end to spaced points on said tank circuit, and means for restoring the balance of said tank circuit upset by the coupling thereto of said variable reactance by said concentric line including, an additional conductor connected to the outer conductor of said concentric line at a point intermediate its ends to form with said outer conductor a resonant line of a length equal to ,/4.

10. In a signalling system, a balanced oscillator including two tubes having anode electrodes and cathode electrodes with lines coupling said anodes in a push-pull tank circuit balanced with respect to said cathodes, and means for modulating the frequency of the generated oscillations in accordance with signals including a variable condenser, and a concentric cable coupling said variable condenser to said lines, and means for restoring the balance of said circuit upset by coupling said condenser thereto and for preventing generated currents from reaching said condenser over the surface of the outer member of said concentric cable including, a cylindrical conductor the length of which is ,/4 surrounding said concentric cable, and a low impedance connection between the end of said cylindrical conductor and a point on the outer member of said concentric cable between said element and said tank circuit, said cylindrical conductor form ing with said outer member of said cable a resonant circuit the length of which is i/4 to supply at said point a current substantially equal and opposite to the generated .current flowing at said point on said line toward said reactance element.

EDWARD CECIL CORK. JOSEPH LADE PAWSEY.

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