US2151921A - Method of modulating high frequency transmitters - Google Patents

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March 28, 1939. E. KRAMAR ET AL` METHOD 0F MODLATING HIGH FREQUENCY TRANSMITTERS l Filed Aug. 2l, 1955 2 Sheets-Sheet l fik lLSQ QS E. KRAMAR ET AL March 28, 1939.

METHOD OF MODULATING HIGH FREQUENCY TRANSMITTERS Filed Aug. 2l, 1955 2 Sheets-Sheet 2 N SHQ Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE IVIETHODl F MODULATING HIGH FREQUENCY TRANSMITTERS Application August 21, 1935, Serial No. 37,122 In Germany February 5, 1935 4 Claims.

In various cases, such as with sound modulated transmitters for feeding radio beacons, the problem arises of keeping the degree of modulation constant. Let rst the problem be briefly ex- 5 plained which exists for instance with radio beacons. It has been proposed vvth a radio beacon for ultra short waves to feed a dipole continuously from the transmitter and to key two reflector dipoles alternately in the dot-and-dash rhythm. This beacon arrangement if erected at a suitable place may be used not only for lateral direction nding but also for landing according to the slip-way method. However, the application of the slip-way method which as is well known works with an intensity indication, the

descent being along a curve of constant field intensity requires that on. the receiving side the loudness be not subject to variations. Since the loudness for a' constant amplitude of the modulation frequency depends on the modulation degree the problem here is to keep constant the amplitude of the frequency which is to be modulated. In order, moreover, to obtain a large range of transmission, while ensuring that the transmitter be well utilized, the transmitter is modulated Very intensely, namely to a degree beyond 90%.

It is relatively simple to maintain the constancy of a modulation frequency employed for characterizing the beacon, such as a frequency of say 1000 C. P. S. The amplitude of the carrier frequency however is less easy to maintain constant. If now for instance the amplitude of the carrier frequency decreases while the modulation frequency continues to be of a constant amplitude, then an overmodulation occurs because as stated a very intense modulation is provided for. Although in this way the transmitter will not in any case be fully overmodulated yet at least harmonies or over-tones will occur, which interfere very seriously with the navigation. As is well known, in addition to the main beacon so-called pre-signals are employed with radio beacons, such pre-signals being based for instance on the same wavelength while working with dilerent characteristic tones. These pre-signals serve for example to characterize the commencement of the landing or serve to indicate the boundary of the landing ground. If now harmonic waves arise due to overmodulation of the beacon transmitter then there is the danger that these indieating devices for the other signals, such as the said pre-signals, are caused to respond, which should of course be avoided.

According to the invention the problem of maintaining the modulation degree constant is solved by controlling either the bias of a stage connected in advance of the modulation stage or the bias of the modulation stage itself.

Two embodiments of the invention are shown 5y by way of example in the accompanying drawings,

Fig. 1 being a diagrammatic representation of one of these embodiments,

Fig. 2 is a diagrammatic representation of the 10 other embodiment.

In Fig. l the rst three stages I, II, III of a multistage transmitter are shown.

The rst stage I is arranged to operate with quartz control; in the second stage II a control for effecting the constancy of the modulation degree is active, while in the grid circuit of the third stage III the modulation is eiected. The modulation frequency is impressed over the transformer T upon the-push-pull tubes Rm by 20 grid voltage modulation. In the anode circuit an inductance L1 is connected which is located in the grid circuit of the next stage. Further, through the inductance L2 a rectifier G, which in the case represented is a tube the grid of which 25 is connected to the anode, is coupled. The rectified voltage is used for controlling the grid bias of the second stage over a condenser Cg and a choke Dg. The control is eiiected with the aid of the resistance V and W. Included in series with V is a choke Dn. At -G-v a constant negative bias is given. The other parts represented in the drawings are Well known. They have per se nothing to do with the invention.

The mode of action of the arrangement is as 35 follows: The modulation frequency arriving over T is practically constant. The carrier frequency already modulated is rectied in the output circuit of the third stage by the rectiiier G. The resultant continuous voltage is a means for 40 measuring the magnitude of the carrier, for as is well known the mean value of a modulated oscillation does not vary if the transmitter is otherwise constant. The voltage relations are now so selected that the Agrid bias of the stage II shall 45 always be so controlled that the carrier ampliiied in stage II shall always be of the same magnitude. The only requirement is that the control is effected in a stage in which the magnitude of the carrier alone can be influenced, 50 as otherwise the mentioned phenomena of overmodulation cannot be avoided.

Instead of* the rectier tube here employed rectiers of other kind, such as oxide rectiers, may of course be used. Furthermore the con- 55 trol may of course be effected in another manner customary in the high frequency art.

The further invention, to Which Fig. 2 relates, proposes to rectify part of the non-modulated carrier frequency and to use the resultant continuous voltage for controlling the bias of one of the prestages of the transmitter. In order to effect this method, preferably a tube rectifier is modulation is accomplished. In the output cir-V cuit of the stage III the continuous control voltage isrderived by the rectifier G. The modulation frequency is impressed over the transformer T upon the push-pull tubes Riv by grid voltage modulation. The rectifier G is coupled through a condenser C. 'Ihe continuous voltage which the rectiiier G produces at the resistance W across the choke Di is led, for instance through a resistance V and a choke D3, to the grid of the tube of stage II. 'I'he grid of the rectifier tube is caused by the condenser K to have the same high frequency potential as has the anode, and

hence is short-circuited with respect to the high frequency. Over the choke D2 a constant continuousvoltage is given to the grid of the rectifier. If in the usual manner the grid were connected directly with the anode then the grid bias of the stage II, i. e. the grid bias having to be regulated in order to ensure a reliable operation ofthe transmitter, and the setting Yor adjusting of the rectifier working point, would not be independent of each other. This is avoided by the provision that the grid of the rectifier is caused by condenser K to have the high frequency potential of the anode, while over the choke D2 continuous voltage is available for enabling the rectifier working point to be set'at will, so that a regulation of the stage II by the rectified carrier occurs only at a definite value, that is to say, it is this value Which is kept constant. Over the resistance W a constant continuous voltage is led to the grid of the stage II as shown.

The bias on the grid of the rectifier may be controlled continuously, such as by the modulation tone or in dependency upon the network voltage. In the rst case one has to rectify the modulation tone and to employ the resultant continuous voltage for controlling the grid.

In the foregoing description referring to Fig. 1, in order to maintain the modulation degree constant, the assumption has been made that the modulation toneris in itself of constant amplitude, this being in many cases obtainable by simple means. If such is not the case however, then for rendering the loudness in the receiver constant the modulation tone may as stated be rec tied and thus the bias of the rectier be controlled. The field intensity of the transmitter, it is true, Will then not be constant, but at least the modulation degree will be so, so that variations 'in the loudness will be prevented with the receiver'adjusted for a definite sensitivity. The same method may be adopted also if variations in the network voltage occur. In this case the rectifier will be controlled in dependency upon the network voltage.

What is claimed is:

1. In a sound modulated transmitter having several high frequency stages including a modulation stage preceded by non-modulating stages and having means for rectifying a part of the output current of one of said high frequency stages,the method of keeping constant the degree of modulation which comprises applying a modulating signal of constant amplitude to the modulating stage and utilizing the direct current potential derived Vfrom said rectification to control the gain of a high frequency stage preceding that from which the direct current potential was derived.

2. A method according to claim 1, wherein the direct current potential is derived from the output of said modulation stage.

3. Afmethod according to claim 1, wherein the direct current potential is derived from the output of the modulation stage and is filtered prior to employing said voltage for controlling the gain of a preceding high frequency stage.

4. A method according to claim 1, wherein the direct current potential is derived from the output of one of said non-modulating stages.

ERNST KRAMAR. ERICH I-IEINECKE.

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