US2719268A - Modulated carrier wave transmitters - Google Patents

Description

Sept. 27, 1955 H. F. BARTLETT MODULATED CARRIER WAVE TRANSMITTERS Filed Oct. 5, 1951 United States Patent MODULATED CARRIER WAVE TRANSMITTERS Hugh Frederic Bartlett, Chelmsford, England, assignor to -Marconis Wireless Telegraph Company Limited, London, England, a company of Great Britain Application October 5, 1951, Serial No. 249,994

Claims priority, application Great Britain October 13, 195i) 3 Claims. Cl. 332-43 This invention relates to modulated carrier wave transmitters and more specifically to modulated carrier Wave transmitters employing so-called Class B modulation with a high frequency modulated stage in push-pull. The ob ject of the present invention is to simplify and reduce the cost of such transmitters without detriment to the performance thereof.

The invention is illustrated in the accompanying diagrammatic drawings in which Fig. 1 shows a typical known circuit and Fig. 2 embodies the present invention.

In order that the invention may be the better understood the essentials of a typical known push-pull Class B modulated high frequency stage will first be described, together with the defects which the present invention seeks to avoid.

This typical known circuit is shown in Fig. 1. In this circuit a Class B modulator which is represented by the valves V1, V2 has the anodes of said valves connected to the opposite ends of the primary P of a modulation input transformer T, H. T. potential for the said anodes being applied from HTl through the mid-point M of this primary. The audio frequency currents for operating the modulation system are applied to the primary winding shown at A of audio frequency input transformer D having secondary winding E connected at opposite ends to the grid electrodes of valves V1 and V2. The cathodes of these valves are earthed. A mid-tap connection in secondary winding E leads to the grid bias means, designated at GB, to the earth as shown. The transformer secondary S is earthed at one end through a suitable blocking condenser C7 and is connected at the other end through a low frequency choke LFC to a source of H. T. potential at I-ITZ. The said other end is also connected through a low pass filter LPF to the two anodes of the valves V3, V4 of the high frequency modulated stage, the connections of these anodes containing the customary high frequency chokes HFCI, HFCZ, one in each lead. The high frequency input to valves V3 and V4 is applied through primary winding K of high frequency transformer L whose secondary winding N is bridged by the two variable condensers C8 and C9 connected in series and with the midcap 0 therebetween connected to earth. The opposite ends of secondary winding N connect through condensers C10 and C11 with the grid electrodes of valves V3 and V4, respectively. The grid bias system includes source G, B connected through resistors R1 and R2 to the grid electrodes of valves V3 and V4 as shown. Connected between these two valve anodes is a center tapped tuned circuit each end of which is connected to the appropriate anode through a blocking condenser C1 or C2. The tuned circuit usually consists as shown of a center tapped tank coil TC in parallel with two similar, ganged tuning condensers C4, C in series, the point between these condensers being connected to the center tap of the inductance through a further high frequency choke HFC3. The center point between the two tuning condensers is directly connected to the two cathodes of the valves and earth. C6 designates a smoothing condenser. The output of the sys- "ice tem consists of tuned circuit W consisting of inductance X, coupled with coil TC and tunning condenser Y in shunt with inductance X, leading to the output circuit designated at terminals Z.

With this arrangement the largest part of the total ca pacity shunted across the modulation transformer is provided by the sum of the capacities of the two blocking condensers C1, C2 and, if these be of values also represented by C1 and C2 the shunt capacity is, approximately, and neglecting other smaller capacities (such as those provided by the customary neutralizing condensers not shown) C1+C2. In practice these condensers must be comparatively large and expensive for they have to offer very low impedance to the fundamental high frequency and must be able to withstand the D. C. supply potential plus the radio and audio frequency potentialsan aggregate potential which, in a high power transmitter, is large. Moreover, the provision of high capacity in shunt across the modulator is objectionable since, at the higher audio frequencies, it makes the modulator load depart materially from a resistive character with consequent distortion and fall in response at the upper frequencies. Indeed, it is due to the seriousness of this distorting effect that the low pass filter LPF is provided between the modulator and the high frequency stage and such a filter is, of course, costly.

The present invention seeks to avoid the foregoing defects and to provide an improved arrangement wherein the total capacity in shunt across the modulator is substantially reduced as compared to the known arrangements and wherein, moreover, the blocking condensers (such as C1 and C2 of Fig. 1) may be made smaller and cheaper and the need for a low pass filter (LPF of Fig. 1) between modulator and high frequency stage eliminated.

According to this invention a push-pull modulated high frequency stage with class B modulation and having a center-tapped tuned circuit connected through blocking condensers to the anodes of the valves of the stage is characterized in that the center tap of the tuned circuit is connected to the cathode point of the said valves through an additional condenser shunted by an impedance which is permeable to direct current and of high value in relation to the reactance of said condenser at modulation frequencies.

Fig. 2 shows a preferred embodiment of the invention.

In Fig. 2 the same references denote the same parts as in Fig. 1. As will be seen the difference (apart from the omission of the filter LPF between Figs 1 and 2 is that, in the latter circuit, instead of providing a direct connection between the two cathodes (and earth) of the valves V3, V4 and the center tap of the tuned circuit, an additional condensed C3 shunted by a leak resistance R is inserted in the connection. With this modification, if C1 and C2 are, as before, the values of the blocking condensers and C3 is the value of the additional condenser designated by that reference, the shunt capacity across the modulation transformer is, approximately, (and neglecting the same capacities as before) In a properly balanced push-pull circuit the additional condenser C3 has to carry only the second harmonic of the fundamental high frequency and therefore its capacity may be made much less than C1 and C2 so that the total shunt capacity becomes much less than C1+C2. This a substantial reduction of distortion due to the modulator load becoming reactive at high audio frequencies, so that there is no need for a low pass or second harmonic filter (such as LPF of Fig. 1) while the degree of linearizing negative feed-back which can be employed is greatly increased. Also, since most of the audio frequency potential is now across the additional condenser C3, the po- 3 tential which the blocking condensers C1, C2 must be capable of withstanding is reduced and these condensers can be made physically smaller and cheaper. I claim:

1. A push-pull modulated high frequency stage with class B modulation comprising a pair of valves each having at least a cathode, a control grid and an anode; a center tapped tuned circuit coil having one end connected to one of said anodes through a blocking condenser and the other end connected to the other of said anodes through another blocking condenser; a pair of ganged tuning condensers connected in series across said coil; a high frequency choke connected between the center tap on said coil and the junction point of said tunning condensers; and an additional condenser shunted by a leak resistance connected between said junction point of said tuning condensers and the cathodes of the values; said leak resistance being of high value in relation to the reactance of said additional condenser at modulation frequencies.

2. A modulated push-pull high frequency stage comprising a pair of valves each having at least a cathode, a control grid and an anode, a center-tapped tuned high frequency circuit connected at its ends through blocking condensers to the anodes of said valves, a circuit for effecting class B modulation of said stage, said circuit including an anode potential supply path to the anodes of said valves and a modulation input impedance connected to a point in said anode potential supply path, said blocking condensers providing capacity effectively across said impedance, a connection including an additional condenser providing a capacitative path for high frequency between the center tap of said tuned circuit and the cathodes of said valves, and a direct current permeable impedance of high value in relation to the reactance of said additional condenser at modulation frequencies in shunt across said additional condensers.

3. A modulated push-pull high frequency stage as set forth in claim 2 wherein the cathodes of the stage are earthed and said direct current permeable impedance is a leak resistance.

References Cited in the file of this patent UNITED STATES PATENTS 2,034,899 Ditcham Mar. 24, 1936 2,447,701 Hings Aug. 24, 1948 2,538,715 Van Weel Jan. 16, 1951 FOREIGN PATENTS 457,568 Germany Apr. 27, 1929

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