US2243193A - Modulation system - Google Patents

Description

May 27, 1941.

N. H. CLOUGH MODULATION SYSTEM Filed May 6, 1939 2 Sheets-Sheet 1 11 4 .5 [A v! m v2 w c y? INVENTOR NEWSOME HENRY LUUGH BY kg ATTORNEY 1941- N. H. CLOUGH 2,243,193

MODULATION SYSTEM Filed May 6, 1939 2 Sheets-Sheet 2 H 6 2 m w. r m w 2 T 7 r E E o W n I A .0 S V m m Y N a I\ I 2 2 A V 2 A R m A E; c w 1 m. 1= S H D H H l 1 T r C W 5 ll 1/ S S B V m T mm c m M v 1 A A R n K 1 W Patented May 27, 1941 warran- MODULATION SYSTEM Application May 6, 1939, Serial No. 272,676 In Great Britain April 22, 1938 7 Claims.

This invention relates to modulator circuit arrangements and has for its object to provide improved modulator circuit arrangements of high eficiency, comparative simplicity and cheapness, and comparative immunity from those known leakage resonance and similar disadvantageous efiects which occur in known high efficiency.

choke between said stages and four so-called class B modulator tubes so connected as to constitute alternative valve paths from the D. C. source to each stage, two of the modulator tubes operating to secure modulation for positive half waves and the other two operating to secure modulation for negative half waves, the whole ar rangement being such that in effect the high frequency stages are made to take power in parallel from the direct current source at the peaks of modulation.

The invention is illustrated in and further explained in connection with the accompanying drawings.

In the drawings Fig. 1 illustrates somewhat diagrammatically my modulation system wherein two class C stages are connected in series by a choke coil and the stages are per se shunted by modulator tubes with each stage and the choke coil shunted by other modulator tubes, the modulator tubes operating class B and being modulated differentially in pairs to thereby modulate the potential on the high-frequency stages and, consequently, their ability to convert direct current to alternating current power.

Figs. 2 to 5, inclusive, are modifications of the arrangement of Fig. 1. In Fig. 2, which is substantiallysimilar to Fig. l, the modulating circuits are shown and include means for applying more modulating potentials to a pair of the modulator tubes than to another pair. In Fig. 3 a pair of the modulator tubes have their input electrodes excited by the outputs of the other modulator tubes. In Fig. 4 the modulating potentials are applied in somewhat the same manner in which they will apply in Fig. 3. .In Fig. 4, however, radio-frequency chokes are interposed between the high frequency stages and the modulation potential choke. In Fig. 5 the modulating potentials are applied through sub-modulators.

In order that the invention may be the better understood there will first be described with reference to Figure l a highly simplified circuit which may be regarded as descriptive of the basic principle of the invention.

In the theoretical basic circuit of Figure 1 there is connected across a source HT of direct current potential, a series circuit consisting, in the order stated, of a class C high frequency amplifier stage HFl, a low frequency choke CH, and a second class C high frequency amplifier stage HFZ. -The side of the stage HFI adjacent the positive terminal of the source HT is marked A, the other side of said stage is marked B the remaining side of the choke CH is marked C and the side of the stage HF2 adjacent the negative terminal of the source HT is marked D. A modulator tube VI has its anode connected to point Band its cathode connected to point D and a second modulator tube V2 has its anode connected to point A and its cathode to point C. The tubes VI and V2 operate in class B and are arranged to be conductive simultaneously on the same half cycle. In order to simplify Figure 1 the speech input circuits for the modulator tubes are not shown. The circuit constants are such that in the carrier condition the two high frequency stagesHFl I-IF2 contribute equally to a.

common output and the potentials between points high frequency stages HF] and HF2, will besimultaneously increased. In the limit, when the internal resistances of Vi and V2 become zero,

the whole voltage of the source HT will be applied across each high frequency stage and the equivalent of peak-100% modulation will be obtained. In the basic circuit, as so far described, only the two modulator tubes VI V2 are referred to, but two further correspondingly connected tubes Vi V2 are provided to deal with the other half waves of modulation the two said further tubes being connected between points A and B and between points C and D respectively. It will usually be found desirable to connect the negative end of the source, or some other point thereon, or the centre point of the choke CH to earth.

In the above basic circuit it should be noted that as the four modulator tubes require the.

same grid-filament modulation input, two of the tubes, VI and V2 whose filament-earth potential is not constant, being dependent on the modulationwill require different grid-earth input voltages from those required by the other two tubes VI and V2 whose filaments are either earthed or at constant potential with relation to earth. A practical circuit in which this fact is taken into account is shown in Figure 2 and comprises a direct current source HT, represented by two generators in series two high frequency stages HFI HFZ operated in class C and a choke CH all connected as in the basic circuit, and four modulator tubes operated as before in class B and designated respectively VI, VI, V2, V2. Modulation input is applied by a transformer T having a secondary S with a centre tap ST through which bias potential may, if required, be applied or which may be connected directly to earth. One end of the transformer secondary is connected, if required through a negative bias source as shown, to the grid of the tube VI and the other end of the transformer secondary is connected, also through a bias source, if required to the grid of the tube V2. Intermediate taps ITI, 1T2 on the transformer secondary symmetrically disposed with relation to the mid-point ST are connected, one, through a negative bias source if required to the grid of VI and the other,

through a similar negative bias source if required to the grid of V22 The anodes of VIand V2 are connected to the positive terminal of the source HT and the cathodes of VI and V2 are connected at the negative terminal thereof. The cathode of V2 is connected to the anode of V2 and to the point C. The cathode of VI' is connected to the anode of VI and to the point B.

It will be seen that with this circuit the grid of tube V2 must be moved in the positive direction by the amount by which the grid of tube VI has been moved plus the amount by which the point C is moved, if the valves have equal characteristics and identical bias settings and the grid transformer taps are correctly disposed to achieve this result.

In the description as so far given, it has been assumed throughout that the points B and C at the ends of the choke are the only points of connection of the choke to the modulator tubes. This, however, is not a necessary feature and in order to ensure in a simple manner that the gridfilament potential of V2 shall move in the correct phase and amplitude relation with respect to that of VI with minimum difiiculty due to stray capacity efi'ects the circuit now to be described with reference to Figure 3, may with advantage be adopted. In this circuit two class C high frequency stages I-IFI and HF2 and a choke CH are connected as before across the direct current source HT. The choke CH, however, is provided with two intermediate taps B and C equidistant from B and C respectively. The grid of VI is connected through a negative bias source to B, and the cathode of W is connected to B; the grid of V2 is connected through a negative bias source to C and the cathode of V2 to C'. The anode-cathode space of VI is connected between B and D and that of V2 between C and D. Speech or other modulated input is applied through a speech transformer T whose secondary S has its ends connected respectively to the grids of VI and V2, the mid-point of the secondary being connected through a negative bias source to the point D. In this arrangement the tube V2 and, of course, also the tube VI is controlled automatically by voltage set up in the choke CH. By properly choosing the location of the points B C a modulation depth of 100% is obtainable.

In another arrangement shown in Figure 4 a series circuit consisting in the order stated of a high frequency choke HFCI, a high frequency class C amplifier valve HFI, a second high frequency choke HFC2, a low frequency choke CH, a third high frequency choke HFCS, a fourth high frequency choke HFC4, and a second high frequency class C amplifier valve HFZ is connected between positive and negative terminals of a source HT of potential. The anode of the valve HFI is connected through the choke HFCI to the positive terminal of the source HT and the cathode of the valve HF2 is connected direct to the negative terminal of the said source. In shunt across the series circuit consisting of the choke I-lIF'CZ the choke CH and the choke HFC3 are two fixed condensers KI, K2. The anode of HFI is capacity coupled by condenser K3 to one end and the anode of the tube HF2 is capacity coupled by condenser K4 to the other end of a parallel tuned circuit comprising an inductance L in parallel with the usual three plate condenser CP having in effect a mid-tap. The mid-tap of the tuning condenser is connected to the junction point between the two fixed condensers KI, K2, and the tuned circuit inductance L is coupled to the output coil LI. The low frequency choke CH is between the points B, C, and has a centre tap CHS which may be connected to earth if required, and intermediate taps B C equidistant respectively from B and C. The grid of a modulating tube VI is adjustably tapped upon a resistance RI which is in series with a condenser CI and a negative bias source GBI between the point B on the one hand and the cathode of the tube VI on the other, one plate of the condenser CI being connected to B. Similarly the grid of a tube V2 is adjustably tapped upon a resistance R2 which is in series with a condenser C2 and a negative bias source GB2 between C and the cathode of V2 one plate of the condenser C2 being connected to C. The tube VI has its grid connected through a negative bias source to one end of the secondary S of a speech input transformer T and tube V2 has its grid connected through a negative bias source to the other end of said secondary. The said secondary has a centre tap ST which is connected to the point D (this is the same as the negative terminal of the direct current potential source HT) through a negative bias source. The anodes of the tubes VI and V2 are connected to the point A (the positive terminal of the direct current source HT); the cathodes of the tubes VI and V2 are connected to the point D; the anode of tube VI is connected to the cathode of tube VI and to the point B; and the anode of the valve V2 is connected to the cathode of valve V2 and to the point C.

Figure 5 shows a further preferred modification in which, however, sub-modulator tubes VSI and VS2 are employed, the grid potentials (both steady and varying) for the modulator tubes VI and V2 being obtained from across resistances ARI, AR2, in the anode circuits of the sub-modulators VSI VS2 respectively. The grids of valves VSI VS2 are driven in phase with, but usually carry smaller amplitudes than the grids of VI and V2 respectively, 1. e. the grids of VSI VS2 are driven in phase opposition. The drive may be obtained in any of a variety of ways. For exam ple (this is not shown) speech input may be effected by means of a transformer having two push-pull secondaries, oneconnected between theridsjof'VJ andVZ' andthe se'cond'connected betweenlthe' grids of 'VSI andVSZ; the i centre points of the two push-pull secondaries being tiometers Pl P2 connected as shown, may be provided for adjusting the relative amplitude of the speech inputs to V5! and VS2 on the one hand and to VIVZ' on the other, and speech input to VSIVS2VIV2' be effected from sub-sub-modulator tubes VSSIVSS2 (which are fed from the speech input transformer T) by resistance-capacity coupling effected by elements KRAI, KRAZ, and KCIKCZ.

In certain of the figures batteries are shown as providing anode and grid bias voltages. For example in Figure 5 a battery is represented as supplying anode voltage to the sub-sub-modulators (VSSI and VSS2) and grid bias to these tubes and to the sub-modulators (VSI and VSZ). Obviously any other suitable potential sources may be used with or without smoothing filters and/or potentiometer adjustment means as may be required.

It is not absolutely necessary that the outputs of the two high frequency stages be combined in a single circuit. For example, in a case where the same programme is required to be transmitted on two different wave lengths simultaneously, the two high frequency tubes may operate on different wave lengths with individual high frequency input drives and, of course, individual output circuits.

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. In a modulation system, two class C high frequency amplifiers, a source of direct current potential, a modulation potential choke coil, a circuit connecting said high frequency amplifiers and said choke coil in series with the choke coil between said high frequency amplifiers, means connecting said series circuit in shunt to said direct current source, two pairs of electron discharge modulator systems each having electrodes between which impedance exists, means connecting the impedances of a pair of said electron discharge systems in series, means connecting the junction point between said last named series-connected impedances to a point on said choke coil, means connecting the said impedances of the other pair of said electron discharge systems in series, means connecting a point between the said last named series impedances to another point on said choke coil and means for modulating substantially in phase opposition the impedances of pairs of said systems at modulation potential frequency to modulate said high frequency stages on positive and negative half cycles of the modulation frequency.

2. A system as recited in claim 1 wherein said last named means includes means for modulating the impedances of a pair of said systems through a range greater than the range through which the impedances of another pair of said systems is modulated.

3. In a modulation system, two class C high frequency amplifier stages, a source of direct current potential, a choke coil having a point thereon vconnected'to one of said 'high frequency stages and a second point thei'eon connectedto another.

of said high frequency stages to form with said high frequency stages a series circuit, means connecting said series circuit in shunt to said source of direct current potential, two pairs of electron discharge modulator tubes each having input electrodes and output impedances, means connecting one of the tubes of a pair of said tubes in shunt to one of said high frequency stages and the other of the tubes of said one pair of tubes in shunt to the other of said high frequency stages and said choke coil, means connecting one of the tubes of the other pair of said tubes in shunt to one of said high frequency stages, means connecting the other of said tubes of said other pair of tubes in shunt to the other of said high frequency stage and said choke coil, and means for impressing modulating potentials substantially in phase opposition on the input electrodes of said pairs of tubes.

4. A system as recited in claim 3 wherein said last named means includes means for impressing modulating potentials on a tube of each pair of tubes of greater amplitude than the modulating potentials impressed on the other tube of each pair of tubes.

5. In a modulation system, two class C high frequency amplifier stages, a source of direct current potential, a choke coil having a point thereon connected to one of said high frequency stages and a second point thereon connected to another of said high frequency stages to form with said high frequency stages a series circuit, means connecting said series circuit in shunt to said source of direct current potential, four electron discharge modulator tubes each having input electrodes and output impedances, an impedance connecting two of said electron discharge modulator tubes input electrodes in pushpull relation, means connecting points on said last named impedance to the input electrodes of the other two of said tubes to connect the input electrodes thereof in push-pull relation, means connecting one of said first two tubes in shunt to one of said high frequency stages and the other of said first two tubes in shunt to said one of said high frequency stages and said choke coil, means connecting one of said other two tubes in shunt to the other of said high frequency stages and the other of said other two tubes in shunt to said last named high frequency stage and said choke coil and means for impressing modulating potentials on said impedance connecting the input electrodes of two of said tubes in push pull relation.

6. A system as recited in claim 5 wherein said four tubes have their input electrodes biassed for class B operation.

7. In a modulation system, two class 0 high frequency amplifier stages, a source of direct current potential, a choke coil having a point thereon connected to one of said high frequency stages and a second point thereon connected to another of said high frequency stages to form with said high frequency stages a series circuit, means connecting said series circuit in shunt to said source of direct current potential, two pairs of electron discharge modulator tubes each having input electrodes and output impedances, an impedance connecting two of the electron discharge modulator tubes input electrodes in pushpull relation, means connecting points on said choke coil to the input electrodes of the other two of said modulator tubes, means connecting one of said first two of said tubes in shunt to one of said high frequency stages and the other of said first two tubes in shunt to said one of said high frequency stages and said choke coil, means connecting one of said other two tubes in shunt to the other of said high frequency stages and the other of said other two tubes in shunt to said last named high frequency stage and said choke coil, and means for impressing modulating potentials on said impedance connecting the input electrodes of said first two tubes in push-pull re- 5 lation.

NEWSOME HENRY CLOUGH.

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