US2504469A - Valve modulator - Google Patents

Description

April 18, 1950 J. R. TILLMAN 2,504,469

VALVE MODULATOR Filed Nov. 13, 1946 Patented Apr. 18, 1950 VALVE MODULATOR John Richard Tillman, North Harrow, England, assignor to Telephone Manufacturing Com pany Limited, London, England Application November 1s, 1946, Serial No. 709,655 In Great Britain November 14, 1945 9 Claims.

This invention relates to thermionic valve modulating systems and has for its object to provide an improved form of modulator which is stable :in operation and is less dependent upon variations of the supply potentials and ageing of the "valves than has hitherto been possible. The in- "vention also has for an object the provision of a :system for modulating one signal voltage by a second, in which the results of modulation, other than the sum and difference frequencies, may be i-Ilargely or completely eliminated from the output of the modulator.

The present invention makes use of a multi- -electrode valve of a type in which (a) the cathode current is dependent upon the potentials of two grids with respect to the cathode and substanstially independent of the potential of any other ielectrodes and which varies approximately liniearly over at least a portion of the permissible range of variation of voltage of the first grid and =(b the fraction of the cathode current which zreaches the anode is dependent on the potential (of a third electrode (usually considered as a secstond control grid) and is approximately linear rover at least a portion of the permissible range .o'i variation of this electrode. Most modern .multi-electrode valves, with suitable operating potentials, satisfy these two requirements.

With the present invention there is provided :a thermionic valve system for modulating together two signal voltages, comprising a thermi onic valve of the type described, means for apply- ;ing said signal voltages to the said control grids, and means for applying a negative feedback volt- :age of value, whereby the component of the output current of the valve of a frequency the same :as that of one of the signal voltages is eliminated.

The invention also comprises a thermionic valve system for modulating together two signal voltages, comprising two thermionic valves of the type described, means for applying one of the signal voltages in phase opposition to the corrersponding control grids of said valves, means for applying the other of said signa] voltages in the same phase to the other of said control grids of :said valves, and means for applying negative feedback to said control grids whereby the operation of said modulator is stabilised.

It is to be understood that modulation comprises broadly the combination of two signal volt- :ages in a manner to produce at least some com ponents of frequencies which are related to the frequencies of both the signal voltages. lhus the term modulation includes, for example, the demodulation of a suppressed-carrier wave signal.

In the accompanying drawing are illustrated certain features of the invention including one practical embodiment thereof. In this drawing, Figure l is a simplified circuit arrangement, employing a. single valve, from which circuit conthrough a condenser Cl.

siderations may be deduced; Figure 2 is a. preferred practical arrangement embodying two valves, and Figure 3 shows a circuit modification.

In the arrangement shown in Figure 1, the valve Vi comprises a cathode K, an anode A, and three interposed grids GI, G2 and G3, although further grids may be included, subject to the conditions set out above that the cathode current is controlled by the potential of grids GE and G2 with respect to the cathode and is substantially independent of the potential of the grid G3, while the fraction of the space current which reaches the anode is determined by the potential of grid G3, both grids GI and G3 having a linear rate of control over at least a portion of the permissible range of variations.

The valve Vi may be of the pentode type in which case G! is the normal control grid, G2 the screening grid and G3 the suppressor grid, but for convenience the suppressor grid may be referred to as the second control grid. If the valve is a hexode or octode valve, the grids GI and G3 are the two control grids and grid G2 is the screening grid.

The valve Vl is adapted to have a suitable voltage impressed upon the grid G2 from a source- Bl, upon the grid G3 from a source B2 and upon the anode from a source B3. Signal voltage may be applied from terminals in to the grid GI The cathode current of the valve is passed through two serially connected resistances RK and RN, and a grid resistance RE is returned to the junction of these resistances. It is to be noted that the resistances RK and RN are not by-passed by condensers and that the input circuit includes both resistances. The valve operates as a negative feedback amplifier for signal voltage E1 impressed across terminals it] in View of the potential drop established across the resistances by the cathode current component of the signal frequency. A signal voltage may also be applied from terminals II, in series with the source 132, to grid G3 and the output of the valve may be derived from terminals [2.

Within the linear range of variation of the control grid GI, and with a constant potential VG2 impressed on the grid G2 by the source Bl. the cathode current IK is given by:

It follows that which an adjustable portion may be shunted by condensers C2, C3 respectively; these condensers must have sufficiently low impedance at the working frequencies to ensure that the effective values of the resistances are those of the unshunted portions thereof. The junction of resistances R2 and R3 is connected to the negative terminal of a voltage source B4, of which the positive ter-. minal is earthed. This source may be of the order of 24 volts.

The second signal voltage is applied in parallel, that is, in the same phase, to the two control grids G3 from input terminals [3, the input circuits including isolating condensers C4, C5, and a tapped resistance R4 connected in the manner shown for the purpose of varying slightly the relative inputs to the two valves. The steady potentials upon the second control grids are alsomade adjustable, in this case by the inclusion of tapped resistances R5, R6 to the taps of which the grids are connected through high resistances R1, R8, the resistances R5 and R6 being connected across the terminals of the source B4.

The anode circuits of the valves are completed through the primary windings of the output transformer T2 of which the secondary winding supplies the load circuit.

It will be understood that the circuit described with reference to Figure 2 is illustrative only,-

and that it may be modified in details in known manner. In practice, for example, the voltage sources B5 and B6 may be combined into a single source.

In operation, where it is desired to modulate a carrier wave with another signal voltage, for example in a carrier wave telephone system, the carrier wave may be applied either to the transformer Tl or to the terminals I3, the latter being preferred.

The output transformer T2 must be so wound that the two primary windings are as similar as possible and should have the same number of turns in order that the voltages induced in the secondary winding due to the input signal at the second control grids of the valves shall have relative amplitudes and phases which enable sufficiently good balancing of the output signals of the same frequency as that signal (q/21r) to be achieved.

If this condition is not exactly fulfilled, improvement can be effected by connecting a condenser of suitable value across one or other of the windings.

The primary-secondary turns ratio will be determined by the load requirements.

The ratio of the input transformer Tl will also be determined by the impedance of the primary and secondary circuits. It will be understood that where antiphase signals are available or are derived by other means, the input transformer may be dispensed with. A device other than a transformer may be used to combine the anode currents of the two valves, thereby dispensing with the output transformer T2.

In one arrangement as shown in Figure 2, and using pentode valves, the voltage applied to the input transformer Tl produced at each of the first control grids a voltage of the order of 1.2 volts R. M. 8., and the other signal voltage was about 3; the bias on the grids G3 with respect to the cathodes was about 12 volts negative, the potential of grids G2 being 100 volts, as previously described, and that of the anodes 200 volts.

With this particular arrangement, it was possible to obtain an output of the side band components stable to 10.1 db. with changes of i20% of the cathode heater voltage of the valves. The unwanted components, of frequency 1 /21 and q/21r, were each 40 db. below the value of either side band at the optimum settings.

lThe mathematical treatment given with respect to Figure 1 is subject to the limitation that: the applied voltage variations are confined to the limits of linearity of the two control grids.'

In practice, this is a condition which by suitable choice of valves can be approximated sufliciently closely, particularly as regards the first control grid, for a large number of applications, but if for any reason the magnitude of the signal applied to the second control grid involves opera-- tion beyond the linear portion of the curve, the

stability of the side-band components and the balancing out of the component of frequency 10/21:- is unaffected.

The operation of the modulator in the non-. linear area will give rise, as is well known, to the production of various components of fre--' quencies (11.1) :b mq

where m and 1:. have a wide range of integral values. If operation is non-linear for the second i control grid only, to which is applied the signal of frequency q/2vr, n is largely restricted to the:

values it has in the analysis given previously.

The production of these other components may or may not be useful, and the circuit operated accordingly.

If the signal applied to the second control grid is so large as to drive the grid up to or beyond the point of anode current cut-oil or into gridcomponent of frequency q/21r can be eliminated obtained by decreasing the dependence upon the mutual conductance of the valve is in the ratio of (the) but this improvement may be increased by making R of value greater than as required for the elimination of the output component corresponding to cos.pt. Normally this will result in the reintroduction of this component in the output, but this may be again eliminated by introducing a balancing signal into the second control grid.

Such a modification is shown in Figure 3, which corresponds to Figure 1 except for the incorporation of a transformer T3 having its primary winding supplied from terminals l0, and of which the secondary winding is included in series with the input to the second control grid, in the correct phase to cancel the unwanted component in the output.

7 In thisqarrangement 5132..aznaayv :be, substantially: larger than it]: n improving stability "bllllPIOdllCihg in "the output a component-of frequency-p/21r. .By suitable adjustment'of the voltage injected into 'the'circuit ofithe grid G3 however, .this component can again:

b efbalanced out. "If, for any reason, Ris required to, be'less than thetransformer' T3;may alsobe used;.with therappropriate phase of itspsecondary windingzto efiect the,desiredbalancing:out'of theunwantedsignal.

lclaim: l. A thermionic valve system fonmodulatingtogether two signal voltages, comprising thermionic valve 'includinga pair of icontrol gridsand. a third grid cooperating with acathode and any anode, means for applying said signal voltagesxto the said control grids,.and means to eliminate from the valve output currentxcomponents of the frequency of one of the signal voltages; said component eliminating ;means including "means for impressing upon a control grida negativeieedback voltage of .the frequency of-the signalvoltage applied to that-grid.

2. A thermionic valve ;system :for :modulating together two signal voltages, comprising two. thermionic valves each including a pair: of :con-

trol grids and a third grid cooperating "with :a.

cathode and. an: anode, means for applying oneof thesignal volta es; .in.i.phase opposition to the;

corresponding control-grids of .saidvalveaimeans for applying the other ofsaidcsignal voltages in thesame phase to the other: of" said control grids: of said valves, ,andmeanszfor :applyingnegative:

feedback to said controlgricls-whereby theioperationofsaid modulator is stabilized.

thermionic valves each including aupairm of econ- .3. .A- thermionic. valve system :for: modulating; together two signal.,v0ltages, comprising two.

trol grids and a .third grid cooperating wi-th1a 45 cathode andan anodameansfor applyingaone' of.

the signal voltages in phase opposition to the corresponding control grids :of said ,valves means 1 for applying theotherof saidsignallvoltages.in:

the same phasetothe otherv ofz-saidicon-trolgrids of, said valves, and meansionapplying individ uallyadjustable negative .feedbackrto. the :respeetive valves whereby thezicperation .OfiSfiidEmQdlI-r" latoris'stabilized.

.4. 1A thermionic valve system according; :toclaim 2, .whereinsaid negative feedbackmeans ineludes circuit elementsof such-value'as to eliminatefrom the output. of said valvesj-apomponent of frequencyiequal to that of the-voltage .applied'l in phasez opposition .to said valves.

.5. Athermionic valvesystem according to claim: 2, wherein means are provided to :eliminatazin-a;

commonoutputjrom,saidvalves, a componentof frequency equalto that. of *the voltage applied ato said valves in the. same phase.

.6. A thermionic valve system'according to claim 2, wherein said valves are operated within the: limitsof linear'vari'ation of theanode current-by, said control grids, and wherein meansarepm-t vided'to substantially eliminate from a common 5 outputfrom said valves all.components=except. those of the sum anddifference frequencies ofzsaid.

signal-voltages.

71A thermionic valve system according to' claim 1; wherein anoutput circuit is connected between said cathode and said anode, and:said negative feed back voltage means-comprises'an impedance in said output circuit and connected between said cathode and the control grid upon' which is impressed said signal'voltage which is eliminated in the output circuit.

'8; A thermionic valve system according to claim 2, and comprising means for adjusting relatively the degree of negative feedback ap plied to said valves, and the amplitude of' thesignal voltage applied in the same -=phase= to' said valves.

' 9: A thermionic valve system according to claim 8 and comprising means for varying relatively; the amplitude of the signal voltage applied in phase opposition to said valves.

JOHN RICHARD TILLMA'N.

REFERENCES CITED 'T'Ihe following references are of" record .in the file of this patent:

UNITED STATES. PATENTS Number Name Date 19263875 Llewellyn -Sept. 12,"1933- 1,977,536 Tubbs Oct."16, 1934' 2,074,440 Usselman Mar. 23,1937 $201,016 Usselman z j-May=14, 1940 2 ,278,159 Wheeler 'Mar.'31, 1942

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