US1969329A - Radio frequency wave reception - Google Patents

Description

1934- H. o. ROOSENSTEIN 1,969,329

RADIO FREQUENCY WAVE RECEPTION I Filed Jan. 6, 1935' f 1 r I I l 4 @g i L l k INVENTOR- HANS OTTOROOSE TElN ATT'ORNEY- Patented Aug. 7, 1934 UNITED STATES PATENT oFF ce RADIO FREQUENCY WAVE RECEPTION Hans Otto Roosenstein, Berlin, Germany, as-

signor to Telefunken Gesellschaft fiir' Draht lose Telegraphic m. b. H., Berlin, Germany Application January 6,

1933, Serial No. 650,465

In Germany February 8, 1932 5 Claims.

SL-5 21) show curves explaining the operation of the invention and Fig. 2 shows a circuit embody- .ing the invention.

In co-pending patent application Serial No. 615,647, filed June 6, 1932 a rectifier circuit 0 scheme has been disclosed which allows of the 5 rectifying characteristic, i. e., the curve which represents the instantaneous value of the currentJ leaving the rectifier as a function of the input radio frequency voltage E, is representable by the relation:

' 25 fied current in accordance with the said formula will vary between these limits The size of the variation may be calculated thus 1 w chosen shape of rectifier characteristic, the

said fluctuation is a function only of the degree of modulation (modulation percentage), butnot of the amplitude E of the incoming signal; In a circuit arrangement of the kind disclosed in the parent patent the suggestion, inter alia, had been made to use as rectifiers tubes of a k nd whose characteristic, starting from a certain bias potential value, exhibits an essentially logarithmic form so that the relationship between instantaneous value and applied or impressed voltage and ensuing strength of current, would follow a logarithmic lawthroughout the positive half-wave, while the current during the other half-cycle of the potential would be constant.

Tubes of this kind having a characteristicof the sort shown in Fig. 1 are realizable in practice only with great difficulties, inasmuch as the requirement is made that the characteristic thereof should present a concave curvature in reference to the abscissa axis. Discharge char acteristics of the kindfound in thermionic tubes, as will be noted, invariably exhibit a characteristic clinging, at it were, to the abscissa, in other words, a characteristic which presents a convex curvature towards the abscissa axis.

Tubes have been disclosed of a type whose characteristic, inside the range of working of negative biasing potentials,:present a shape answering approximately a negative logarithmic law so that the characteristic has a convex curvature towards the abscissa. Such tubes are readily producible in the shape of screen-grid tubes equipped with grid electrodes insuring a variable gain reciprocal (12 Such tubes have been described by Ballantine and Snowin the I..1=t; E; of-December, 1930. These tubes have been described for amplifier'work and gain regulation.

For the purpose of eliminating the effects. of fading, in the senseof the aforesaid patentap plication, there is used according to this invention for the rectifier responsive only to the degree (percentage) of modulation of the incoming waves an amplifier tube in a connection of a kind Whose static characteristic, inside the working range comprised roughly between 0 and 20V, is a negative logarithmic characteristic, the latter, consequently, within the working range, presenting towards the abscissa no infinitelyhigh slope (mutual conductance) values'and always presenting a convex curvature.

Referring to Fig. 2, 1 is a tube for which the relationship between the plate current Ja and the grid voltage Eg, inside the working range of negative biasing voltages, answers approximately this equation:

(5) Ja=-P log B(VEg) This function is illustrated in Fig. 2a. The straight line Eg V is an asymptote to this curve. The current and the mutual conductance assume infinitely high values when the gridpotential has the value E: +V. Although the theoretical'characteristic is practically not realizable, Graph II, Fig. 2a shows the shape of curve which is obtainable in the case of tubes and which, inside'the range of negative biasing potentials, comes sufficiently closely to curve III.

In the scheme shownin Fig. 2, tube 1 is. used amplitude Whence the grid voltage (7) E =-e(1-cos wt) By substituting 7) in (5),, thisequation is found for the plate current The mean value Jm for the plate current J a, inside period T, follows to be:

E=e cos wt while the dependence of the mean plate current upon the amplitude of the incoming signal is 1cos wt Evaluating this integral there is obtained de T T+2%/' Now, assume that the signal supposed to be sinuous and of an amplitude e is modulated in accordance with the equation:

where (01,18 the modulation frequency, en the mean amplitude of e (carrier wave), and m the modulation degree of percentage. As a result of the impinging of such a'modulated signal, also the mean plate current Jm will be caused to vary in frequency 001. The strength of current Jm is a function of the amplitude ez According to the Taylor series, there thus becomes e=eo(l+m sin @115) The seriation is broken off after the second term, the assumption being that the degree of modulation m is so small that higher terms may be disregarded. By substitution of the expression found in Equation (11) for in Equation (l l) there is obtained What this formula indicatesis that the mean plate current varies in the frequency 1 with an vIf 60 V this amplitude is practically independent of'the amplitude 60 of the carrier wave of the incoming signal, though proportional to the degree or percentage of modulation m.

Fig. 2b shows graphically the inter-dependence of the sensitiveness of reception (response) and the carrier-wave amplitude e0 represented by the factor of proportionality covering a case in which the tube characteristic shown in Fig. 2a holds good.

As will be seen a fluctuation of e at the ratio of 1:100 (and this occurs quite frequently in practice) occasions a change in amplitude of the demodulated signal at the ratio of only 1:2.

In the light of what has been pointed out above, it would be necessary that the characteristic of the tube should come as closely as feasible to a logarithmic curve. In practice, however, the vol ume output of the receiver exhibits a more favorable independence of the incoming amplitude in many instances whenever there prevail certain departures from this curve-shape.

The cause underlying this fact resides in that the assumption has been made as regards the basis of the calculation that, owing to the rectification, the peaks or crests of "the alternating voltage touch the line Eg=0, and that as a consequence the grid alternating potential is representable by ('7):E =e(1-cos wt). In reality, the peaks of the grid alternating voltage will not exactly coincide with Eg=0, in fact, they will project into the range of positive potentials so'much more, the higher e. As a result, the maxima of the plate current, during the period of the impressed frequency, become greater and the mean value of the plate current is caused to rise. Where very large amplitudes are dealt with, such arise A in the mean plate current may even compensate 5 the decrease due to rectification. This disturbing action could be overcome by that, for markedly negative grid voltages, the slope (steepness) of the Ia.Eg characteristic is rendered greater than would correspond to the logarithmic function (see 420 graph III, Fig. 2a,). What results as a consequence is a supplemental rectification for high grid alternating voltages, which compensates the error due to the said rise in the crest values of the plate current. E

In order that such a circuit scheme may preserve this property even with the use of different external resistances, it is of advantage to use tubes of a kind having an internal resistance which is high contrasted with any external re- 130 sistance associated therewith. But this end is realizable, in the case of sensitive tubes, only when at the same time the gain-reciprocal (1: 1) is made low. At any rate, the gain reciprocal should be less than 5%, a limit readily attainable l35 with screen grid tubes. This latter type of ,tube moreover ofiers the advantage that it is possible to influence extensively the form of the characteristic by using a control grid comprising two or more parts whereby the gain-reciprocal becomes -14O different from the screen grid. In Fig. 2 the control grid is shown in two parts 4 and 5 spaced apart by omitting one or more intermediate turns as shown in the aforesaid Ballantine and Snow article. The tube also includes a positive screen145 grid and a suppressor grid at cathode potential. That is to say, the tube 2 is of thepentode type.

What I claim is: a g I Y 1. In a modulated radio frequency wave receiving system, a rectifier network for securingproportionality of audio frequency output amplitude and degree of modulation, said network including a detector tube having an input circuit upon which waves are impressed, the electrodes of said tube being so geometrically related that the plate current-control grid voltage characteristic of the tube is substantially logarithmic for small negative grid voltage values, the droop of the characteristic being relatively more rapid for large negative grid voltage values.

2. In a modulated radio frequency wave receiving system, a rectifier network for securing proportionality of audio frequency output amplitude and degree of modulation, said network including a detector tube having an input circuit upon which waves are impressed, means in said input circuit providing detection by grid rectification, the electrodes of said tube being so geometrically related that the plate current-control grid voltage characteristic of the tube is substantially logarithmic for small negative grid voltage values, the droop of the characteristic being relatively more rapid for large negative grid voltage values.

3. In a modulated radio frequency wave receiving system, a rectifier network for securing proportionality of audio frequency output amplitude and degree of modulation, said network including a detector tube having an input circuit upon which waves are impressed, the electrodes of said tube being so geometrically related that the plate current-control grid voltage characteristic of the tube is substantially logarithmic for small negative grid voltage values, the droop of the characteristic being relatively more rapid for large negative grid voltage values, said tube being of the pentode type wherein the gain reciprocal is less than 5 percent.

4. In a system as in claim 1, said tube including a screen grid, the control grid'of said tube being so constructed with respect to the screen grid that the action of the latter upon the former differs for different points of the control grid.

5.- In a modulated radio frequency wave receiving system, a wave rectifier network including an electron discharge tube provided with a wave input circuit and an output circuit, said tube being provided with at least a cathode, anode, a control grid and a positive screen shielding the grid from the anode, said input circuit including a resistorcondenser network, for rendering the tube operative as a grid circuit rectifier, said tube having its control grid so geometrically related to the

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