US2224200A - Circuit for amplifying carrier frequencies - Google Patents

Description

Dec. 10, 1940; R. SCHIENEMANN 3 CIRCUIT FOR AMPLIFYING CARR'LER FREQUENCIES Filed May 1-2, 1958 2. Sheets-Sheet 1 l J i I l l I i .58 MC INVENTOR 9000M SCH/ENEMA/VN ATTORNEY I 3 R lllllllll [1 [M0 f p: a F 1 I IV H A a A a l Dec. 10, 1940. R SCHIENEMANN 2 CIRCUIT FOR AMPLIFYING CARRIER FREQUENCIES Filed May 12, 1958 2 Sheets-Sheet 2 I l l I l s l I l I l INVENTOR RUDOLF SCI/IENEMA NN ATTO RNEY Patented Dec. 10, 1940 UNITED STATES PATENT OFFICE 2,224,200 CIRCUIT FOR AMPLIFYING CARRIER FREQUENCIES tion of Germany Application May 12, 1938, Serial No. 207,563

In Germany May 13, 1937 3 Claims.

The invention relates to circuits for amplifying carrier frequencies which comprise resonant circuits tuned to different frequencies. Such arrangements are necessary, e. g., for television or telephony purposes in order to amplify the band of frequencies to be transmitted.

The invention more particularly is concerned with the problem of obtaining approximately constant amplification over the whole band with a given width of the band of frequencies to be amplified. The independent variables which are available for obtaining this desired uniform amplification are the resonance frequencies of the different circuits and their attenuations. At the outset it was assumed that, with a view towards insuring the desired straight frequency characteristic, the distances or intervals between the separate resonance frequencies had to be made equal, and that also the attenuations of the resonant circuits had to be equal. At most it might have been anticipated that the optimal frequency interval and the optimal attenuation would have to be increased or decreased steadily in the direction from the lower towards the higher fre- 2 quencies within the band to be amplified.

-The invention is based on the knowledge that a constant amplification is obtained, on the contrary, when either the distances of the separate resonance frequencies or their attenuations are made to decrease towards the upper and lower cut-off limits of the frequency band to be transmitted. Preferably the distances as well as the attenuations should simultaneously decrease.

For a better understanding of the invention 35 reference will be made to the accompanying drawings wherein Fig. 1 is a graphical representation on a linear frequency scale of a plurality of resonance curves which serve to explain the invention; Fig. 2 is a similar representation on a logarithmic scale, Fig. 3 is a graphical representation of the resonance curves when the distances between the resonant frequencies of the several circuits are decreased toward the limits of the band but with equal attenuations of the circuits; Fig. 4 is a graphical representation of the resonance curves when the attenuations of the several circuits are decreased towards the limits of the band but with equal intervals between the resonant frequencies of the circuits; and Fig. 5 is an amplifier circuit embodying the features of this invention.

A rough idea of the most favorable conditions is given by the curves shown in Fig. 1 which are based upon the calculated values and wherein the most favorable position of the resonance curves R1 to R6 of the different circuits ascertained in a given instance is plotted against a usual linear scale for the frequency f and the corresponding attenuations at the same time are A at least qualitatively'indicated by the magnitude 5 of the resonance maxima. The curve V indicates the course of the amplification factor for this position of the different curves. The ordinate in Fig. 1 is designated by a. The result is extraordinarily surprising in that the crowd- 10 ing together of the resonance maxima or crest values towards the both ends or limits of the frequency band and the simultaneous decrease of the damping values or attenuations of the corresponding circuits, were not to be expected.

The technical rule forming the object of the invention, namely to decrease the intervals between the resonance frequencies and/or the at tenuations of the resonant circuits towards the both ends or limits of the frequency band to be 0 transmitted, shall be illustrated and explained in more detail in what follows by reference to Figures2, 3 and 4, wherein a logarithmic scale is used for the frequency axis. Such a logarithmicscaleoffers the advantage that the resonance 25 curves will then be symmetrical to their resonance frequency and that their shape remains the same or unvaried with the same or constant attenuation, regardless of the magnitude of the resonance frequency.

The case illustrated in Fig. 1 appears as shown in Fig. 2 when logarithmic scales are used for the frequency axis as well as for the ordinate axis. The range at the limits of which the curve V falls off to the amount 0.7 is called the width b of the band to be amplified. The corresponding attenuations d are indicated at the different curves R1 to Re.

With reference to Fig. 3 it shall first of all be pointed out that with equal attenuations of 40 the different resonance circuits, the distances be tween the resonance frequencies must be decreased towards the limits of the band to be transmitted in order to obtain an approximately constant amplification factor. It is to be noticed, thereby, that the amplification factor for a single frequency within the band to be transmitted'is proportional to the product of the amplitudes of all resonance curves at this position. Thus, for instance, when proceeding from the frequency is to the frequency ii, the resonance curves R2 to Re fall off, while only the resonance curve R1 rises. Hence, in order that the amplification factor at the frequency f1 may again have approximately the sam value as at the frequency 12, the increase of the amplitude of the curve R1 must approximately compensate the decrease of the amplitudes of the curves R2 to Re. If, however, one proceeds from the frequency is to the frequency f2, it is only the curves R3 to Re which decrease (as compared with the curves R2 to Rs proceeding from f2 to f1), while two curves increase namely R1 and R2 (in contrast to only the curve R1 when proceeding from f2 to h). In other words, in order that the amplification factor at the frequency f2 may again have the same value as at the frequency is, the decrease of the curves R3 to Rs (in contrast to R2 to Rs in the former case) must at least approximately be compensated by the increase of R1 and R2. What follows therefrom is that the maximum or crest value of R2, i. e., the frequency f2, may be spaced farther apart from f3 than the frequency f1 from the frequency 12. Moreover, it will be noted that the curves R3 to Re have a greater amplitude at the frequency f2 than at the frequency f1 so that also by this reason the distance f2-f1 must be smaller than the distance fa-fz. Referring to Fig. 4, in similar manner, it can be demonstrated that also by decreasing the attenuation of the diiferent resonant circuits towards the ends or limits of the band to be transmitted, with equal frequency intervals of the resonant circuits, an approximately constant amplification factor within the frequency range to be transmitted may be obtained. Again assume that it is the frequencies ii to f3, at which the maxima of the resonance curves R1 to R3 are situated. Proceeding from the frequency f2 to the frequency f1, the curves to Rs decrease, while only the curve R1 increases. In order that for frequency 1 the prodnot of all amplitudes of the curves R1 to Rs may have again approximately the same value as at 0 the frequency f2, the increase of the curve R1 must approximately compensate the decrease of all other curves R2 to Re. For this aim a distinct magnitude of the maximum of R1, the position of which on the frequency scale is assumed to be fixed in this case, is necessary. If, however, proceeding from the frequency is to T2, it is only the curves R3 to Re which decrease (in contrast to R2 to R6 in the former case), while the both curves R1 and R2 increase (in contrast to only R2 in the former case). Therefore, in

order to obtain at the frequency f2 again approximately the same amplification factor as at the frequency is, the maximum of R2 ought not to be as great as the maximum of R1 in the first treated case. Besides, also in this case the'curves R3 to R6 have smaller amplitudes at the frequency f1 than at the frequency f2 so that also for this reason the attenuation of the circuit R1 must be smaller than that of the circuit R2. Particularly favorable conditions, i. e., a scarcely perceptible decrease of" the amplification factor up to the neighborhood'of the both limits of the band, are obtained, if one makes both the intervals between the resonance frequencies and the attenuation of the respective resonant circuits decrease towards the ends of the band. This fact shall be demonstrated by calculation in a manner as hereinafter to be outlined, without however a proof thereof being furnished.

For the curve V th following equation may be derived, wherein 75 is the ratio of inductance to capacity of each of the several resonant circuits, 1L their number and, as above, the mid frequency of the band is designated by f and the width of the band by b.

L i v- C (f*+ n/2 f If the multiplications in the term under the root of this equation are carried out and the resulting expression is arranged according to the powers of 1, all coefiicients of except that of the highest power of f may be made equal to zero by suitable choice of the quantities In 702 Ion 2 which are coeflicients dependent on the dampings and separations in frequency of the pairs of circuits. This results in the curve V being rendered nearly fiat over the band of frequencies b. Once the values of the coefiicients k1 k2 Ian/2 which produce the above result have been determinedby solution of the simultaneous equations resulting from setting each of the coefficients above mentioned equal to zero, the frequency intervals A between the resonant frequencies of each pair of symmetrical tuned circuits and the damping d of each pair may be determined from the following equations, in which the value of used is that whose subscript corresponds to the pair of circuits in question.

wherein A signifies the distance between every two neighboring resonant circuits of equal attenuation (indicated in Fig. 2). The attenuation is herein as above the so-called parallel attenuation, i. e., it is calculated by the formula wherein R is the resistance in parallel to the resonant circuit L and C. Once the damping d for a pair of circuits is known, the value of R is determined since is made the same arbitrarily chosen value for l2 and I3 are inserted. These coils represent -65 amplifier, in the plate circuits of which the coils together with the capacities in parallel to them,

preferably with the plate-earth capacities C10 and C11 drawn in dotted lines, the resonant circuits. The requisite attenuation may be obtained, e. g., by suitably dimensioning the parallel resistances I4, I5 or the inherent resistances of the coils l2 and I3. The terminals l6 and H are connected to the positive pole of the plate-voltage source,

while a negative bias is applied to the terminals l8, IQ of the grid-leak resistances 20, 2| of the control grids fed through the capacities.

As already mentioned in the beginning, the invention inter alia also may be applied to the amplification of speech currents. If, e. g., along a telephone line eight repeater stations comprising four tubes each are provided, according to the invention all 32 resonant circuits forming part of them may be differently tuned and damped. Likewise one may perhaps distribute the resonance frequencies of the first 16 circuits over the whole band according to the rules given by the invention and choose the frequencies of the second 16 circuits in the same manner as the frequencies of the first circuits. In this manner also the subdivision of the whole amplifying channel even may be carried on by forming four groups comprising eight resonance circuits each or eight groups comprising four differently tuned and/or damped circuits each. Within each group then the frequencies and/ or the attenuations are graduated according to the invention. With eight groups comprising four differently tuned and/or damped resonant circuits each of the four circuits are differently designed, but the various repeater stations are conformable to each other. Within the scope of the invention one may likewise place the first four circuits of the first four stations in the first repeater station, the second four circuits of the first four stations in the second repeater station and so on. Then the first station contains throughout circuits equal to each other, the second station likewise nothing but circuits equal to each other, which, however, are different from the circuits of the first station etc. For all that the first four stations of the whole channel represent then an embodiment of the inventive idea, as in the first four stations a circuit arrangement for amplifying carrier frequencies is provided, which comprises resonant circuits tuned to different frequencies.

I claim:

1. A circuit for substantially uniformly amplifying a relatively wide band of carrier frequencies, comprising a plurality of cascaded resonant circuits which are tuned to diiferent frequencies within said band, characterized in that the distances between the resonance peaks of successive resonant circuits progressively decrease in each direction from the center of the frequency band to be transmitted towards the limits of said band.

2. A circuit for substantially uniformly amplifying a relatively wide band of carrier frequencies, comprising a plurality of cascaded resonant circuits which are tuned to different frequencies within said band, characterized in that the attenuations of the several resonant circuits progressively decrease in each direction from the center of the frequency band to be transmitted towards the limits of said band.

3. A circuit for substantially uniformly amplifying a relatively wide band of carrier frequencies, comprising a plurality of cascaded resonant circuits which are tuned to different frequencies within said band, characterized in that both the distances between the peaks of successive resonant circuits and the attenuations of said circuits progressively decrease in each direction from the center of the frequency band to be transmitted towards the limits of said band.

RUDOLF SCHIENEMANN.

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