US2450182A - Frequency modulation at ultra high frequencies - Google Patents

Description

. Sept. 28, 1948.. 2,450,182

FREQUENCY MODULATION AT ULTRA HIGH FREQUENCIES Filed March 26, 1943 F. L. BURROUGHS 2 Sheets-Sheet 1 Cav s w m N a mM/m 3% w 5 %A A. m m

' F. 1.. BURROUGHS 2,450,182

2 SheeisSheet 2 LTL Sept. 28, 1948.

FREQUENCY MODULATION AT ULTRA HIGH FREQUENCIES Filed March 26, 1943 FIIIIITIIIIIIIL I Patented Sept. 28, 1948 FREQUENCY MODULATION AT ULTRA HIGH FREQUENCIES Franklin L. Burroughs, Emporium, Pa., assignor to Sylvania Electric Products Inc., Emporium, Ta, 2. ccrporationof Massachusetts Application March'26, 1943,, SerialNo. 480,611

4 Claims.

This invention relates to frequency modulation, particularly to that of oscillation circuits generating frequencies of the order of several hundred megacycles.

The attempts at applying frequency modulae tions to carrier frequencies of several hundred mega-cycles have not been successful. Conven tional reactance tube modulating circuits cannot be used, partly because of the erratic behavior of lumped impedances at these frequencies, partly as a result of the unavoidable straycapacities and inductances .of the circuit leads, and finally due to the fact that anode and grid voltages at these frequencies are no longer in phase opposition. This latter change of the phase relations between input and output voltages of a tube is attributable to transit time effects, and it may be said that the increase of the electron transit time with frequency has formed a barrier for the application of frequency modulation to the higher frequencies.

The invention is based on the principle of usefully employing these same electron transit time effects for obtaining frequency modulation at those high frequencies for which up to now such effects have formed a barrier.

It is, accordingly, a principal object of the invention to provide means for utilizing electron transit time effects for the purpose of frequencymodulating ultra-high frequency carriers.

One object of the invention relates to the translation of transit time effects into variation of a shunt capacity applied to a tuned transmission line section.

Another object of the invention relates to the modulation of the input capacity of a modulator tube, part of which is effectively in parallel with a fixed shunt capacity applied to a tuned transmission linesection.

It is an object of the invention to provide means for applying an ultra high frequency'voltage to the input electrode of an electron discharge tube and to change the transit time of the electrons between these input electrodes by means of a low frequency voltage, whereby the frequency of the ultra high frequency voltage may be changed within wide limits in an associated tuned transmission line circuit.

It is another object to provide an ultra high frequency oscillation circuit whose frequency is varied in response to low frequency voltages applied to the input electrodes of an auxiliary discharge tube.

.It is :a still further object of the invention to utilize the variation of the input capacity of :an

- changing the electron discharge tube, caused by variation of the transit angle of the electrons traveling from one input electrode of this tube to another, for the purpose of modulating the frequency of a tuned transmission line section.

One feature of the invention relates to the de tuning of an oscillation generator, consisting of a tuned quarter-wave transmission line shunted by the plate to grid capacity of an oscillator tube, which detuning is carried out by properly coupling the variable input capacity of an auxiliary modulator tube to the electrodes of the oscillator tube.

A still further feature relates to modulating the frequency of a tuned transmission line sectionfby transit angle of the electrons between two electrodes in one or more of the tubes which are coupled to the transmission line.

The lowest natural frequency 710 of a transmission line section depends on its length s and theboundary conditions. For instance, a quarterwave section is formed by a transmission line of length s, closed on one end and open on the other. Similarly, a half-wave section is formed by a transmission line of length s whose ends are either both open or both short-circuited. The frequency no is connected with the wave lengths he by the well known relation no=clo, where c is the velocity of light. Another example of a transmission line section whose frequency can easily be determined is the transmission line of length 8, closed on one end and shunted at the other by a capacity KO. This type of transmission line section is usually called a quarter wave by a capacity. A simple formula can be written for the natural frequency of this type of transmission line if the unit length capacity K of the trans-mission line is known. The natural frequency of such a shunted line section is then given by the expression (1) a tan a: KS/Ko where w is equal to 21rnos/ 0. Equation 1 shows that a variation of the shunt capacity K0 will vary the frequency of the transmission line section.

If K0 consists of the interelectrode capacity between the plate and grid of an oscillator tube and the input electrodes of a modulator tube which is coupled to the former by coupling condensers, the total shunt capacity K0 can be varied by varying the input capacity of the modulator tube.

According to the invention, the input capacity of the modulator tube is varied by superimposing a low frequency modulating voltage on the ultra high frequency voltage applied to the modulator line shunted grid. The input capacity of the modulator tube varies as a function of the transit time of the electrons traveling from cathode to grid and of the ultra high frequency voltage applied to the grid.

The product of the ultra high circular frequency o and the transit time T is usually called the transit angle If the applied ultra high frequency voltage amplitude is small compared with the effective D. C. voltage between cathode and grid, the effective input capacity Cv can be expressed in the form where Cov is the cold cathode to grid capacity (of a triode) M is usually of the order of A; and the phase angle {3 is O, for =O, and increases slowly with increasing transit angle up to a limiting value of about 31r/2. (The exact values of M and ,8 depend on the tube construction and the operating conditions, and they are not significant for an understanding of the invention. For limitations and the derivation of Equation 2, see, c. g., the article by H. Rothe, P. I. R. E., July, 1940, pp. 325 if.)

The invention will now be described in connection with the drawing, in which,

Fig. 1 is a curve of the input capacity as a function of the transit angle.

Fig. 2 is a simplified equivalent circuit diagram of the circuit elements shown in Figs. 3 and 5.

Fig. 3 represents a circuit according to one embodiment of the invention.

Fig. 4 is an exact equivalent capacity diagram corresponding to the embodiment shown in Fig. 3.

Fig. 5 represents another embodiment of the invention.

Fig. 6 is an exact equivalent capacity diagram corresponding to theembodiment shown in Fig. 5.

The curve of Fig. 1 shows the general character of the variation of the input capacity cv of an electron discharge tube (e. g., a triode) as a function of the transit angle Let the transit time T of the electrons from cathode to grid be decreased,- which may be achieved by making the effective D. C. voltage more positive. If the ultra high frequency superimposed on this D. C. voltage remains constant, equal to we, the variation of the transit angle corresponding to a variation of transit time dT will be do=wodT. In the circuits shown in the drawing, a variation of the transit time changes the circular frequency from we to wo-l-dwo because, according to the invention, part of the input capacity of the modulator tube is in parallel with the shunt capacity of the transmission line. Thus the total change of transit angle becomes By inspection of Fig. 1, it can be seen that for moderate values of 11 the value of the slope of cv, i. e.,

In Fig. 3 are two tubes, tube A with cathode F1, grid 01, and plate P1, and tube B with corresponding cathode F2, grid 92, and plate P2, with the grid 92 of B coupled to the plate P1 of tube A. Thecapacity between F2 and gm of tube B is variable capacity Cv, while that between P1 and yr of tube A is constant capacity C1. In Fig. 3, the microphone M is connected through a suitable coupling unit T, which may include a preamplifier, to the transmission line L shunted by a condenser K. The line L may in conjunction with condenser K, be adjusted to act as a quarterwave line and also to serve as an ultra high frequency trap between the device 'I' and tube B while allowing the signals from device T to be impressed upon the said tube. If desired, each of the line sections L may be an adjustable coaxial line. One side of the quarter-Wave line is connected to grid g2, and the other side of the line is connected to cathode F2 through a suitable grid bias battery B or the like. Preferably, battery B is so poled as to normally bias the grid 92 negatively with respect to the cathode F2. As the signal voltage from device T are impressed on grid 92, the transit angle between 92 and F2 is correspondingly varied as above described, thus varying the input capacitance between 92 and F2. The plate or anode electrode P2 is connected to a suitable source of steady plate voltage +B through the high frequency choke CH. The inherent electrostatic capacitance between the plate and grid is represented as Bpg, and the inherent electrostatic capacitance between the plate and cathode is represented by Bpf. The variable input capacitance between 92 and F2 is represented by Cv. The grid 92 is coupled through condenser C01 to the plate or anode electrode P1 of tube A which is likewise supplied with steady plate voltage through the high frequency choke coil CH1. The grid g1 in returned to ground through the grid leak resistor GL in series with the choke coil CH2. Tube A is connected to act as a high frequency oscillator by reason of the couplling between the plate P1 and 91 by means of the tuned transmission loop S, one side of which is connected to the :plate P1 through the block'condenser BC. The loo S may be of an adjustable length type forming in effect an oscillator loop having the desired self-inductance and capacitance for controlling the generation of the oscillations at the desired frequency. A coupling loop Lp is located adjacent to line loop S and the loop 11p is connected to a suitable di-pole antenna D Fig. 5 is a modification of Fig. 3 and the parts thereof corresponding to those of Fig. 3, bear the same designation numerals. The main difierence between the embodiment of Fig. 5 and that of Fig. 3 is that the grid of tube B is coupled through the condenser C02 to the grid Of tube A instead of to the plate of the latter tube.

Fig. 2 is a simplified diagram of the capacity coupling corresponding to the two embodiments of the invention indicated in Figs. 3 to 6. As can easily be found by a comparison of Figs. 2 to 4, the capacity Cl. of Fig. 2 is identical with capacity C1 of Fig. 4. Capacity C'v of Fig. 2 isthe eifective capacity of Cv in Fig. 4, in parallel with the two series connected capacities Bpg and Bpf. Ch of Fig. 2 corresponds to C of Fig. 3

and Ck2' of Fig. 2 is equivalent to Ca of Fig. 3. The capacity of Cpl of Fig. 3 i assumed to be negligible in this comparison.

It is thus evident that the embodiments shown in Figs. 3 to 6 may be represented by the simple diagram shown in Fig. 2. In Figs. 3 to 6, the capacity C01 and CO2 are external coupling capacities coupling one of the electrodes of the oscillator tube A to the grid of the modulator tube B. Bpg and Bpf are the plate to grid and the plate to cathode capacities of tube B respectively. In these figures, the line S may, for example, be a Lecher Wire system, coupling in loop LP to antenna DP.

According to the invention, grid 92 is preferably at a fixed bias and modulated by low frequency voltages which may be fed from the microphone M or any other low frequency source. The coupling capacities C01 and Cc2 are properly chosen, and tube types must be selected which will yield a reasonable variation of the effective capacity in response to variations of the capacity Cv.

The embodiments shown in the figure are given for the purpose of illustration only and not by Way of limitation. Modifications which offer themselves to those skilled in the art are considered to be Within the scope of the invention. In place of a circuit using a single modulator tube, another circuit may be substituted by the proper combination of two push-pull modulators, and the coupling capacities may be applied at appropriate points of the tuned transmission line rather than at the ends as shown in Figs. 3 and 5. The proper choice of the tube types incorporating the characteristics concerning the influence of the efiective coupling capacity on the deviation ratio, the application of the methods for preventing frequency drift and similar changes, desirable from a practical point of view in a given application, may be carried out without deviating from the spirit of the invention. In many cases, it will be satisfactory to make tube B an ordinary pentode or triode whose cathode to grid capacity remains positive during modulation. For larger deviations a tube may be used with a space charge grid located between the cathode and control grid and the operating voltages are chosen so that the effective input capacitance varies between positive and negative values.

What I claim is:

1. In combination, an oscillator tube, a modulator tube, and a tuned transmission loop having connected thereto a frequency-determining capacity network comprising four capacitances, one

of the capacitance being in shunt to the loop the other three capacitances being in series with each other, the series capacitances also being in shunt to said loop, said one capacitance comprising the interelectrode inherent capacitance between two of the electrodes of the oscillator tube, said other three capacitances comprising in series respectively the grid-cathode capacitance of the oscillator tube, the grid-cathode capacitance of the modulator tube and a separate condenser, and means to vary the bias on the grid of the modulator tube to vary thereby the effective capacitance of said network and thereby varying the frequency of the oscillations in said loop.

2. The combination according to claim 1 in which the said condenser is connected between the plate of the oscillator tube and the grid of the modulator tube.

3. The combination according to claim 1 in which the said condenser is connected between the grid of the oscillator tube and the grid of the modulator tube.

4. The combination according to claim 1 in which a tuned quarter-wave transmission line is included between the grid-biassing means and the modulator tube and acts as a Wave trap for the high frequency oscillations.

FRANKLIN L. BURROUGHS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,777,410 Jones (Dot. 7, 1930 1,945,545 Finch et al. Feb. 6, 1934 1,945,547 Pray Feb. 6, 1934 1,950,759 Terman Mar. 13, 1934 2,032,620 Langmuir Mar. 3, 1936 2,231,372 Rothe et a1 Feb. 11, 1941 2,278,429 Crosby Apr. 7, 1942 2,305,882 Lindenblad Dec. 22, 1942 OTHER REFERENCES Ultra-High Frequency Techniques, by Brainerd, et al., published by Van Nostrand (30., New York city, pages 82, 83, 296, and 297. Copy in Division 51.

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