US1950759A - Variable reactance circuit - Google Patents

Description

March 13, 1934. F. E. TERMAN VARIABLE REACTANCE CIRCUIT Filed May 25, 1931 INVENTOR, FREDERICK E. TERM/W ATTORNEY Patented Mar. 13, 1934 UNITED STATES PATENT OFFICE This invention relates to variable reactanccs, and particularly to reactances which are cyclically variable at high frequency, and which may be utilized as modulators to produce side- 5 band frequencies without carrier frequencies, in accordance with the method set forth in my application for United States Letters Patenton a Frequency changing system, filed October 20, 1930, Serial No. 489,917.

Among the objects of my invention are: first, to provide a circuit having an effective reactance which may be cyclically varied at high frequency; second, to provide a circuit adaptable for use in a frequency changing system of the character-set forth in my above identified application;' third, to provide a circuit having a reactance corresponding either to an inductance or a capacitance in its variation with frequency,

tive phase rotations of the two branches are balanced over wide frequency bands.

Other objects and advantages of my invention will become apparent, or will be specifically pointed out, in the course of this specification.

I have pointed out in my above mentioned application, Serial No. 489,917 that if a modulating wave, such as the electrical equivalent of a sound wave, be passed through an impedance which is cyclically varied at a carrier frequency, there will be present in the circuit currents of frequencies corresponding to the sum and the difference of the carrier frequency and the modulating frequency, but without the presence of the carrier frequency current itself. In that apfrequency, i. e., wherein the positive and nega-- plication, and in my co-pending application, Se-' The of the tube there is inserted a variable resistance element, such, for example, as one of the cyclically variable vacuum tube resistor arrangements described in my above mentioned application, Serial No. 539,655. This resistive 90 element may, if desired, comprise a dynatron tube having'a negative resistance. If the reactance of the first mentioned tube be measured between its cathode and grid terminals, it will be found to have a frequency characteristic of the same character as the coupling between grid and anode, i. e., if the coupling be capacitive, the reactance measured between cathode and grid will decrease with increasing frequency. The magnitude of this reactance will vary inversely with the resistance of the element in its output circuit, and will have the same sign as that resistance. That is to say, if the resistance in the output circuit be negative, as with a dynatron element, the reactance between grid and cathode will be of the same sign as an inductance.

Fig, 1 is a simple form of circuit in accordance with my invention.

Fig. 2 is a preferred form of a cyclically variable reactance circuit.

Fig. 3 is a modification of my invention,

. which may be utilized to produce an effective negative variable capacitance.

One of the simplest formsof such a circuit is illustrated in detail in Figure l. The terminals 1 and 2 of the variable-reactance circuit are connected respectively to the grid 3 and cathode 4 of a triode 5 of ordinary construction. The plate 6 of the tube is coupled to the grid 3 through a capacitance 7, which may be the internal capacitance of the tube, but is preferably an external capacitance connected around the tube. Anode current for the tube is supplied from a source 8 which feeds the plate through a choke coil 10 having an impedance which is high as compared with the tube resistance at all frequencies for which the circuit is designed.

Connected across the choke 10 is a variable resistor 11, in series with which a blocking condenser 12 is'preferably connected, this resistance thus being the effective load circuit of the tube and connected between the anode and cathode.

6. The effective input capacitance of the circuit as a whole under these circumstances is equal to the capacitance C; between cathode and grid, plus (1+u')C2, and where C2 is the capacitance of the condenser 7 and u is the effective amplification of the tube. This eflective amplification u is equal to u, the amplification constant of the tube, times R/(R+r), where R is the resistance of the element 11, and r is the internal resistance of the tube.

It will therefore be seen that as the resistance R of the element 11 is varied, the eflective capacitance of the input circuit will vary in the same manner, or in other words, the effective reactance of the circuit will vary inversely with A preferred method of applying this principle to produce a cyclically varying reactance, is shown in Figure 2. The input terminals 1' and 2' are connected respectively to the grid 3' and cathode 4 of the tube 5' as before. In this case the coupling between anode and grid comprises an inductor 15 in series with a blocking condenser 16, connected across grid and anode terminals as before, in order to produce an inductive reactance in the circuit as a whole. The plate of the tube is supplied with current from the source 8' through the choke 10'.

The load circuit in this case, however, comprises a tetrode 17 whose filament 18 connects with the filament of the tube 5. A choke 19. connects from the filament to the plate 21 to hold the latter at zero bias, the plate 21 also connecting with the plate 6' through a blocking condenser 20. The shield grid 22 may derive its positive bias from a tap on the source 8', while the control grid 25 is supplied from a source 26 of carrier frequency potential.

As disclosed in my copending application, Serial No. 539,655 above mentioned, the tetrode 17, connected in this manner, becomes a resistor whose resistance varies cyclically with the potential of the carrier frequency source. The reactance across the terminals 1' and 2' will therefore be an inductive reactance varying in the same manner as the potential of the carrier frequency source 26.

Other types of resistive elements, either varying cyclically, or of constant value, may be utilized in like manner to give varying eflect. Thus there is shown in Figure 3 a circuit which is operative to give an effective negative variable capacitance across the terminals 1" and 2". The tube 5" whose elements are similarly numbered to those of the tube 5 in Figure 1,

but are distinguished by double accents, has its grid and anode connected by a capacitance '7" as in the first figure.

In this case, however, the resistive output circuit comprises a dynatron 30, the latter being a tetrode whose cathode 31 and control grid 32 are supplied from a carrier frequency potential source 33. The plate 35 connects directly to the anode6" of the triode. The screen grid 36 connects to the filament circuit of both tubes through a source 37 which maintains a screen grid of the highest potential in the circuit. A tap 38 on the battery or other source 3'7 supplies a somewhat lower potential through the choke coil 40 to the anodes of both tubes.

As is well known, if the potentials of the anode 35 and the screen grid 36 be properly adjusted, secondary emission of electrons from the anode will cause the net current flowing in the anode circuit to decrease as the voltage rises, the tube therefore acting as a negative resistance. By varying the potential of the grid 32 by means of a carrier frequency source 33, the magnitude of this negative resistance may be varied in a cyclical manner. Under these circumstances a negative capacitance will be displayed across the terminals 1" and 2", or, if

an inductance be substituted for the capacity 7" in the same manner as the inductance 15 is connected in Figure 2, the circuit will display a negative inductance.

The meaning of these negative reactive quantities may be stated as follows: The magnitude of the current flowing past the terminals 1" and 2" varies in the same manner as in a positive reactance. That is, if the circuit be acting as a negative capacitance, the reactance offered will decrease with increasing frequency. The phase will, however, be reversed as compared to that of the current in a true capacitance, the voltage leading the current instead of the current leading the voltage. If the negatively reactive circuit be inductive, the reactance will increase with increasing frequency, but the current flowing will lead the voltage instead of lagging as in a positive inductive circuit.

The use of reactive elements of opposite phase characteristics to balance each others effect is well known in resonant circuits. In such a circult the reactances, at the frequency of resonarice, are equal but of opposite phase. By using negative reactances of the character above described, such resonant circuits may be made in which the reactive effects are balanced out at all frequencies over which the device is operated. That is, since the positive and negative reactive elements have similar frequency characteristics, a circuit comprising a positive and negative reactance will be in balance at all frequencies if it be in balance at any particular frequency. Resonance, in the sense of zero phase rotation or unity power factor, may therefore be obtained over extremely wide frequency bands.

It the negative reactance be made variable, by varying the negative resistance inthe output circuit of the tube, it is thus possible to utilize a combined circuit, in which positive and negative reactances are balanced, as a modulating circuit in the same manner as a single, unbalanced, reactive circuit is used.

I claim: v

1. The method of operating a triode as a variable impedance element in a circuit including a reactive coupling between the grid and anode elements of said triode, and a resistance connected between the cathode and anode thereof, which comprises varying said resistance to produce consonant variations of the effective anode-grid reactance of said triode.

2. The method of operating 'a triode as a variable impedance element in a circuit including a reactive coupling between the grid and anode elements of said triode, and a resistance connected between the cathode and anode thereof, which comprises cyclically varying said resistance to produce consonant variations of the effective anode-grid reactance of said triode.

3. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid and having reactive coupling between said anode and grid, and a second vacuum tube connected as an output resistor between the anode and cathode of said first tube, said second tube being variable in resistance to produce consonant variations in reactance between the cathode and grid of said first tube.

4. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid, a reactive element connected between said grid and anode, and a second vacuum tube connected as an output resistor between the anode and cathode of said first tube, said second tube being variable in resistance to produce consonant variations in reactance between-the cathode and grid of said first tube.

5. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid, a capacitance externally connected between said grid and anode, and a second vacuum tube connected as an output resistor between the anode and cathode of said first tube, said second tube being variable in resistance to produce consonant variations in reactance between the cathode and grid of said first tube.

6. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid, an inductance connected between said cathode and anode, and a second vacuum tube connected as an output resistor between the anode and cathode of said first tube, said 'second tube being variable in resistance to produce consonant variations in reactance between the cathode and grid of said first tube.

'7. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid and having reactive coupling between said anode and grid, and a second vacuum tube having a negative resistive characteristic connected as an output resistance between the anode and cathode of said first tube, whereby said first tube displays an input reactance bay-- ing frequency characteristics corresponding to the reactive coupling between its cathode and anode and of opposite phase characteristics.

8. A circuit of variable reactance comprising a vacuum tube including a cathode, an anode, and a grid and having reactive coupling between said anode and grid, a second vacuum tube having a negative resistive characteristic connected as an output resistance between the anode and cathode of said first tube, and means for varying the efiective negative resistance of said second tube.

9. A variable reactance circuit comprising a vacuum tube including a cathode, an anode, and a grid, and having reactive coupling between said anode and grid, a second vacuum tube connected as an output resistor between said anode and cathode, and means for cyclically varying the efiective resistance of said second tube to produce corresponding variations in the efiective cathode-grid reactance of said first tube.

10. The method of employing an amplifier tube having input and output circuits to'vary the impedance of said input circuit which com-

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