US1956134A - Transmitting circuits, with thermionic valves - Google Patents

Description

April 24, 1934. RUSPQLI 1,956,134

TRANSMITTING CIRCUITS WITH THERMIONIC VALVES Filed Sept. 10, 1931 J*:T#a:

5, fluspa/ Patented Apr. 24, 1934 UNITED STATES TRANSMITTING CIRCUITS, WITH THERMIONIC VALVES Edmondo Ruspoli, Paris, France Application September 10, 1931, Serial No. 562,167 In Italy March 30, 1931 3 Claims.

This invention has reference to thermionic valve oscillators.

It is well known that oscillations of very constant frequency may be generated in thermionic valve oscillators in which the piezo-electric action of a quartz (or similar) crystal creates a condition of the grid circuit allowing reaction and grid excitation to take place. But it is also known that the power which may safely be applied to such an oscillator is very low. Beyond this power limit, the increased mechanical vibrations, voltages and currents cause damage to the crystal.

Other thermionic valve oscillators are known where a crystal is used to stabilize the frequency, but where a certain degree of reaction and grid excitation are provided independently of piezoelectric action. If the crystal is removed, the circuit will oscillate without stabilization, or will be near the starting point of oscillations.

But here again, if the crystal is to be kept free from danger when it is effectively stabilizing the frequency, the power limit must be kept low, in the types of oscillators generally used.

The object of the present invention is to greatly reduce the strain usually applied to a quartz crystal in an oscillator, without losing the important benefit of the constancy which its use confers to the frequency. Considerably greater power can then be safely handled by the oscillator, allowing stages of amplification to be dispensed with in many cases, with a decided advantage from the standpoints of weight, simplicity and economy of first cost and operation.

The invention consists of a vacuum tube oscillator, more especially destined for use in radio transmission, in which the proper degree of reaction, necessary for grid excitation, is provided through the combined effects of two devices, one

of these utilizing the piezo-electric properties of a crystal, and the other utilizing a common method of reaction. The effect of each device is limited by appropriate means and in a measure calculated, on the one hand, to insure sufiicientfrequency stability, and on the other hand to reduce crystal action to within safe margins, even when said crystal is cut for frequencies higher than 7,000 kilocycles (when it is very thin and more subject to damage) and high plate power is applied, for example 200 watts.

The diagram of Figure 1 shows one of the possible known circuits in which the piezo electric action of a crystal creates a condition of the grid circuit in which grid excitation and oscillation are possible.

Diagram of Figure 2 shows one of the possible known arrangements where a crystal used to stabilize the frequency of an oscillator, but where a certain degree of reaction and grid excitation are provided independently of piezo-electric action. If the crystal is removed from these circuits, they can still oscillate, without stabilization, or at least are near the starting point of oscillations.

The diagram of Figure 3 shows a well known form of oscillator in which power is fed back from plate to grid through the internal tube capacities. Condenser C may be emitted if the grid-filament capacity suffices to give circuit 01 a correct natural frequency. In some cases in fact, this correctnatural frequency may be obtained by placing in series a capacity (not shown on the figure) in the grid filament circuit.

Diagram Figure 4 illustrates, by way of example, one way in which the invention may be carried out.

The curves in Figure 5 show oscillation frequencies, plotted against capacities of plate tuning condenser CV, for different circuit arrangements.

To better explain the present invention, we will recall two of the conditions which any vacuum tube circuit must fulfil in order to generate oscillations of a given frequency.

Condition A.A coupling of the plate and grid circuits must be provided through which the grid potential will receive an adequate impulse for every oscillation of the plate potential. 1

In certain oscillators this coupling consists only of the internal tube capacity, in others an additional coupling is provided.

Condition B.-The grid potential must oscillate easily, at the chosen frequency, relatively to the filament potential. In other words, a resonance effect in the grid circuit must allow grid oscilla ing potentials to build up, at the frequency of the above mentioned impulses. depend on the device through which the grid is connected to the filament.

A complete failing of either of the conditions A or B must necessarily prevent any grid excitation (and consequently, any generation of con tinuous oscillations), but a lessening of one of the two may, within certain limits, be compensated by an increase in the other.

If the oscillator is to work with good output efliciency, a certain degree of grid excitation must nected between the grid and the filament.

The piezo-electric mechanism depends on certain elastic deformations of the crystal, which are alternatively effects or causes of potential differences between its faces. These deformations have a very marked tendency to follow a characteristic fundamental oscillatory rhythm, of extremely constant frequency, and sometimes other tendencies as Well, but less marked, to follow certain secondary frequencies.

At the fundamental frequency of the crystal (and eventually at the secondary frequencies also), this resonance phenomenon will bring about the building up of the grid potential oscillations required by condition B. But, at the other frequencies, even very close to those above mentioned, there will be no resonance response or building up of these potentials and therefore no possibility of sustained oscillations.

The universally known circuit shown in Figure 1 illustrates this method. The plate-grid capacity of the tube provides the coupling required by condition A; it is represented in the diagram as a, and is shown connected with dotted lines. This coupling carries voltage impulses from the plate to the grid, and as the grid-circuit oscillates by the piezo-electric effects of the crystal (condition B), the grid voltages build up until continuous oscillations are maintained. A satisfactory output efficiency may be obtained in the load resistance R, which may be replaced by an an tenna or amplifier coupled to the plate coil. The

degree of grid excitation will depend on the difference between the natural frequencies of the plate circuit and of the crystal, and can therefore be adjusted by plate condenser CV. It is found that oscillation can take place for a wide band of adjustments of this condenser. The amount of excitation depends on the more or less difference between the natural frequencies of the plate and grid circuits. The frequency is controlled by the crystal and remains very constant throughout these adjustments. This is illustrated by the curve 1.1 in Figure 5, where oscillation frequencies are plotted against values of plate condenser CV, when these values decrease from maximum towards minimum. This circuit is subject to the general drawback of crystal oscillators, as a plate power of the order of 10 watts is the utmost which may be applied without damaging the crystal.

Method II.Another method of fulfilling condition B is to connect the grid and the filament to different points of a tuned circuit 01 having an adequate resonance frequency. An example of this method is illustrated in Figure 3. The grid potential can oscillate easily at this, and at all other very close frequencies. If the plate circuit is tuned closely enough to these, the potential impulses reaching the grid through the tube capacity will build up the grid voltages high enough for oscillations to be maintained. The amount of grid excitation, as in the case of Method I, can be adjusted by means of condenser CV, and the tube may oscillate for a wide range of these adjustments. But this time, variations of CV will cause corresponding variations in the oscillation frequency, as may be seen in curve 2-2 in Figure 5.

The present invention consists in fulfilling condition B by a combination of the devices, characterizing Methods I and II (a crystal and a tuned grid), while providing adequate means for limiting and adjusting the extent to which the resonance effect of each device will affect the grid excitation. These adjustments must allow the total grid excitation to reach its optimum value without exceeding it. If then the excitation by the crystal is increased, the excitation by the tuned grid circuit must be correspondingly decreased and vice versa.

Figure 2 shows, by way of example, a known vacuum tube oscillator arrangement, above referred to, comprising a tuned grid circuit and a crystal for stabilizing the frequency when the natural frequency of the plate circuit, tuned by the condenser CV is included within certain limits. The feed back of said device is such that, if the crystal were removed, the circuit might still oscillate or have suificient grid excitation to almost oscillate.

The method of carrying out the invention illustrated by way of example in Figure 4 may be looked upon as derived from the circuit of Figure 2 to which have been added appropriate devices for adjusting the effects of crystal resonance and grid circuit resonance to certain preferred ratios, allowing the oscillator to handle more power with less strain on the crystal (by applying equivalent devices to other circuits of the same kind, other forms of the invention would be obtained).

Grid excitation by the crystal will be adjusted by means of a capacity Q (Figure 4) or suitable resistances (not shown) or an inductive coupling (not shown). Excitation by grid circuit resonance can be cut down and adjusted by one of the following means: 7

(a) By adjusting plate condenser CV, thus bringing the plate tuning farther from or closer to the grid tuning,

(b) By giving a suitable damping to the tuned grid circuit, by means of one or more of the auxiliary resistances such as R1, R2, R3, R4, R5, Re. It will not be necessary to employ them all simultaneously. In reality, some of them will always have values close to zero or practically infinite. In selecting them, one will have to take into consideration the natural resistances of all parts already present in the grid circuit;

(0) By adjusting the inductance-capacity ratio of the grid circuit.

With the arrangement described, it is found that, for decreasing capacities of CV, the oscillation frequency varies as shown by curve 3 in Figure 5. Up to point P3, curve 3 practically coincides with curve 2. The frequency varies as if no crystal were present. Beyond P3, the stabilizing action of the crystal commences, and curve 3 separates from curve 2 and tends to coincide with curve 1 from P1 to P2. At P2, stabilization ceases abruptly, and curve 3 ascends vertically to P4, to coincide again with curve 2 from there on. A very stable frequency is obtained between points P1 and P2. These points will be brought closer together if capacity Q is reduced. The distance between them might correspond to differences in plate tuning of the order of 0.1% to 5% in frequency.

For a given degree of total or combined excitation of the grid circuit, it is found:

1That when the ratio excitation by crystal action excitation by tuned grid circuit is increased:

(a) The frequency is stabilized for an increased range of plate condenser tuning;

(b) R. F. currents flowing through the crystal increase.

2That decreasing this ratio will bring about the reverse phenomena.

In view of these facts, the devices characterizing the invention will be so adjusted as to give a value to said ratio which will obey the following conditions:

l--It must be sufficiently high to allow frequency stabilization to take place on a plate tuning range large enough not to be exceeded by accidental detuning causes (mechanical. movements and vibrations, variations in power supply, modulation, etc.

2It must be low enough for the R. F. currents flowing through the crystal and voltages on same to be small, allowing the power input and output to be increased without danger for the crystal.

In order to judge these values correctly, a radio frequency milliameter A may be connected in series with the crystal. By adjusting capacity Q, one will then broaden the stabilization range P1P2 (Figure 5) as much as possible without letting the current in A exceed reasonable read ings, indicating crystal safety.

With low power, a high ratio of excitation by crystal action excitation by tuned grid circuit may safely be used, with a broad stabilization range.

With increased power, the ratio must be lowered, with consequent narrowing of the stabilization range.

ExampZe.The following typical results have been given by the circuits of Figures 1 and 4, using a watt tube (radiotron U. V. 211 of the Radio Corporation of America). The load R was replaced by coupling a zeppelin antenna to the plate coil. The wave length was 42 meters.

In both cases, the same order of stability was maintained and greater power has frequently been handled, crystals giving service for several months.

The circuit of Figure 2, which has been chosen :as a basis for the method, that has been discussed .above, for carrying out the present invention, is proposed as a nonlimitative example. By treating other circuits in an equivalent manner, other forms of the invention would be arrived at.

What I claim is:

1. An electric oscillation generating device in cluding a three electrodes vacuum tube, means for applying a suitable potential to the plate, an oscillatory plate circuit, an oscillatory grid circuit, a piezo-electric crystal, means for connecting said crystal in parallel with the oscillatory grid circuit, means for adjusting the degree of the grid excitation caused by the crystal and means for adjusting the degree of grid excitation caused by the oscillatory grid circuit.

2. An electric oscillation generating device including a three electrodes vacuum tube, means for applying a suitable potential to the plate, an oscillatory plate circuit, an oscillatory grid circuit, a piezo-electric crystal, means for connecting said crystal in parallel with the oscillatory grid circuit, an adjustable condenser, means for connecting said condenser in series with the crystal and means for adjusting the degree of grid excitation caused by the oscillatory grid circuit.

3. An electric oscillation generating device including a three electrodes vacuum tube, means for applying a suitable potential to the plate, an oscillatory plate circuit, an oscillatory grid circuit, a piezo-electric crystal, means for connecting said crystal in parallel with the oscillatory grid circuit, means for adjusting the degree of the grid excitation caused by the crystal and adjustable resistances connected in the grid oscillatory circuit in order to adjust the degree of the grid excitation caused by said oscillatory circuit.

EDMONDO RUSPOLI.

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