TRAVELING-WAVE OSCILLOSCOPE

The problem of oscillographic recording of electrical oscillations at frequencies in the range of \(10^8\)—\(10^{11}\) Hz at the present time, when microwaves are being used ever more widely in physical research, radiolocation, and other branches of radio engineering, is a very important one. Detector methods of recording make it possible to measure the energy of oscillations, but not to judge the shape of the current or voltage curve. The difficulties encountered when using an electronic oscilloscope in this frequency range are reduced mainly to the following phenomena:

a) Change in the voltage deflecting the electron beam during the time of flight of the electron between the deflecting plates.

b) In the case where the voltage under investigation is applied to both pairs of plates—the change in voltage during the time of flight from the first pair of plates to the second leads to an additional phase shift that distorts the figure observed on the screen (in Stekolnikov’s terminology—the \(\Theta\)-phenomenon).

c) It is not possible to use a repetitive linear time sweep in recording very high frequencies. On the other hand, it is impossible to use photographic recording with a single passage of the beam over the screen, or to observe the oscillogram on the screen in the case of ordinary electronic oscillographs, since at a beam velocity over the screen of the order of thousands of kilometers per second the intensity of the phosphor glow is very small.

In view of the above-mentioned difficulties in the field of oscillographic recording of very rapid processes, a number of definite ways of overcoming them have been outlined. The first to be mentioned is an increase in the accelerating voltage, i.e., the construction of high-voltage oscillographs. It is quite obvious that, owing to the fact that already at energies of the order of tens of kilovolts the electron velocity is comparable with the velocity of light, effects connected with the time of flight cannot be eliminated in this way. In addition, high-voltage oscillographs are usually dismountable metal structures, bulky, complex, and inconvenient in operation.

As was shown in recent years by Stekolnikov\(^{1,2,3}\), the method he developed of impulse, brief overvoltage of ordinary sealed-off oscillographic tubes makes it possible to obtain recording speeds on the screen reaching \(10{,}000\) km/sec and to record an oscillation with a frequency up to \(10^8\) Hz with a single passage of the beam over the screen.

A natural, although technically very difficult, method of reducing the influence of transit time was that implemented in so-called micro-oscillographs\(^{4,5}\). The use of a very narrow beam of electrons, formed by an electron-optical system analogous to that of an ordinary electron microscope with several stages of reduction, makes it possible to reduce the geometrical dimensions of the deflecting plates and of the screen and to obtain oscillograms of microscopic dimensions, from which, however, it is quite possible to judge the oscillations under investigation thanks to the correspondingly reduced size of the writing spot on the screen. In this case the single passage of the beam across the screen (photographic plate) is also usually used. A very unpleasant phenomenon, affecting the size of the writing spot and consequently the accuracy of the entire oscillogram, in this case is the mutual spreading of the electrons in a beam of high intensity (current density) and small diameter. Using a three-stage reduction system with magnetic lenses, Li\(^{6}\) was able to record oscillations at frequencies up to \(10^{10}\) cycles.

Finally, in 1949–1950\(^{7,8}\) the idea appeared of using in an oscillograph the principle of the “traveling wave” (see, for example, the review by V. M. Lopukhin\(^{9}\)). As is known, the basic phenomenon used in “traveling-wave tubes” is the interaction of an electron beam with an electromagnetic wave propagating along a slowing device (spiral), which lowers the phase velocity of the wave in the direction of the axis of the device by several times in comparison with the velocity of propagation of electromagnetic waves in vacuum.

Fig. 1.

In the oscillograph constructed by Owaki and others\(^{8}\), this principle is used for deflecting an electron beam perpendicular to the axis of the tube. The construction of the electron-beam tube in this case, with the exception of the deflecting plates, does not differ from the usual one. Instead of deflecting plates, wire electrodes, bent many times in one plane, are used (Fig. 1); as in an ordinary oscillograph, one pair of electrodes is perpendicular to the other. An electromagnetic wave \(U_m \sin \omega T\) propagates from \(P\) to \(Q\) along parallel wires.

conductors. The electron flies along the line \(AB\) between them. The phase velocity of the electromagnetic wave along the axis \(AB\) is

\[ U=\frac{a}{l}\cdot c, \]

where \(a\) is the distance between neighboring conductors, \(l\) is the length of the conductor between neighboring points on the axis \((0\text{–}1,\ 1\text{–}2,\) etc.), and \(c\) is the speed of light.

Let us suppose that the velocity of the electrons in the beam along the axis \(AB\) is constant and equal to \(v_0\), with \(v_0=u\). If this condition is observed, the elec-

Fig. 2.

Fig. 3.

trons passing between the deflecting elements are acted upon by a field constant in direction and amplitude (if the attenuation of the wave in the line is neglected). The time of flight of the electrons between the deflecting elements is equal to the time of propagation of the wave along them.

In our note we shall not touch upon the mathematical consideration of the dependence of the oscilloscope sensitivity to signal voltage on the ratio \(v_0\) to \(u\), nor upon the influence of the inevitable attenuation of the wave caused by the author himself. We point out that these questions had already been analyzed to a considerable extent before the appearance of his work in Stekolnikov’s monograph\(^{11}\) and in Lopukhin’s surveys\(^{9,10}\). The most important circumstance is that, when the equality \(u=v_0\) is exactly satisfied, the sensitivity does not depend on the signal frequency.

We shall present some of the first results of the application of an oscilloscope with a traveling wave. In view of the brightness of the beam, insufficient for the use of single-shot recording, the well-known Lissajous-figure method was employed for investigating microwave oscillations, with the difference from the usual case that the phase difference of the oscillations on the horizontal and vertical deflecting elements depends on the flight time of the electrons between them. Thus, the traveling-wave oscilloscope in the case described, while eliminating one of the effects of the electron flight time, does not eliminate the other.

In the case when the oscillation contains the \(n\)-th harmonic, the additional phase difference due to the flight time is equal to \(\omega \tau\) for the fundamental frequency and \(n\omega \tau\) for the \(n\)-th harmonic (\(\tau\) is the time of flight of the electron between the first and second pairs of deflecting elements). Fig. 2 shows oscilloscope traces of oscillations including the 3rd, 5th, and 7th harmonics at the fundamental frequency \(3 \cdot 10^8\) cycles. With a small amplitude of oscillation of the fundamental frequency, interpretation of the oscillograms is somewhat difficult (Fig. 3).

In conclusion we note that, from our point of view, it would be especially valuable to combine in a single oscilloscope the principles of pulse overvoltage, which would make possible increased brightness, the use of single-shot recording and linear sweep, instead of the Lissajous-figure method, with beam control by means of a traveling wave. It would also be very important to achieve the elimination of \(\theta\)-phenomena, i.e., the effect of the flight time between the deflecting plates.

V. Vavilov

CITED LITERATURE

1. I. S. Stekolnikov, Elektrichestvo, No. 11—12 (1944), p. 27.
2. I. S. Stekolnikov, DAN 56, No. 6 (1946).
3. I. S. Stekolnikov, Impulse Oscillography and Its Applications, Academy of Sciences of the USSR, Moscow, 1949.
4. Hollmann, Hoch. Frequenz. Technik 54, 188 (1939).
5. Hollmann, Proc. of the IRE 28, 213 (1940).
6. G. M. Lee, Proc. of the IRE 34, 121 (1946).
7. J. K. Pierce, Electronics 22, 97, November (1949).
8. Owaki et al., Proc. of the IRE 38, 1172 (1950).
9. V. M. Lopukhin, UFN 36, 456 (1948).
10. V. M. Lopukhin, UFN 40, 592 (1950).
11. I. S. Stekolnikov, The Electronic Oscilloscope, Gosenergoizdat, 1949.