Barkhausen short oscillation

Schott M tube triode from Jena from 1917, which was used by Barkhausen and Kurz as a transmitting and receiving tube .

The Barkhausen Kurz oscillation , also known as electron dance oscillation , formerly known among radio amateurs as electrical electron dance and BK oscillator, is a form of excitation of high frequency oscillations in the frequency range between a few 10 MHz and a few GHz in electron tubes . The oscillation frequency only depends on the distance between the electrodes and the operating voltage; an external resonator is not required. The name is derived from the German physicist Heinrich Barkhausen and Karl Kurz , who first described this effect in 1917 and who researched it at the Technical University of Dresden in the 1920s . For the first time, oscillators in the meter and decimeter wave range could be built. In comparison to the generation of the Gill-Morell oscillation , the Barkhausen-Kurz oscillation does not in principle require an external resonator, but Barkhausen and Kurz used Lecher lines for measurement, which also served as a resonator.

description

A PC88 electron tube, wired as a braking field tube without an external resonant circuit, generates Barkhausen-Kurz oscillations with a voltage-dependent frequency

Incandescent lamp without filling gas with uncoiled filament in which Barkhausen-Kurz oscillations can occur as a disruptive effect

The electrons emanating from the cathode are accelerated to the grid in a triode connected as a braking field tube , the control grid of which is therefore at a more positive potential than the anode . Part of it gets through the positively charged grid into the space between grid and anode. There the electrons reach the field area of ​​the negative “anode” (better: reflector, see reflex klystron ) and turn around there. After this detour, some of them hit the grid and discharge it, the rest of them return to the space charge area. The transit time determines this pendulum frequency only depends on the effective distance between the cathode (space charge cloud) and the anode (repulsion) and the effective acceleration voltage . Electrons with the wrong speed are caught prematurely by the grid and do not contribute to the excitation of vibrations. "Correct" electrons are formed by the speed modulation of the vibrating lattice and form space charge clouds during flight, which in turn reload the lattice effectively and in the correct rhythm. ${\displaystyle d}$ ${\displaystyle U}$

Since the electron paths, depending on the tube construction, can also go around the cathode or also propagation paths with different path lengths are possible, the oscillation is usually broadband and very noisy. Since the size of the space charge cloud around the cathode also depends on the heating power, the mean path length and thus the frequency are also slightly dependent on this. There is proportionality between the excited frequency and the square root of the applied grid voltage. ${\displaystyle f}$

The following applies approximately:

${\displaystyle f={\frac {1{,}5\cdot 10^{5}\cdot {\sqrt {U}}}{d}}}$

application

The studies and calculations for the theory created the conditions for later time-of-flight tubes .

Barkhausen-Kurz oscillators were used in laboratories and also at the beginning of the Second World War as UHF oscillators and transmitters.

The brake field tube did not acquire any practical significance later, apart from a few "disruptive effects". So could Zeilenendröhren in television sets under certain error conditions in Barkhausen short vibrations fall - whose anode is negative while the line as the screen grid. The image of the error was recognizable on the screen as vertical narrow curtains at the edge of the image over the entire height of the image - while the sawtooth-shaped current curve of each line is running through, the oscillations always occur in the same way and, depending on frequency and amplitude, interfere with reception.

The Barkhausen Kurz oscillations can also occur as a high-frequency disruptive effect in vacuum “filled” incandescent lamps with long, smooth filaments in the form of a trap. Such incandescent lamps were common in the early days of electrical lighting, but are still made today. The Barkhausen short oscillation of the lamps in the vicinity of the glowing incandescent lamp can interfere with VHF radio reception.

The Barkhausen-Kurz oscillation can be frequency-stabilized with an external resonator (often a line circuit ); it is then no longer broad, but very narrow-band. Such an arrangement can be called a precursor for all oscillators with time-of- flight tubes ( reflex klystron , magnetron ). Radio amateurs and hobbyists used the effect to advance into frequency ranges that were at least previously not achievable with available components.

literature

• Heinrich Barkhausen: Electron tubes, 3rd volume feedback . 4th edition. S. Hirzel, Leipzig 1931.
• Technical & Scientific Literature Department, J. Jäger: Data and Circuits of Television Receiver Valves . In: Series of Books of Electronic Valves . IIIc. NV Philips' Gloeilampenfabrieken (Philips Industries), Eindhoven, NL 1953 ( PDF, 15MB ).

Individual references / footnotes

1. ↑ H.Barkhausen: electron tubes . 6th edition. tape 3 . S. Hirzel, Leipzig 1951, p. 102 .
2. ↑ H. Barkhausen and Karl Kurz: The shortest shafts that can be produced with vacuum tubes . In: Physikal. Magazine . tape 21 , no. 1 . Leipzig 1920, p. 1-6 .
3. ↑ Archive link ( Memento of the original from July 14, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Page 134ff.  
4. ↑ Incandescent lamp as VHF jammer thread / forum posts
5. ↑ http://www.elektronik-labor.de/Notizen/Laufzeitoszillator.html Experiments with short oscillations and standard tubes