Pot circle
A pot circle is a coaxial line circle that is closed on all sides and behaves like a parallel resonant circuit . In contrast to an oscillating circuit made of discrete components, its resonance frequency is not clear, since the resonance property is caused by the formation of standing waves along the inner conductor to the cylinder wall. The pot circle is therefore related to structures such as coordinated antenna wires or Lech circles .
description
At high frequencies (above about 100 MHz ) the wavelength is so small that the dimensions of the components, especially the coils and capacitors, can no longer be neglected and increased radiation losses occur because the components act like antennas. If you want to achieve low losses and high quality factors at these frequencies , closed shields must be used. It is therefore advisable to consider the large-area inner wall of this shielding as part of the resonant circuit, because the large area drastically reduces the current density and thus the losses due to the skin effect . However, cavity resonators are only small enough for installation in normal devices above about 2000 MHz. Therefore, between about 100 and 2000 MHz, pot circles with a central rod or tube are used as oscillating circles.
In the picture you can imagine the capacitor between the right end of the rod and the surrounding shield. The electric field is strongest there, which is why the resonance frequency can be changed by a slight variation in the distance. The magnetic alternating field is at its maximum at the left end of the rod, here the energy can be inductively coupled out by means of wire loops.
No-load quality factors of 1000 can be achieved with pot circles, because they are closed on all sides, a large surface is involved in the power line and no energy can be radiated.
Dimensions
A pot circle without additional discrete capacitance at the right end has an overall length of almost exactly λ / 4. Since a standing wave forms on the central rod of length L, resonances can also occur at other wavelengths. That is what the relationship is for
with n = 1,2,3, .. (see also line theory # short-circuited line and line circuit ).
Electrical Properties
In order to reduce the geometric length and to enable fine tuning, a variable capacitor ( trimmer ) can be connected to ground (outer wall) from the free end of the inner conductor . If the frequency has to be changed significantly, the geometric length can be changed using a short-circuit slide on the left end of the rod.
The coupling to other circuit parts is either capacitive or inductive (tap near the base or by a wire loop). In the case of cup circles connected together as a band filter , an opening or a short-circuit clip between the two circles serves as a coupling.
Disc triodes and pencil tubes are specially developed electron tubes for use in pot circles.
Embodiments and examples
1 - Trimmer for center frequency and frequency deviation
2 - Disc triode (GS13-1)
3 - Decoupling loop
At very high frequencies, pot circles with inner conductors open on both sides are also built - the resonance length of the unconnected inner conductor is then λ / 2. Then these circuits can be wired with capacitors on both sides, for example in some UHF tuners (receiving parts in television receivers) on one side with a trimmer, on the other with a capacitance diode .
Disk triodes and also transistors are operated in pot circles in a lattice base circuit or base circuit. The grid connection of disc triodes is brought out as a ring and is at ground potential (pot bottom). Transistors are built with two base connections.
In the example of an older FM radar oscillator ( picture ), the two line circuits are double coaxially one inside the other and are connected to one another by a coupling opening (feedback). The frequency variation is done by a small motor-driven sector, which causes a capacitive frequency detuning in the form of a triangular curve.
See also
Web links
- radartutorial.eu
- Instructions for a pot circle with resonance at 435 MHz
- Illustration of several cup-circle filters
Individual evidence
- ↑ Meinke , Gundlach : Pocket book of high frequency technology. Volume 2: Components. Edited by Klaus Lange and Karl-Heinz Locher. 5th, revised edition. Springer-Verlag, Berlin et al. 1992, ISBN 3-540-54715-0 , p. L46.