Distributed-element circuit: Difference between revisions

Revision as of 00:50, 24 October 2019

Latest revision as of 23:23, 22 May 2024

(29 intermediate revisions by 22 users not shown)

Line 1:

{{Short description|Electrical circuits composed of lengths of transmission lines or other distributed components}}

[[File:LNB circuit.jpg|thumb|upright=1.5|alt=Satellite-TV block-converter circuit board|A [[low-noise block converter]] with distributed elements. The circuitry on the right is [[lumped element]]s. The distributed-element circuitry is centre and left of centre, and is constructed in [[microstrip]].]]

'''Distributed-taco manelement circuits''' are [[electrical circuitscircuit]]s composed of lengths of [[transmission line]]s or other distributed components. These circuits perform the same functions as conventional circuits composed of [[Passivity (engineering)|passive]] components, such as [[capacitor]]s, [[inductor]]s, and [[transformer]]s. They are used mostly at [[microwave]] frequencies, where conventional components are difficult (or impossible) to implement.

Conventional circuits consist of individual components manufactured separately then connected together with a conducting medium. Distributed-element circuits are built by forming the medium itself into specific patterns. A major advantage of distributed-element circuits is that they can be produced cheaply as a [[printed circuit board]] for consumer products, such as [[satellite television]]. They are also made in [[coaxial cable|coaxial]] and [[waveguide (electromagnetism)|waveguide]] formats for applications such as [[radar]], [[satellite communication]], and [[microwave link]]s.

Line 35:

=== Coaxial ===

[[File:Koaxrichtkoppler.jpg|thumb|alt=Photograph|A collection of coaxial [[directional coupler]]s. One has the cover removed, showing its internal structure.]]

[[Coaxial cable|Coaxial line]], a centre conductor surrounded by an insulated shielding conductor, is widely used for interconnecting units of microwave equipment and for longer-distance transmissions. Although coaxial distributed-element devices were commonly manufactured during the second half of the 20th century, they have been replaced in many applications by planar forms due to cost and size considerations. Air-[[dielectric]] coaxial line is used for low-loss and high-power applications. Distributed-element circuits in other media still commonly transition to [[coaxial connector]]s at the circuit [[Port (circuit theory)|ports]] for intrconnectioninterconnection purposes.<ref>{{multiref|Natarajan, pp. 11–12|}}</ref>

=== Planar ===

Line 42:

=== Waveguide ===

{{main|waveguideWaveguide (electromagnetism)}}

[[File:Waveguide-post-filter.JPG|thumb|alt=Rectangular waveguide filter with five tuning screws|A [[waveguide filter]]]]

Many distributed-element designs can be directly implemented in waveguide. However, there is an additional complication with waveguides in that multiple [[waveguide mode|modes]] are possible. These sometimes exist simultaneously, and this situation has no analogy in conducting lines. Waveguides have the advantages of lower loss and higher quality [[resonator]]s over conducting lines, but their relative expense and bulk means that microstrip is often preferred. Waveguide mostly finds uses in high-end products, such as high-power military radars and the upper microwave bands (where planar formats are too lossy). Waveguide becomes bulkier with lower frequency, which militates against its use on the lower bands.<ref>Ghione & Pirola, p. 18</ref>

Line 74:

=== Dielectric resonator ===

{{main|dielectricDielectric resonator}}

A dielectric resonator is a piece of dielectric material exposed to electromagnetic waves. It is most often in the form of a cylinder or thick disc. Although cavity resonators can be filled with dielectric, the essential difference is that in cavity resonators the electromagnetic field is entirely contained within the cavity walls. A dielectric resonator has some field in the surrounding space. This can lead to undesirable coupling with other components. The major advantage of dielectric resonators is that they are considerably smaller than the equivalent air-filled cavity.<ref>Penn & Alford, pp. 524–530</ref>

Line 84:

{{see also|Fractal antenna}}

[[file:Hilbert resonator.svg|thumb|upright|alt=diagram|Three-iteration Hilbert fractal resonator in microstrip<ref>Janković ''et al.'', p. 197</ref>]]

The use of [[fractal]]-like curves as a circuit component is an emerging field in distributed-element circuits.<ref>Ramadan ''et al.'', p. 237</ref> Fractals have been used to make resonators for filters and antennae. One of the benefits of using fractals is their space-filling property, making them smaller than other designs.<ref>Janković ''et al.'', p. 191</ref> Other advantages include the ability to produce [[wide-band]] and [[Multi-band device|multi-band]] designs, good in-band performance, and good [[out-of-band]] rejection.<ref>Janković ''et al.'', pp. 191–192</ref> In practice, a true fractal cannot be made because at each [[Iterated function system|fractal iteration]] the manufacturing tolerances become tighter and are eventually greater than the construction method can achieve. However, after a small number of iterations, the performance is close to that of a true fractal. These may be called ''pre-fractals'' or ''finite-order fractals'' where it is necessary to distinguish from a true fractal.<ref>Janković ''et al.'', p. 196</ref>

Fractals that have been used as a circuit component include the [[Koch snowflake]], [[Minkowski island]], [[Sierpiński curve]], [[Hilbert curve]], and [[Peano curve]].<ref>Janković ''et al.'', p. 196</ref> The first three are closed curves, suitable for patch antennae. The latter two are open curves with terminations on opposite sides of the fractal. This makes them suitable for use where a connection in [[cascade connection|cascade]] is required.<ref>Janković ''et al.'', p. 196</ref>

Line 94:

=== Distributed resistance ===

Resistive elements are generally not useful in a distributed-element circuit. However, distributed resistors may be used in [[attenuator (electronics)|attenuator]]s and line [[electrical termination|terminations]]. In planar media they can be implemented as a meandering line of high-resistance material, or as a deposited patch of [[thin-film]] or [[thick-film]] material.<ref>{{multiref|Maloratsky (2012), p. 69|Hilty, p. 425|Bahl (2014), p. 214}}</ref> In waveguide, Aa card of microwave absorbent material can be inserted into the waveguide.<ref>Hilty, pp. 426–427</ref>

== Circuit blocks ==

Line 112:

A directional coupler is a four-port device which couples power flowing in one direction from one path to another. Two of the ports are the input and output ports of the main line. A portion of the power entering the input port is coupled to a third port, known as the ''coupled port''. None of the power entering the input port is coupled to the fourth port, usually known as the ''isolated port''. For power flowing in the reverse direction and entering the output port, a reciprocal situation occurs; some power is coupled to the isolated port, but none is coupled to the coupled port.<ref>Sisodia & Raghuvansh, p. 70</ref>

A power divider is often constructed as a directional coupler, with the isolated port permanently terminated in a matched load (making it effectively a three-port device). There is no essential difference between the two devices. The term ''directional coupler'' is usually used when the coupling factor (the proportion of power reaching the coupled port) is low, and ''power divider'' when the coupling factor is high. A power combiner is simply a power splitter used in reverse. In distributed-element implementations using coupled lines, indirectly coupled lines are more suitable for low-coupling directional couplers; directly- coupled branch line couplers are more suitable for high-coupling power dividers.<ref>Ishii, p. 226</ref>

Distributed-element designs rely on an element length of one-quarter wavelength (or some other length); this will hold true at only one frequency. Simple designs, therefore, have a limited [[Bandwidth (signal processing)|bandwidth]] over which they will work successfully. Like impedance matching networks, a wide-band design requires multiple sections and the design begins to resemble a filter.<ref>Bhat & Khoul, pp. 622–627</ref>