Waveguide filter
A waveguide filter is a passive filter used in microwave technology in the form of a specially equipped waveguide . It is used to suppress certain frequencies of the electromagnetic wave propagating in the waveguide or to allow it to pass. Applications include, among other things, as an assembly in larger transmitter systems , radar systems and communication satellites . For example, waveguides are used as part of a diplexer to separate the transmit and receive frequency bands on an antenna .
General
Due to their structure, waveguide filters, in contrast to passive filters, which consist of individual electronic components such as capacitors and coils , are limited to the frequency ranges of the vibration modes that can be used in the waveguide . Due to the principle involved, it is not possible to implement low-pass filters with this , since technically customary waveguides below 1 GHz can no longer be used. The upper range of application is limited by the usual vibration modes such as TE 10 in the rectangular waveguide, which means that these filters represent bandpass filters or bandstop filters in the work area. The specific frequencies depend on the application and are usually in the range between 1 GHz and 1 THz.
The advantages of waveguide filters are the low losses and the possibility of being able to be used even with high transmission powers. Transmission capacities of up to a few megawatts are therefore technically feasible. Disadvantages are the mechanical dimensions given by the frequency and the associated wavelengths , the weight due to the metallic structure and the high expenditure in the manufacture and adjustment of the filters. Since waveguide filters cannot be miniaturized except by selecting higher frequencies, other technologies such as stripline filters are mostly used in areas such as cell phones .
Construction variants
The structure of waveguide filters usually follows a certain basic structure in terms of geometry, which is repeated several times and thus realizes the desired transfer function . These basic structures represent resonators matched to the respective frequencies; cavity resonators or dielectric resonators are common . The connections for coupling in and out are designed as with waveguides; in addition, a structure for adapting the characteristic impedance or for coupling a coaxial cable can be present.
Depending on the design of the filter, the individual filter structures include adjustment screws that are accessible from the outside, as is usual with cavity resonators and dielectric resonators. In cavity resonators, the metallic adjustment screws protrude into the interior of the waveguide and allow the geometric shape of the metallic edge structure in the resonator to be changed. In dielectric resonators, where dielectric materials such as barium titanate are used, the position and location of the dielectric element is changed by the adjusting screw.
Before using the filter, the individual adjustment elements must be set to the desired transfer function. In the case of the fully assembled filter, the desired frequency response of the filter is set in an iterative process with the aid of measuring devices such as network analyzers . Since this adjustment is essential for the function, the adjustment elements are usually fixed mechanically in the position with screw locking varnish.
In addition to resonators, so-called diaphragms are also used in waveguide filters. These are metallic plates, which are inserted into the waveguide at a certain distance with a certain contour. Diaphragms are primarily used to prevent certain modes from spreading in the waveguide.
Waveguide filters can also be implemented as absorbing filters, because normally frequency components that are in the stop range of the filter are reflected back to the source. This behavior can be undesirable in some applications, such as waveguide filters for suppressing harmonics in a transmission output stage, since the reflected components would lead to additional heating in the transmission output stage. To avoid this, waveguide filters can be used with dielectric elements that have a high dielectric absorption . The dielectric material may have to be cooled from the outside, since the absorbed wave is converted into heat . Alternatively, frequency-selective couplers can be used in combination with a wave sump to terminate the line.
literature
- Ian Hunter: Theory and Design of Microwave Filters . Institution of Engineering & Technology (IET), 2000, ISBN 978-0-85296-777-5 .
- Walter Fischer: Digital television and radio technology in theory and practice. MPEG baseband coding, DVB, DAB, ATSC, ISDB-T transmission technology, measurement technology . 2nd edition, Springer Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-88187-2 .
- Rainer Geißler, Werner Kammerloher, Hans Werner Schneider: Calculation and design methods of high frequency technology 2. Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig 1994, ISBN 978-3-528-04943-0 .
- H. Meinke, FW Gundlach: Taschenbuch der Hochfrequenztechnik Volume 3 Systems, 5th Edition, Springer Verlag, Berlin / Heidelberg 1992. ISBN 3-540-54716-9 .
- Dietmar Benda: Electronics without ballast. Basics of electronics easy to understand. Franzis Verlag, Poing 2008, ISBN 978-3-7723-5380-2 .
- Holger Heuermann: high frequency technology. Linear components of highly integrated high-frequency circuits, 1st edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig 2005, ISBN 978-3-528-03980-6 .
Web links
Individual evidence
- ^ Hari Singh Nalwa: Handbook of Low and High Dielectric Constant Materials and Their Applications . Academic Press, 1999, ISBN 0-08-053353-1 .