These TEM mode filters consist of inductively coupled short circuited stub resonators. These high Q structures enable the realization of high selectively, narrowband filters with very low passband insertion loss, which is important for receiver front-end or high power transmit applications. In addition, the steep attenuation skirts are ideal for separation of closely spaced signals. By using resonators that are less than 45 degrees in electrical length, the upper frequency limit of the stopband may be extended with high levels of ultimate attenuation.
Freedom Microwave has researched the suppression of intermodulation products in high power filters. Through careful process control and component selection, we have devised specialized design techniques to satisfy these requirements.
Pseudo elliptic designs can be used to reduce the number of sections required to meet a specified attenuation response. This reduction in the number of sections results in a smaller filter with lower passband loss. Similar techniques can also be used to provide passband group delay equalization or increase attenuation over a specified stopband region.
To reduce weight and minimize passband loss, Freedom Microwave cavity filters are generally machined from lightweight aluminum alloys and plated with a high purity silver. Proprietary design techniques are used to ensure minimal temperature drift. Freedom Microwave has designed and manufactured cavity filters with temperature drifts of less than 1 ppm/deg. C.
For systems that share a common antenna, such as Tx/Rx communications systems, multiplexers can be formed by combining these bandpass structures to a common port. Both contiguous and non-contiguous multiplexers are available. In addition , by combining two or more filters at both input and output ports, devices with two or more passbands are achievable.
Combline structures are used for bandwidths between 0.2% and 70%. Depending upon the particular specifications, combline filters can provide stopbands that extend beyond 10 times the center frequency. Additionally, these structures can be made extremely compact and are generally considered to be one to the most versatile topologies for use at microwave frequencies.
For very wide passband applications, interdigital structures are used. Interdigital filters provide a geometrically symmetric amplitude response for wideband applications, whereas combline responses become increasingly asymmetric as the percent bandwidth increases. The quarter wavelength resonators used in interdigital filters, however, result in a decrease in upper stopband performance.
High selectivity, low loss bandstop filters are realized by coupling stub resonators to a common transmission line.
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