Analog and Digital Circuit and Component Design - CEI-Europe
Low loss and highly selective filters and multiplexers are key components in the wireless networks that surround us. A low loss diplexer allows the transmitter and receiver of a basestation to simultaneously share the same antenna. The same filter must also guarantee that co-located basestations using competing transmission standards do not interfere with each other. Many of these filters and multiplexers are based on cavity combline technology, which is relatively simple to manufacture. Others are based on dielectric resonator (DR) technology that can realize a high quality factor (Q) filter in a smaller volume. Introducing non-adjacent couplings (cross-couplings) into a microwave filter can generate transmission zeros in the lower and or upper stopbands. It is the filter order and the clever placement of these transmission zeros that generates the selectivity needed for wireless applications. The theory of cross-coupled filters was first introduced in the 1960's. It was then adopted for satellite multiplexer applications in the 1970's and for wireless applications in the following decades. EM simulation is also an essential component of modern cavity filter design. We now have the ability to model and optimize complete filter structures in the EM domain. These virtual prototypes have greatly reduced the number of hardware prototypes that must be built and tuned. Occasionally, we find unexpected spurious couplings in our virtual EM prototypes that prevent us from tuning the filter to the desired response. These spurious couplings would be very difficult and expensive to diagnose after the hardware is built.
Filters are one of the fundamental building blocks used in integrated microwave assemblies, along with amplifiers, oscillators, mixers and switches. Depending on the frequency range and bandwidth we might use printed distributed filters, printed pseudo lumped filters, chip and wire lumped element filters and in some cases, cavity combline filters. Switched filter banks are common and sophisticated multiplexers are used in some systems. Many broadband microwave down converters and up converters are built using thin-film technology on ceramic substrates. The substrates are placed in a channelized housing which isolates the various signal paths from each other. The front end, band select filters may be as broad as octave bandwidth, while the IF filters are typically much narrower. Filters used to clean up harmonics in the LO chain may be narrower still. It the past decade there has been a trend to use more printed circuit board technology when possible and even use commercial off the shelf (COTS) parts in military systems. EM simulation is also an essential component of filter design for military systems. Distributed filters in a cut-off waveguide channel excite, and couple to, evanescent modes in the channel. The net result is the measured bandwidth of the filter is radically different with the cover on and the cover off. If the channel dimensions change, the filter must be redesigned. A design procedure that incorporates EM simulation is needed to include all the filter layout details and the coupling of the filter layout to the waveguide channel.

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