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Although RF circuits are generally considered to be circuits that operate from tens of MHz up to several GHz, and microwave circuits at frequencies beyond that, boundaries based purely on frequency are rarely appropriate. Analog integrated circuits based on lower-frequency design methodologies can now operate well into the microwave range, purely because of smaller feature sizes that are now available in CMOS and silicon-germanium technologies. BiCMOS integrated circuits that operate in the microwave frequency range, designed using low frequency architectures, are now abundant. However, classical microwave circuit design techniques are still important to model and understand problems arising from noise, mismatch, circuit losses, and limited bandwidth. We will focus on the design of discrete circuits that are differentiated from their historically lower-frequency counterparts by several features. In RF and microwave design, the phase shift of the component is significant because its size is comparable with a wavelength, its reactances and parasitics must be accounted for, and reflections occur between elements. These all limit bandwidth. We also need to consider circuit losses that degrade the Q of an element as well as introduce noise, and nonlinearities that introduce distortion into the signal path. Electromagnetic radiation and capacitive coupling will also be features of such circuits. Such 'RF and microwave' effects are most commonly observed when using discrete or custom devices, or when assembling integrated circuits together at higher frequencies into systems.
Data Converters suffer from a wide range of non-ideal effects such as mismatch, parasitics, finite gain / bandwidth / slew rate, offset, noise and so on and so forth. Many of these effects are random effects of statistical nature. They also depend on first and second order characteristics of the process technology, layout, packaging, pinout etc. It is practically impossible to describe all these effects for all possible architectures, sizings, layouts on device (“SPICE”) level and to compare all the simulation results to decide on the best implementation. It is thus necessary to build higher level models which include all relevant effects, at least in the first order, but are still simple enough for quick modifications and efficient study of various layouts, topologies etc. Simulation times must not go beyond minutes. The relevant effects must, of course, be understood to be able to simplify and model them. When Data Converter models are coded and simulated, when first hardware samples come in, and finally during production, they have to be analyzed and their performance determined. Even if the converter should not be directly measured during production test, direct access is required for first characterization, to establish correlations between analysis and production test, and in case of problems. Time versus frequency domain, static versus dynamic characterization – the test methodology must yield the specified variables. Without thorough understanding of the underlying basics misinterpretations and large errors are possible.
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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