Devices that do not contain semiconductors in their RF paths are classified here as passive. Typical items falling into this category include coaxial or waveguide terminations, attenuators, power splitters and combiners, isolators, and filters [1|. Although completely mechanical RF switches do exist and arc frequently used as well, that topic is postponed to Section 4.3, where active modules are discussed in detail.
4.2.1 Terminations
The purpose of an RF termination is to provide a well-known and stable impedance to a Transmission line port, which might belong, for example, to an amplifier or to a power divider. The key performance figures arc the impedance mismatch, the bandwidth across which that value is maintained, and the power-handling capability. Very high power terminations arc often called dummy loads, because their main usage is in PA and TX testing. Naturally, we must find a termination having the suitable mating interface (e.g., an appropriate coaxial connector or a waveguide flange). Coaxial terminations are available for 50- and 75-ohm systems, but only exceptional designs use the latter variant. Most commercial types can withstand CW power up to 100 mW or slightly more and have SWRs better than 1.2 to 26 GHz. Some waveguide terminations are specified below 1.1 but are usable only within the respective guide bandwidth [2]. Microstrip and stripline systems generally do not have ready-made termination modules available but the designer has to use surface-mount device (SMD) resistors having the correct resistance and negligible inducrancc or capacitance. Precision terminations are mainly used as calibration standards for network analyzers and related equipment. For these metrology-grade applications sliding terminations are also manufactured. They enable one to adjust the phase of the residual reflection so that its effect on the overall measurement uncertainty can be estimated.
Generally, terminations do not cause too much trouble in systems design, if devices of adequate quality and bandwidth are purchased. On some occasions
voltage transients may destroy a termination even though we do not exceed the maximum power. The main reason for damage is improper handling. This is particularly true of small coaxial 1.8-mm, 2.4-mm, 3.5-mm, SMA. or K-type connector devices, which do not withstand excessive force or misalignment during mating. Coaxial modules come as male and female alternatives for the best impedance matching.
4.2.2 Attenuators
The main goal of an RF attenuator is—as the name implies—attenuation. However, we must rake into account, the frequency range, the unavoidable mismatch at both of the ports, the phase or group delay response, and the power-handling capability. Commercially manufactured devices arc often processed in a series of 1-2-3-5-10 dB and from there on, in steps of 10 dB up to about 100 or 120 dB. Both fixed and step attenuators are available, and their control can be either mechanical or fully electronic. Some applications require a continuously variable attenuator, the range of which is typically from 0 to about 50 or 60 dB [3], The actual lossy elements can be simple high-quality tcsistors, pin diodes, or lossy fin-like designs in waveguides. Fast pin-diode attenuators act like amplitude modulators, if needed, and one of their additional parameters is switching time, which can be further divided into settling time and active time. Where mechanical step attenuators require 10 to 20 ms to change state, all-electronic counterparts operate within 10 \is or less and pin switches even in the nanosecond class. All step and variable attenuators have the same kind of settling uncertainty and residual attenuation, which is present even if we select 0 dB. Mechanical devices do quite well with a typical residual term around compensate for the attenuation by adding an amplifier of similar gain. Actually, this scheme often gives somewhat better isolation values due to the fact that the amplifier, too, has some reverse isolation characteristics, and, if necessary, we can measure the performance and adjust the parameters accordingly. Special high isolation amplifiers are available as well. If wc have to improve matching as well, we can try to divide the attenuation into two parts, one before and the other after our amplifier.
4.2.3 Power Dividers and Combiners
in terms of theory, power dividers also act as combiners, but this is not necessarily the ease in real life. The name describes the wanted function very well. Wc may need to give the same signal to several different processing elements or wc perhaps want to feed several signals to the same antenna. This is just a perfect place for a good RF combiner or divider. Different constructions are available having a sum port and something up to 32 or 64 individual channels to be summed up or divided into. The division itself will cause a respective decrease in the power level; for example, a two-way divider has output signals at-3 dB, but additional losses are unavoidable—typically 1 to 2 dB per division depending on the frequency ranpe and vendor. Resistive power dividers have further attenuation due to their operating principle but often give wider bandwidth and better matching.
When selecting a power-dividing or power-combining clement for a system, we naturally have to look at the number of ports needed and take care of the frequency range. The power-handling capability is limited, too. Summing devices have to withstand much more than the single input signal |3J. Phase-coherent systems behave in this respect differently to noncoherent designs because voltages may add up in-phase.
In many cases, the phase imbalance between the ports is important. This figure depends on the frequency and may be one of the limiting factors (e.g., in adaptive antenna arrays); see Figure 4.3. Although there might be no reason to ask for isolation between ports when considering the initial input signal, the overall system performance surely benefits from it. Wilkinson dividers found in the coaxial and microstrip worlds give easily more than 20 dB; waveguide structures, on the other hand, do not necessarily yield very much. Isolation and matching of individual ports also depends on the impedance conditions of the remaining inputs or outputs.
Libro: Circuits and Components for System Evaluations and Design
Autor: Pekka Eskelinen
Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia
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