Circuits and Components for system evaluations and design
4.2.5 Directional couplers
Many RF systems have blocks or functions that need to know how much energy propagates to one specific direction in a transmission line. Such data is necessary in defining the impedance mismatch at a junction or when adjusting the amplifier output in a TX (source leveling) so as not to exceed a specific voltaje limit in these cases.
One example of scientific use is shown in figure 4.8. basically, two distinct types exist, one-way and two-way. The slightly more complex two-way directional coupler can be used to measure both the forward and relfected signal levels.
The main parameters used to describe this block are the directivity, coupling, and main line loss, of course, as a function of frequency. As with any transmission line elements, matching is very important for proper operation. Additionally, we may need respective phase responses as well. Special high-power couplers are needed in real TX measurements due to the unavodable losses.
Suitable comercial modules are avaliable for coaxial and waveguide systems, and some very low- frequency surface mount components can be found in the market. The loss in the through mode is typically less tan 1 dB.
Coupling values are available as a series from 6 to 40 dB, the most typical being probably 20 dB. THe more difficult thing is the directivity, because theoretically and in the system designer's eyes it should be infinite, but 40 to 50 dB is a practical máximum and one has top ay a fair amount already for that.
Very wideband wave guide couplers are complicated and cannot be purchased easily for higher microwave frequencies. There, typical products are limited to respective waveguide frequency ranges ore ven less. An example of such two-way construction is show in figure 4.9. if posible, use coaxial versions instead, but note that their directivity will be compromised below 20 dB or so.
Figure 4.8 an example of the usage of a directional coupler. here we are interested in the output power into the sample ( a log) and in the amount of signal reflected back from it.
Figure 4.9 A two-way directional coupler (in the middle) can be used, for example, to monitor the forward and relfected power of a horn antenna. the feeding dielectric resonator oscillator (DRO) oscillator with its ferrite isolator is shown the left.
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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