4.3.2 Switches
Actually, RF switches are available as all-mechanical blocks with nothing "active" in them, but today, partly due to computer control, most switches have at least their driving system converted into an electronic scheme. Just like ordinary low-frequency electronics hardware, a number of switching configurations exist, ranging from the simple single-pole single-throw (SPST) to complicated multiple-pole monsters.
The main applications of RF switches are in signal routing, of course. For example, we want to have an internal calibration generator in our new RX, and this can be connected into the RF input through a single-pole dual-throw (SPDT) switch as indicated in Figure 4.14. Alternatively, we might want to be able to temporarily insert some functional module into our system flowchart (for example, a low-noise preamplifier, as illustrated in Figure 4.15). The main parameters of switches arc insertion loss, isolation, and impedance matching. Switching speed is important, too, but it is a relevant parameter in solid-state devices only. Also, many mechanical switches must have a specification of the minimum number of switching actions and the respective repeatability of the characteristics along their entire lifetime. Most RF parameters are a function of the operating frequency, and many switches have a relatively limited bandwidth. Power-hand ling capability is not infinite either.
Electromechanical waveguide or coaxial switches typically have the lowest insertion loss—below 0.1 dB for VHF and increasing up to 2 dB or so for the low millimeter-wave region. Pin-diode devices can have losses up to 6 dB already at 3 to
4 GHz. The isolation of solid-state elements is often around 30 dB at 2 GFIz, for example 112], when coaxial devices provide about 100 dB to 18 GHz and 50 dB to 40 GHz. While wear, contamination, and corrosion can severely degrade the performance of mechanical RF switches, however, semiconductor devices maintain rheir specs throughout their entire lifetime. The ability of a switch to handle RF power depends very much on the switching condition. MFlor" switching with RF power going across the terminals is more difficult than "cold" switching, where the RF is not present. Normal laboratory-type mechanical switches and associated hardware usually handle 1 ro 10W, but very robust hardware for broadcasting and radar TXs is manufactured, too, with specifications up to several hundreds of kilowatts. Semiconductor switches have the inherent problem of maintaining their operating point within acceptable limits also with the RF signal applied. Many manufacturers recommend keeping the signal level below +20 dBm.
The matching of an RF switch is actually not a simple thing at all, because the momentary switching state should also be taken into account. First, check if the switch has an internal 50-ohm termination for the port that is not selected. What happens at the time of transition (break-before-make)? Often switching transients cause unwanted oscillator instability and amplifier overshooting, because RF devices tend to be wideband in nature, when compared to mechanical processes. Thus, 10 ms of unknown SWR at the moment of switching may be disastrous. Also the static SWR figures must be observed. Mechanical switches arc often specified from 1.1 to 1.5 at all ports whereas some diode switches may have as high as 30 in the temporarily unused connector!
Electromechanical RF switches may serve 5 or 10 million cycles or they may—according to author's own experience—fail after the twenty-first action. They are precious pieces of fine mechanics, and sometimes the manufacturing process has perhaps been less perfect. Anyhow, when needing the best RF performance and if speed is not a major limitation, choose mechanical devices. Diode switches are fast and quite reliable, excluding, of course, signal-related destruction. They arc the fastest choice, being able to react in less than 10 ns, if necessary. This high speed means that the designer has to carefully analyze arrangements where he or she needs several simultaneous or precisely timed switching operations. One such case is demonstrated in Figure 4.16. A further benefit of diode switches is their superior shielding against dust and dirt; waveguide systems, in particular, can benefit from their smaller size and weight.
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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