London Digital PR

Digital PR

Latest News

Use of RF Switches


RF switches can either be absorptive or reflective. Absorptive switches are considered more suitable for high-power applications, such as satellite communications, radars, and wide range Internet, since they absorb the incoming waves and do not generate potentially damaging reflections. Reflective switches have low insertion loss ( = loss of power upon inserting an external device), but a high power loss in the circuit, because reflected waves bounce back and forth through the transmission line, dissipating power. They are suitable for use in broadband communications and instrumentation applications, such as measuring devices. The low insertion loss means the measuring device will be unaffected when hooking up a device to assess its performance.

There are two major configurations that employ RF switches:
1. Using solid state devices, such as PIN diodes and Field-Effect Transistors (FET). The principle of operation of a PIN diode switch is as follows: Applying a DC current to a PIN diode in forward bias will make the diode conduct. This in turn will let an RF source pass a signal through the diode to a load. Applying a reverse DC bias turns off the diode by widening the diode depletion layer, and thus blocks the RF signal.

A Field Effect Transistor Switch can allow electrons to flow either from source to drain or vice-versa. FETs are therefore used for sending several message signals, combined into a single signal, sent over a single wire. It thus has uses in communications, or mixing signals and/or analog or digital sound (as in a synthesizer).


PIN diode and FET switches have a number of advantages as switches:


• Linear behavior: When operating as RF switches, they don’t create as many spurious products as regular PN diodes.
• Greater isolation: Isolation is the ratio of the power level when the switch is ON to the power level measured when the switch is OFF. Good isolation means stray signals won’t leak into the main signal. Isolation is important in measurement devices: if these stray signals were allowed to get through, measurement integrity is severely compromised.
• Switching speed: A PIN diode switch can switch much more quickly (e.g., 1 microsecond) than a mechanical relay operating by means of an electromagnet.


A drawback of these switches is that such devices have a high insertion loss if the operating frequency is > 1 GHz.

2. Using Microelectronic Mechanical System (MEMS). MEMS switches use electrostatic forces to move a beam or membrane that either changes the resistance or capacitance of a circuit. Because the electrostatic force drops off abruptly—by Coulomb’s law, according to the inverse square of the distance between the two charges—it allows making a switch of very small dimensions. Thus a MEMS switch has significant advantages owing to its small size as measured in the operating wavelength.

Thus, between the various configurations of RF switches, they can be employed in a wide range of electronic devices, including:
• Cellular phones: MEMS RF switches can be set up as multiplexers that can test signals of 100 cellphone devices at once. They can then send the test result signals to a single RF analyzer, which stores it and analyzes the data.
• Wireless pagers: RF switches are employed in pagers, which can be used by a handicapped person in summoning help.
• Wireless computer networks: A wireless router can use an RF Switch to take IP signals and route them to various destinations.
• Satellite communications: RF switches have uses in frequency converters and translators, amplifier systems, and video products employed in satellites.
• Cable television equipment: RF switches can be used with wireless cable TV, or to hook up a playstation to the cable network.
• Radar systems: RF microelectromechanical systems employ RF switches in radar sensors.
• In transceivers: RF switches can be used to toggle the contact between the transmitter and the receiver to the antenna.

 

Bookmark and Share

© 2009 London Digital PR Ltd - Designed by The Creative Clinic