In a perfect world, an RF design engineer would like to see a switch with a variety of near ideal specifications. These would include low insertion loss, high isolation, extreme linearity, long term reliability, a wide frequency bandwidth, high power handling capability, low actuation voltage, very low cost, small size and a fast switch time. For real world RF design, variances on ideal specifications can be tolerated for many applications. In general, an insertion loss much less than 1 dB, isolation much greater than 20 dB and linearity with IP3 much greater than 60 dBm will meet many current day RF applications. Throw in a frequency bandwidth of between 0.4 and 6 GHz, power handling capability much greater than a watt, an actuation voltage of 2.7 volts and a switch time less than 5 microseconds, and the RF switch could be used for almost any wireless application.
Ideally cost, size and reliability of a switch would also be minimized to the point of being inconsequential. So the designer looks forward to switches that cost much less than a dime, have a size well below 0.5 square millimeters and will last at least three years for typical consumer operating conditions.
Although semiconductor RF switches can be optimized to come close to the ideal for certain specifications, this requires sacrifices in other performance areas. Alternatives to RF-MEMS Switches such as PIN Diodes and Solid State Switches have good high frequency passing capabilities. But PIN Diodes suffer from an increase in resistance (skin effect) above the X-band range (8-10 GHz) as well as higher current consumption due to individual bias requirements for each Diode. While less expensive than PIN Diodes, Solid State FET Switches have unique problems with insertion loss and isolation at frequencies above 6 GHz unless the switch is designed as an absorptive or T-switch.
And their limitations don't stop there. Other problems include their frequency limitations above the 1 GHz range, where their insertion loss and isolation tend to breakdown. Another problem is their non-linear switching behavior and related signal to noise issues.
Overcoming RF-MEMS Actuation Voltages
Actuation voltage has been an issue with RF-MEMS, traditionally, because of the nature of the technology and the types of microstructure used. RF-MEMS have in the past required an actuation voltage of anywhere between 20 volts and 120 volts, which is far too high to be practical in many portable applications without the use of voltage multiplier or charge pump circuits. On the other hand, PIN diodes and FETs have been generally available with switch voltages in the same order as CMOS circuits (anywhere between 1.5 Volts and 5 Volts) – without the need for any extra circuitry.
In order to overcome the actuation voltage issue with RF-MEMS, designers have come up with different mechanical switch schemes that show it is possible to reduce the actuation voltage to levels below 10 Volts. Furthermore since electrostatic driven MEMS devices do not require significant current to actuate them, the higher voltage generation problem is simplified. Requisite voltages can be provided by a very simple and low cost 'charge pump' circuit (the charge pump circuits, which can be integrated into the chip, are used to convert the 3 Volt CMOS supplies to the 20 plus volts needed to actuate the micro electro mechanical relay). Such a circuit is usually very small and low cost since high power and current is not required.
There are other options as a result of new low-cost MEMs packaging techniques. The new packaging enables the MEMS devices to be built on top of the low-cost silicon used to perform control logic and voltage generation functions.
sábado, 20 de marzo de 2010
Taking a Closer Look at RF-MEMS Switch Specifications
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 20:45
Etiquetas: 3 Emmanuel Rodriguez