RF Switch can efficiently transmit signals in the transmission path. The functions of such switches can be characterized by four basic electrical parameters.
Although multiple parameters are related to the performance of RF Switch, the following four are considered critical parameters due to their strong correlation: isolation, insertion loss, switching time, and power handling capability.
The degree of isolation between the circuit input and the output is the measure of the effectiveness of the cut-off. Insertion loss (also called transmission loss) is the total power dissipation when the switch is on. Since the insertion loss can directly lead to an increase in the system noise figure, the insertion loss is the most critical parameter for the designer.
The switching time is the time required for the switch to transition from the "on" state to the "off" state and from the "off" state to the "on" state. The time up to the number of high-power switch microsecond, down to the low-power high-speed switch several nanoseconds. The most common definition of switching time is the time required for the input control voltage to reach 50% of its final RF output power to 90%. In addition, the power handling capability is defined as the maximum RF input power that the switch can withstand without any permanent electrical performance degradation.
(1) power cord is an important way to EMI access circuit. Through the power line, external interference can be passed into the internal circuit, affecting the RF circuit indicators. In order to reduce the electromagnetic radiation and coupling, the DC-DC module requires the primary side, the secondary side,RF Switch the load side loop area is the smallest. Power circuit no matter how complex the form, the high current loop should be as small as possible. Power lines and ground are always very close to the place.
(2) If the circuit uses a switching power supply, switching power supply peripheral device layout to meet the principle of the shortest flow path of the power. The filter capacitor is close to the switching power supply pin. Use a common mode inductor, close to the switching power supply module.
(3) The long-distance power cord on the board can not be accessed at the same time or near the output of the cascade amplifier (gain greater than 45dB) and near the input. To avoid the power line to become RF signal transmission path, may cause self-excitation or reduce the sector isolation. Long-distance power cord at both ends need to add high-frequency filter capacitor, and even the middle also add high-frequency filter capacitor.
(4) RF PCB power input at the combination of three parallel filter capacitor, the use of the three different advantages of the capacitor were filtered out of the power line, low, medium and high frequency. For example: 10uf, 0.1uf, 100pf. And in accordance with the order from large to small in turn close to the power input pin.
(5) with the same set of power supply to the small signal cascade amplifier, should start from the last level, followed by power supply to the former level,RF Switch so that the end of the circuit EMI generated on the impact of the smaller. And each level of power filter at least two capacitors: 0.1uf, 100pf. When the signal frequency is higher than 1GHz, to increase the 10pf filter capacitor.
(6) commonly used to low-power electronic filter, filter capacitor close to the transistor pin, high-frequency filter capacitor closer to the pin. Transistor selection cutoff frequency is low. If the transistor in the electronic filter is a high-frequency tube, work in the amplification area, the external device layout is unreasonable, in the power output is very easy to produce high-frequency oscillation. The linear regulator module may also have the same problem because there is a feedback loop in the chip and the internal transistor operates in the amplification zone. In the layout requirements of high-frequency filter capacitor near the pin, reducing the distribution of inductance, damage to oscillation conditions.
(7) The copper part of the POWER part of the PCB conforms to the maximum current flowing through it and takes into account the margin (generally referred to as 1A / mm line width).
(8) The input and output of the power cord can not cross.
(9) Note that the power decoupling, filtering, to prevent different units through the power line interference, power supply wiring between the power lines should be isolated from each other. The power cord is isolated from other strong interference lines (eg CLK).
(10) small signal amplifier power wiring needs to copper and ground isolation, to avoid other EMI interference fleeing, thereby deteriorating the level of signal quality.
(11) different power layers in space to avoid overlap. Mainly in order to reduce the interference between different power supply, especially between the voltage difference between a large number of power supply, power plane overlap problem must try to avoid, it is difficult to avoid when considering the interval formation.
(12) PCB board distribution to facilitate the subsequent follow-up wiring processing, for a four-layer PCB board (WLAN commonly used circuit board),RF Switch in most applications with the top of the circuit board components and RF lead, the second layer as Systematically, the power supply part is placed on the third layer, any signal line can be distributed in the fourth layer.