With the increase of mobile phone functions on the PCB board design requirements have increasingly high, with the round of Bluetooth devices, cellular phones and 3G era, allows engineers are increasingly concerned about RF circuit design techniques. Radio frequency (RF) circuit board design due in theory, there are many uncertainties, it is often described as a "black art", but this view is only partially correct, RF board design, there are many guidelines that can be followed and should not be neglected rule. However, in the actual design, the real trick is useful when these guidelines and rules for various design constraints and can not accurately implement a compromise on how to deal with them. Of course, there are many important issues worth discussing RF design, including impedance and impedance matching, insulation material and laminated board and wavelength and standing waves, so these on the phone EMC, EMI have great influence, following on the phone PCB board to summarize the conditions that must be met in the design of RF layout when:
1. The high power RF amplifier (HPA) and low noise amplifier (LNA) isolated as much as possible open, simply put, is to make high-power RF transmitter circuit away from the low-power RF receiver circuit. More phone features, many components, but smaller PCB space, taking into account the layout of the design process maximum limit, all of which requires a number of design skills is relatively high. This time may need four to six PCB design, and let them work alternately, not simultaneously. High-power RF circuit may also include a buffer and a voltage controlled oscillator (VCO). PCB board to ensure that there is at least one high-power block area, there is no vias at best, of course, copper better. Sensitive analog signals should be as far away from the high-speed digital and RF signals.
2. Design partitions can be decomposed into physical partitions and electrical partition. The main components involved in the physical partition layout, orientation and shielding problems; electrical partition can continue to be broken down into power distribution, RF traces of sensitive circuits and signals as well as grounding partition.
2.1. We discuss the physical partition problems. Component layout is key to achieving a good RF design, the most effective technique is to first fix the components located on the RF path and adjust its orientation to minimize the length of the RF path, away from the output enable input, and as far as possible separation circuit and a high-power low-power circuit.
The most effective method is to stack the board main ground (main ground) arranged in the subsurface of the second layer and the RF line to walk as much as possible on the surface. The RF path through the pore size can not only reduce the path to minimize the inductance, but also can reduce the main floor of the Weld Point, RF energy leakage and reduce opportunities in other areas within the laminated panels. In the physical space, such as a linear multi-stage amplifier circuit is usually sufficient RF isolation between a plurality of open area, but the duplexer, mixer and IF amplifier / mixer is always a plurality of RF / IF signals interfere with each other, so care must be taken to minimize this effect.
2.2. RF and IF traces should go as far as possible the cross, and they are spaced as far as possible in the piece of land. RF path on the performance of the correct piece of PCB board is very important, which is the reason why the components layout is usually accounted for most of the time in the mobile phone PCB board design. In the mobile phone PCB board design, can usually be placed in a low-noise amplifier circuit side of the PCB, and the high-power amplifier on the other side, and finally put them through a diplexer connected to the RF and baseband end on the same side side of the antenna. Need some tips to make sure the hole is not straight through the RF energy from the plate side of the pass to the other side, a common technique is to use both sides of the blind hole. Can be directly arranged on both sides of the PCB area from RF interference by the holes straight through the hole to minimize the adverse effects. Sometimes not possible to ensure a sufficient plurality of circuit blocks between the isolation, in this case it is necessary to consider the metal shield to shield the RF energy in the RF area to be welded to the metal shield to the ground, and components must be maintained an appropriate distance, and therefore need to take up valuable PCB board space. Below the shield as much as possible to ensure the integrity is very important to enter the metal shield of digital signal lines should go inside as much as possible, and the best alignment layer is a layer PCB ground. RF signal line can shield a small gap from the bottom of the metal wiring layer and go on to notch the go, but to be more cloth around at some of the gap to the extent possible, to the different layers can be connected together by a plurality of vias .
2.3. Appropriate and effective chip power supply decoupling is also very important. Many linear integrated circuit chip RF noise on the power supply is very sensitive to each chip usually requires the use of up to four capacitors and inductors to ensure an isolated power supply filter out all the noise. An integrated circuit or amplifier often with an open-drain output, and therefore requires a pullup inductor to provide a high RF impedance load and a low impedance DC power supply, the same principle applies to this side of the power supply inductance decoupling. Some chip requires more power to work, so you may take two to three capacitors and inductors to them are decoupled, the parallel inductor little close together, as this will form a air-core transformer and the mutual induction interference signal, the distance between which corresponds to the height of at least one device, or arranged at right angles to minimize their mutual inductance.
