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Wearable PCB Design requirements focus on basic Materials (3)

Edit:Xing DA Electric Technology Co.,LtdUpDate:Mar 12, 2018

                 Wearable PCB Design requirements focus on basic Materials (3)


        RF / microwave design considerations

        Portable technology and Bluetooth pave the way for RF / microwave applications in wearable devices. Today's frequency range is becoming more dynamic. A few years ago, VHF (VHF) is defined as 2GHz ~ 3GHz, but now we can see UHF UHF applications ranging from 10GHz to 25GHz.


       Therefore, for wearable PCB, the RF part needs to pay closer attention to the problem of wiring, separate the signal separately and keep the high-frequency signal away from the ground. Other considerations include the provision of bypass filters. Sufficient decoupling capacitance, earthing, the transmission line and loop design almost equal.


       Bypass filters can suppress the ripple effect of noise content and crosstalk. Decoupling capacitors need to be placed closer to device pins that carry power signals.


      High-speed transmission lines and signal loops require that a layer be arranged between the signals of the power layer to smooth the jitter produced by the noise signals. At higher signal speeds, small impedance mismatches can cause unbalanced transmission and reception signals. Therefore, we must pay special attention to the impedance matching problem related to RF signal, because RF signal has high speed and special tolerance.


      RF transmission lines require impedance control to transmit RF signals from a particular IC substrate to PCBs. These transmission lines can be implemented in the outer, top and bottom layers, or designed in the middle layer.


     The methods used during PCB RF layout design are microstrip lines, suspended strip lines, Coplanar waveguide or grounding. Microstrip lines consist of metal or tracklines of fixed length and the entire or partial ground plane located directly below. The characteristic impedance in a general microstrip line structure ranges from 50 Ω to 75 Ω.


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       The suspended stripline is another method for wiring and noise suppression. The line is composed of large inner wiring and the center conductor on the fixed width under the plane. The ground surface is clamped on a power supply layer, so it can provide a very effective grounding effect. The wearable PCB RF signal wiring it is a method selection.


        A coplanar waveguide provides better isolation between a radio frequency line and a line that needs to be close to the line. The medium consists of a section of a central conductor and a ground plane on either side or below. The best way to transmit radio frequency signals is by a suspension band. The two methods can provide better isolation between the signal and the RF line.


       It is recommended that a so-called "perforation fence" be used on both sides of the coplanar waveguide. This method provides a row of grounding holes on each metal ground plane of the central conductor. The main line running in the middle has a fence on each side. This method can reduce the dielectric constant of noise level .4.5 related to the high ripple effect of RF signal to the same as the FR4 material of the semi-solidified sheet, while the semi-solidified plate-from the microstrip line, can reduce the dielectric constant of the noise level .4.5 related to the high ripple effect of the RF signal. The dielectric constants of banded or offset banded lines range from 3.8 to 3.9.


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        In some devices using the ground plane, blind holes may be used to improve the decoupling performance of power capacitors and provide a shunt path from the device to the ground. This can achieve two purposes: you not only create shunt or ground, but also reduce the transmission distance of devices with small pieces of land, which is an important RF design factor.