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Design-in of RF circuits

NRFâ "¢ loạt radio chip duy nhất là một cách dễ dàng để có được vào các thị trường mở rộng của truyền thông không dây tầm ngắn. Một bảng mạch in (PCB) với một trong những con chip này cùng với một bộ điều khiển vi phù hợp và một ăng-ten cơ bản là tất cả những gì cần thiết để xây dựng một liên kết truyền thông không dây. | WHITE PAPER Nordic VLSI ASA Design-in of RF circuits 1. Preface The nRF series of single chip radios is an easy way to get into the expanding market of short-range wireless communication. A printed circuit board PCB with one of these chips together with a suitable micro controller and an antenna is essentially all that is needed to build up a wireless communication link. Several application notes are available to help shorten the design time as much as possible. There are however some guidelines that need to be followed to end up with an optimal solution. In this document we give a brief look into common problems that may occur. Special focus is set on the problems related to integrating digital circuitry and a RF chip on the same board followed by some advice to solve these problems. 2. Why are digital and analog circuits potential enemies Analog radio and digital micro controller circuitry might work well for themselves but once put on the same circuit board and the same power supply things might get a little more complicated. Digital signal lines most often swing between ground and positive power supply which for most applications means a peak to peak amplitude of 3V. Switching time of digital signals is fast normally in the nanosecond range. Due to the large amplitude and fast switching time the signal will contain a considerable amount of high frequency components independent of the switching frequency. On the analog side the input signal from the antenna to a radio receiver can be less than 1uVrms. The magnitude difference between the digital and RF signal can therefore be as large as 1 million 120 dB . It is evident that if these signals are not separated or shielded properly the weak RF signal may be corrupted with radio performance deterioration or even malfunction as a result. 3. Common errors combining RF and digital on one PCB. 3.1. Insufficient separation between sensitive and noisy signal lines. As mentioned above digital signals have high amplitudes