EO_482

66 - ELETTRONICA OGGI 482 - NOVEMBRE/DICEMBRE 2019 COMM SHORT RANGE COMMUNICATION Tabella 4– Compromessi nella scelta dei parametric di un sistema radio short range Priority Band Modulation Reasonig Tradeoffs Range lower, mid ASK Assuming a large antenna, lower frequencies allow for better RX sensitivi- ty. ASK commonly has better RX sensitivity than FSk. Midband regulation allows for more radiated TX power Cost battery life, size, simplicity, DR, IR Cost lower ASK Small and simple circuits. ASK is a preferred modulation for a simple TX. ASK RX chips tend to require the fewest peripheral components Range, battery life, DR, IR, tolerance Battery life Lower ASK Lower current drain at lower operating frequencies for both the TX and RX provide longer life from a limited source. ASK only requires a duty cicle % versus constant transmissions for FSK Range, cost, LOS, simplicity, DR, IR Size Mid - If size includes the antenna, then the 868 MHz/915 MHz bands are the best target because small antennas can be used with reasonable aperture sizes and electrical lengths. If there is no restriction on the antenna, then refer to the “Cost” priority Range, LOS Line-of-sight (LOS)/ obstacles Lower FSK Lower frequencies penetrate obstacles, bend around objects more easily, and suffer less absorption than higher frequencies. FSK is less influenced by multipath and possible amplitude changes caused by motion (TPMS example) Battery life, size Simplicity Lower ASK ASK is an easier and more tolerant modulation scheme to handle. Larger wavelengths (lower frequencies) are less influenced by board and com- ponent sizes Range, battery life, DR , IR, tolerance scelta dei parametri di sistema più idonei. Oggigiorno da molti costruttori viene offerta la disponibilità di una grande varietà di trasmettitori, ricevitori e transceiver in single-chip, oltre a tutta la necessaria circuiteria, in genere sono necessari solo pochi componenti esterni per realizzare un completo sistema radio. Inoltre, i co- struttori mettono a disposizione dei progettisti molta documentazione tecnica, come datasheet, tutorial e application note. Ad esempio, la realizzazione di uno schema elettrico di un trasmettitore prevede, oltre al trasmettitore, solamente un microcontrollore o una semplice interfaccia di codifica, un’antenna e una bat- teria. Per il ricevitore devono essere previsti circui- ti di sintonia che dovranno essere configurati per la frequenza operativa e data rate previsti nel progetto, oltre al microcontrollore o interfaccia di decodifica e sistema di alimentazione. Una volta definito lo schema elettrico occorre tenere presente che gran parte dei problemi che potrebbero presentarsi nella fase di test di sistema del progetto potrebbero essere causati da una cattiva progettazione del layout del circuito stam- pato. Prestando la massima cura nella progettazione del circuito stampato si eviterà perdita di tempo nei test e nelle fasi di debug del sistema. Priority Band Modulation Reasonig Tradeoffs Data rare (DR) Higher FSK, PSK spread spectrum Higher data rates will require wider bandwidths for operation and the regulatory requirements are easier in the higher bands. High data rate, sperad spectrum, and the high bands all require more operating current. Smaller aperture and wider baqndwidth negatively impacts the range. Range, cost, battery life, simplicity Interference rejection (IR) Mid, Higher Spread spectrum Spread-spectrum modulation rejects carriers and other interference very well. The wider bandwidths needed for operation are available in the hi- gher bands Range, cost, battery life, simplicity Frequency tolerance Lower _ More important at higher bands. Narrower IF filters will provide better sensitivity and longer range. Absolute frequency accuracy is easier to obtain at lower bands. TCXOs are more expensive than standard crystals Cost, simplicity Size Mid – If size includeds the antenna, then the 868 MHz/915 MHz bands are the best target because small antennas can be used with reasonable aperture sizes and electrical lengths. If there is no restriction on the antenna, then refer to the “Cost” priority Range, LOS Line-of-sight (LOS)/obsta- cles Lower FSK Lower frequencies penetrate obstacles, bend around objects more easily, and suffer less absorption than higher frequencies. FSK is less influenced by multipath and possible amplitude changes caused by motion (TPMS example) Battery life, size Simplicity Lower ASK ASK is an easier and more tolerant modulation scheme to handle. Larger wavelengths (lower frequencies) are less influenced by board and com- ponet sizes Range, battery life, DR, IR, tolerance