Practical Industrial Data Networks:Design, Installation and ...

Radio channel data rateRadio and wireless communications overview 287Data rate across the radio channel defined by the radio and bandwidth capabilities. Notethat these specifications are generally set at the time of manufacture.Minimum radio frequency signal levelShould not be set too low on the receiver otherwise noise data will also be read.Supervisory data channel rateUsed for flow control and therefore should not be set too low otherwise the buffer on thereceiver will overflow. Typically one flow control bit to 32 bits of serial data is thestandard.Transmitter key up delayThe time for the transmitter to energize and stabilize before useful data is sent over theradio link. Transmitter key up delay should be kept as low as possible to minimizeoverheads.17.5.3 Spread spectrum radio modemsSeveral countries around the world have allocated a section of bandwidth for use withspread spectrum radio modems. In Australia and America, this is in the 900 MHz area.In brief, a very wide band channel is allocated to the modem, for example,approximately 3.6 MHz wide. The transmitter uses a pseudo random code to placeindividual bits, or groups of bits, broadly across the bandwidth and the receiver uses thesame random code to receive them. Because they are random, a number of transceiverscan operate on the same channel and a collision of bits will be received as noise by areceiver in close proximity.The advantage of ‘spread spectrum’ radio modems is very high data security and dataspeeds of up to 19.2 kbps. The disadvantage is the very inefficient use of the radiospectrum.17.6 Intermodulation and how to prevent it17.6.1 IntroductionBesides noise and interference that emanates from man-made sources (cars, electricalmotors, switches, rectifiers etc) there are three other main causes of RF interference.These are produced from other radio equipment. The first and the most obvious source, isanother radio user operating close by on the same frequency as the system suffering frominterference. Unfortunately, besides using special coding techniques to minimize theproblem there is little that can be done, short of complaining to the regulatory governmentbody that issues licenses, or finding out who it is and asking them to stop transmissions.The second source of interference comes from noisy transmitters that emit spuriousfrequencies outside their allocated bandwidth. These spurious emissions will tend to fallon other users’ channel bandwidths and cause interference problems. Aging transmittersand those that are not well maintained are normally the culprits.The third source of interference is known as intermodulation. This is normally the mostcommon source of interference and generally the most difficult to locate and the mostcostly to eliminate. The following section will examine this phenomenon in more detail.