Conformity Magazine - January 2009 - (Page 31) more than one transmitter is part of a single device which can transmit simultaneously, there is no formal standard in place. Moreover, in this situation, the radiated emissions cannot be calculated easily, based upon power rating of individual modules. Spectrum Due to Collocation It is understood that collocated transmissions are a non-linear activity, and can produce additional spurious emissions which add to the total RF pollution. When transmitters operate at different frequencies, each transmission has its effects (field strength, power density) independent of the other in the open space (linear), and each frequency will appear separately in the spectrum. Due to close proximity, strong signals form one transmitter pass into another and cause inter-modulation products at the circuit non linearities, which appear in addition to the original transmitted frequencies and their spurious emissions. Peak power at any frequency will not change; however, the spectrum may include more EM field terms. For example E1 + E1spur , E2 + E2spur, E3 +E3spur etc. are the normal EM fields due to T1 and T2, T3, etc. If they are sufficiently far apart, the total power density at some distance away from them will be: Pd = E12/ η + E12spur / η + E22 / η + E22spur/ η ….. and so on However, if T1 and T2 are collocated, additional spurious emissions due to inter-modulation may be generated, thus: Pd (total) = E12/ η + E12spur / η + E22 / η + E22spur/ η …..+ E2Inter-1/ η + E2Inter-2/ η + The E2Inter-i are several inter-modulations products which add up to the total power density spectrum. Thus, the Pd (total) is the sum of all Pds due to each EM field component, including the EM field due to inter-modulation products. North American Regulations on Equipment with Multiple Transceivers In North America, neither the FCC nor Industry Canada directly address the issue of collocated multiple transceivers in any of their applicable standards or related publications. Only a brief reference is found in Industry Canada’s RSS102 [7] about multiple transceivers regarding SAR (specific absorption rate). For the FCC, the definition of collation is also implied in one obscure place. A small note appears in the corner of the FCC grant certificate which reads as follows: “This module may only be installed by the OEM or an OEM integrator. End-user and installers must be provided with antenna installation and transmitter operating conditions for satisfying RF exposure compliance. This transmitter must not be collocated or operated in conjunction within 20 cm of any other antenna or transmitter.” The FCC definition of the collocation is implicit in this note, i.e., if antennas of two transmitters are within 20 cm of each other, then they are considered collocated. SAR and the 5% Rule It will shortly become clear what FCC means by “collocation.” The concern does not appear to be about the adverse effect on equipment functionality, but rather about public RF exposure and safety. No other FCC document refers to the above note or endorses it. For the designers of the equipment it is easy to overlook this, and to send equipment to market with multiple transmitters without further approval. However, the FCC allows collocation of transmitters after some further testing at a test lab and approval. The FCC approval follows a permissive change process. A general guidance for such changes is given in FCC-2-J sec. 932 and sec. 933 [3]. The permissive change (PC, as it is commonly known) is a general procedure which may be applied to multiple radios cases. In the PC process, a new FCC ID is allocated to the new product and the old FCC ID is removed or superseded. Industry Canada, on the other hand, allows collocation as long as SAR limits are not violated. Industry Canada specifies such SAR related rules in the RSS-102, sec 2.5.1 and 3.2 [7] for low power devices. To verify compliance with FCC rules, the power density (Pd) for each device is estimated by measuring spurious emissions and the conducted power output, and adding them to get the total Pd verified according to the SAR limits as specified in the FCC 1.1310 table 1B for low power devices. The Pd is estimated by following relation [1]: Pd = P G / 4 π r2 where P is the conducted power, G antenna gain, and r is the distance from antenna. For large transmitter sites, where many transceivers are collocated, the Pd at some human accessible locations is measured and verified according to the limits. The value of distance-r from transmitter, for SAR evaluation, is not clearly defined by the FCC or by Industry Canada. Only typical exposure limits are given in FCC 1.1310 table 1B. However, the distance for small collocated transmitters can be taken as 3-meters. The SAR rules are meant for exposure due to transmitters with large RF power at human accessible locations. JAnUAry 2009 Conformity 31
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