Conformity Magazine - January 2009 - (Page 29) Adverse Effects on the System Performance Due to Collocation The problem occurs when one transceiver is in transmitting mode and a nearby transceiver is in receiving mode. For example, a transceiver transmits 1 W (30 dBm) of RF power near a receiver with sensitivity S, around -105 to -115 dBm. An isolation of more than 135 to 145 dB is necessary between the two devices to avoid interference, which is difficult to achieve. See Figure 1 for a typical scenario. The isolation between Tx and Rx signals can be provided by three parameters, as indicated by Friss transmission equation: 1) frequency separation, 2) Tx power management and 3) distance between two antennas. In most cases, Tx power and antenna separation are not a control choice, because the range is required and the actual space between the transceivers is defined. So the most important parameter to achieve isolation is frequency separation (or management) between transceivers. When a transceiver T1 transmits at F1, it generates an RF electromagnetic (EM) field Et at a distance r, which results in a power density Wt. For an isotropic antenna, Pt, the transmitted power, and Wt can be related as follows [1]: Et2/ η = Wt (1) characteristics, and high degree of misaligned polarization, which makes the receiver efficiency er low in the above relationship. From (3), received power can be written in the logarithmic form as follows (terms associated with receiver and transmitter, are grouped together, distances are in meters): Pr = -22 - 20 log r + Pt + 10 log (et) + 20 log λ1 + 10 log (er) (4) The signal power Pr in dBW, at undesired frequency F1 received by T2 antenna, passes on to the receiver front end, which may cause LNA compression and result in blocking. Further down, when amplified received currents meet nonlinearity in the circuit, they can generate inter-modulation products (a large collection of sum and difference of the two or more frequencies and their harmonics). Some of these products may fall within the pass band of the receive filters, and result in strong interference. To estimate the received interference power levels due to collocated transceivers, we consider two examples, one with non-identical transceivers, and other with identical transceivers. T1 and T2 are non-identical collocated transceivers operating at F1 and F2. Assume that both antennas have good band pass characteristics at their where η is the free space impedance, and Wt is power density W/m2; and Wt = et Pt / 4 π r2 where et is total antenna efficiency, and Pt is power at T1 antenna input. Notice that, in this relation, only the antenna efficiency et is frequency dependant, which includes efficiencies due to resistive loss, reflection, radiation and other factors. However, the receiving end transceiver T2, r-meters away, receives a fraction of this power Pr, which depends upon the effective area of T2 antenna at the unwanted frequency “F1 or wavelength λ1,” the second term in the brackets: Pr = Wt (antenna effective area, isotropic) Thus: Pr = (et Pt / 4 π r2) (er λ12 /4 π) = et er Pt λ12 / (4 π r)2 (3) (2) Comprehensive EMI Solutions, On Time & On Budget From design, quick-turn protos, and worldwide ISO9001 manufacturing, we are the superlative EMI solutions provider. The cornerstones of our mission are simple: Quality. Integrity. Service. Value. It is this commitment to total solutions without compromise that has led to our established position as a premier designer, manufacturer & distributor of EMI filters and magnetic products. This is a basic form of the Friss equation, showing parameters of each antenna separately, and the efficiency terms, et and er, are key factors in this discussion [1]. The magnitude of the received power Pr at T2 front-end at F1(un-intended) entirely depends upon er of the T2 antenna at F1 = 1/λ1, which should be very low for good isolation. Therefore, for good isolation at undesired frequencies, the receiver antenna should have high reflection coefficients (or VSWR), poor radiation Plug into our comprehensive solutions. Radius Power is a leader in the design & manufacture of power products used to condition, regulate and govern electronic performance. The company provides a broad line of EMI Filters, Power Entry Modules, Power Transformers & Inductors to meet front-end requirements for AC & DC power line applications. We offer free pre-compliance testing for conducted emissions at our labs or yours! Bring us your EMI challenge & we will provide an optimized solution usually within 48 hours. Call Radius today. 714-289-0055 PH 714-289-2149 FAX 1751 N. BATAVIA ST. ORANGE, CA 92865 www.radiuspower.com JAnUAry 2009 Conformity 29 http://www.radiuspower.com http://www.radiuspower.com
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