Conformity Magazine - January 2009 - (Page 30) respective frequencies. Thus, term er for T2 is low when receiving T1 signal at F1. Beyond the antennas, the front-end filters will also help in isolation due to frequency difference. But, even with such band reject characteristics, achieving sufficient isolation (more than 100 dB) at short distance is not easy. Example 1 A GSM cellular modem (1900 MHz) and a Bluetooth (BT) transceiver (2450 MHz) are mounted on the same PCB, working in their respective bands. Assume the distance between the antennas is 10 cm. Also assume GSM sensitivity -105 dBm and Tx power 32 dBm, and the BT sensitivity -85 dBm and Tx power 15dBm. Also assume both transceivers have typical isotropic antennas with et = 0.9, and er = 0.05 (only 5 % efficiency at undesired frequency). All numeric values are arbitrary, but close. (In this article, er is defined at undesired frequency, therefore, low.) Let us consider two cases, T1 transmits T2 receives, and viceversa. Case 1: — GSM in Tx and BT in Rx and distance r = 10 cm — Let power transmitted by GSM at 1900 MHz be Pt = 32 dBm, and antenna eff. et = 0.9 — Let Pr(BT) be power received at BT front-end, and ant. eff. er = 0.05 at 1900 MHz, or (λ1= 15.7 cm) The equation (4) can be written as: Pr(BT) = -22 - 20 log (0.1) + 32 + 10 log (0.9) + 20 log (0.1570) + 10 log (0.05) = 1.14 dBm The Pr(BT) received power at the BT front end is more than 1.14 dBm, which is high. The maximum acceptable out of band blocking interference is -10 dBm, according to Bluetooth Core Specs sec. 4.1.3 [13] in 30 MHz to 2000 MHz band. The BT transceiver has some built in immunity to interference due to frequency hopping architecture, but such large signal can cause blocking of front end and degradation of performance. Case 2: — BT in Tx and GSM in Rx and distance r = 10 cm — Let power transmitted by BT at 2450 MHz be Pt = 15 dBm, and antenna eff. et = 0.9 — Let Pr(GSM) be power received at GSM front-end, and ant. eff. er = 0.05 at 2450MHz, or (λ1= 12.25 cm) The equation (4) can be written as: Pr(GSM) = -22 - 20 log (0.1) + 15 + 10 log (0.9) + 20 log (0.1225) + 10 log (0.05) This results in Pr (GSM) = -16.5 dBm signal, a large spurious emission only 500 MHz away from 1900 MHz. It can cause blocking and inter-modulation products at the GSM front end, and degrade GSM reception. The limit for GSM spurious emissions is -36 dBm. Next, we consider two identical collocated transceivers T1 and T2, with identical antennas and front-end (filter and LNA etc.). Therefore, terms et and er for antennas are the same, and related to the antenna gain (directivity), reflection coefficients, etc., and can be close to unity, for well-tuned antennas. The Pr at receiving transceiver front-end can be relatively large. Example 2 Now let us consider two identical class-1 BT transceivers transmitting 15 dBm, 10-cm apart. Since both radios operate asynchronously, one of the BTs may be receiving a remote weak signal while the other begins to transmit. As both antennas are similar, we have er = et = 0.9. Therefore, equation for power received at the front end of one while the other transmits is: Pr(BT) = -22 - 20 log (0.1) + 15 + 10 log (0.9) + 20 log (0.1225) + 10 log (0.9) = -5.65 dBm This is a strong in-band interference signal. Since the BT specification allows only -10 to -27 dBm, out of band interference signal [13], above interference power passing through antenna to the front end is quite large, which may cause excessive noise and data loss, and even blocking of the front end. Collocation in the North American Regulatory Sense Radiated Emissions and SAR The RF power received at any point in space is result of power transmitted by all RF transmitters surrounding it. The transceivers may interfere with each other’s transmissions, which is not a concern for the regulatory bodies. The regulatory bodies, in particular the FCC, are concerned with public safety, that is, the effects on the human body due to RF radiation. Figure 1: Two collocated transceiver, for example, a cellular phone transmitting 32 dBm and a Bluetooth transceiver receiving at sensitivity level of approximately -85 dBm. 30 Conformity JAnUAry 2009 In North America, the FCC and Industry Canada specify the power and spurious limits for a single transmitter. When
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