Conformity Magazine - January 2009 - (Page 33) Since immunity testing is a major difference between the regulatory philosophies found in North America and the EU, we will look at it in greater detail in the section to follow. Radiated Immunity and its Relation with Collocation Interference The immunity of a system is its ability to function correctly in adverse electrical interference, conducted and radiated, required for radio equipment according to ETSI EN 301 489-1 “Common technical requirements,” sec. 7.2., as part of general EMC testing. The RF radiated immunity is a test of the system ability to withstand RF interferences, somewhat similar to collocation interference. The radiated immunity is tested according to IEC 61000-4-3 [12], which assesses tolerance to EM fields from 1 V/m to 30 V/m (1, 3, 10, 30), with V/m chosen according to declared performance level at frequencies from 80 MHz to 2000 MHz, when the EUT is 3-m away from the disturbance (typically). The standards also provide the exclusion bands, those frequencies within which the radiated immunity is not tested. The exclusion bands (ETSI EN 301 489-1, sec 4.3) extend several percentage points above and below the frequencies band(s) allocated for a particular radio technology. Therefore, in-band collocation interference is essentially not tested. Moreover, the basic idea of testing the RF immunity is not to test the amount of isolation from collocated interference, but rather to actually evaluate protection of systems from mobile phone and other intentional radiators. To get a feel for the magnitude of the received power Pr at the front-end of the equipment under test (EUT) due to the interfering EM field, Et (1, 3, 10 or 30 V/m), we’ve worked out an example below. Example 3 Let the field strength at the EUT (say a GPS receiver) be Et -V/m. It may vary up to +6 dB, depending upon interfering frequencies. We consider the effects on the receiver at the GPS frequency, ~1575 MHz (λ1 =19 cm), and the disturbance a few MHz away from it. We can use the equation (3) to calculate Pr at the GPS front-end, in dBW. The second term in brackets is the GPS antenna effective area, where the value of er depends upon interfering frequency and may vary significantly (here, e.g., it is 0.05): Pr = Wt. (er λ12 /4 π) = ( Et2/ η) (er λ12 /4 π) where Et is the interfering field in V/m; or Pr = 20 log Et -10 log (η) + 10 log(er) +20 log (λ1) -10 log (4 π) in dBW For the values of Et (1, 3, 10 and 30 V/m), the above equation yields Pr (-34, -25, -14.2 and -4.6 dBm). These are quite strong signals at the front end, and the receiver’s capability to reject this depend upon its front end filters. Note that GPS receivers have sensitivities better than -115 dBm, and that GPS modules often use active antennas with gains of 25 to 30 dB. Multiple Transceivers and Compliance Requirements When more than one transceiver is combined in equipment, which can transmit simultaneously, the emissions process becomes non-linear as recognized by ETSI TR 102 070-2 sec. 4.1. As discussed earlier, this is because of inter-modulation effects between two or more transmitters and other oscillators within the circuit, and the interaction of signals with circuit non-linearities. Therefore, total emission from combined equipment cannot be considered as sum of individual emissions of the sub-equipment when working alone. But in the sec. 6.7 of this part for multiple radio equipment, it is not clear if the above theory is applied because it allows separate assessment of the parts within the combined equipment (sec. 6.7.1), and further testing of the combined equipment may not be required. A simple example may explain the test requirements. Example 4 A product M, sufficiently smaller than 20 cm in maximum dimension, contains a GSM transceiver module, a Bluetooth transceiver module, and a GPS radio (receiver), all controlled by a common hardware and receiving their power from a properly designed common power source. What EU standards may be used to demonstrate compliance? There can be at least two scenarios: 1) each transceiver has been previously tested to its relevant standards according to R&TTE Directive; and 2) no-transceiver has been tested before. Figure 2: The FCC addresses only high power transmitter collocated installations for the reasons of RF radiation effects on the public safety. (SAR limits at near ground level). JAnUAry 2009 Conformity 33
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