Generalized Interference Treatment Outline

1. Generalized Interference Treatment Outline ANSI C63.19 Working Group Submitted by Stephen Julstrom for October 2, 2007

2. Method to predict the worst-case weighted level of in-use HA RFI (fast probe) • Measure the worst-case HA RF susceptibility (V/m, A/m) for a 55 dB-SPL IRIL according to the standard. (80% 1 kHz AM; RF field strength reference is the unmodulated carrier.) • For the WD, measure the worst-case output level from the weighted, square-law detected, fast probe. • In a follow-up far-field measurement, substitute an 80% 1kHz modulated carrier at approximately the same frequency as the WD carrier to the same fast probe. Adjust the level of the modulated carrier to produce the same measured level at the output of the square law detector (weighted or unweighted). (20% 2nd harmonic distortion from the square law detector will not materially affect the results.) • In the same far field environment, now remove the modulation from the carrier and replace the fast measurement probe with a calibrated probe and measure the field strength that the measurement probe just received. • With the measured unmodulated carrier strengths of step 1 and 4 presented in dB(V/m) or dB(A/m), calculate the actual worst-case weighted HA IRIL in response to the WD’s RF modulation. (This is what the M-rating summation process effectively does.) weighting x2 weighting x2 reference Worst-case in-use weighted IRIL (dB-SPL) = 2(step4 – step1) + 55 dB-SPL The order of steps 2-4 can be reversed to enable pre-calibration of the fast probe.

3. Method to predict the worst-case weighted level of in-use HA RFI (slow probe) • Measure the worst-case HA RF susceptibility (V/m, A/m) for a 55 dB-SPL IRIL according to the standard. (80% 1 kHz AM; RF field strength reference is the unmodulated carrier.) • For the WD, measure the worst-case output level from the slow probe. • In a first follow-up far-field measurement, illuminate the slow probe with the same WD modulation as was just measured. Adjust the level for the same probe output level as step 2. • In a second follow-up far-field measurement, apply the same WD modulation field strength to a fast probe. Measure the weighted, square law detected output of the probe. • Continue with steps 3-5 of the fast probe procedure. Slow probe Slow probe weighting x2 weighting x2 reference Worst-case in-use weighted IRIL (dB-SPL) = 2(step4 (of previous page) – step1) + 55 dB-SPL The order of steps 2-5 can be reversed to enable pre-calibration of the slow probe for an individual modulation protocol.

4. Effect of the Generalized Method on Presently Studied Modulation Protocols The signals used in the telecoil interference study were visually analyzed and computer-analyzed for average, burst average, and peak envelope power. The first table compares these results with the related results of Annex I. (All values in dB.) The modeled average, burst, and peak values were then compared to the modeled results from applying the new generalized method to the study signals. It is possible to present the outcome as equivalent new “AWF” ratings: The implied “AWF”s based on average power measurements show a rough correspondence with the originally rated C63.19-2007 AWFs for GSM, TDMA, CDMA, both relatively to each other and, coincidentally, in absolute numbers. But the standard presently calls for peak power measurement. Compared to that measurement reference, the new generalized method predicts 12.4 to 28.7 dB less hearing aid IRIL for the protocols studied than implied by the standard’s present measurement method. This allows 6.2 to 14.3 dB greater RF emissions for a given category rating than is presently allowed. (For GSM, 9 to 9.6 dB greater RF emissions, depending on the precise measurement of the peak value.)