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Generalized Method for the Determination of Wireless Device RF Interference Level

Generalized Method for the Determination of Wireless Device RF Interference Level. ANSI C63.19 Working Group Submitted for discussion by Stephen Julstrom January 19, 2008. PINS items being addressed:

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Generalized Method for the Determination of Wireless Device RF Interference Level

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  1. Generalized Method for the Determination of Wireless Device RF Interference Level ANSI C63.19 Working Group Submitted for discussion by Stephen Julstrom January 19, 2008

  2. PINS items being addressed: 2. To provide AWF factors for systems operating in the 698 MHz to 6 GHz frequency range. If possible, develop a generalized treatment based on A-weighting or some other appropriate weighting to replace the AWF table. 7. Determine and specify the power measurement that is most closely linked to user experience, peak, RMS, or other parameter of power Goal of the generalized method: These issues can be resolved through the specification and measurement of a WD’s RF Interference Level. The derivation of this quantity will be justified and its definition given. • The method recognizes that there is no way to predict the acceptability of a given level of detected audio frequency interference from a given modulation characteristic without actually detecting and examining the interference. • Any such method will, at some point, need to incorporate a fast probe that can respond to audio frequency modulation, although an alternate procedure will be included wherein the actual WD measurements can be made with a slow probe. • The fast probe needs to be paired with a square-law detector in order to simulate the detection mechanisms in hearing aids. Measurements on hearing aid microphones (internal FET impedance converter), amplified telecoils (bipolar transistor microcircuit), and individual bipolar transistors have confirmed their square law detection characteristics. The square law assumption is built into the standard, as is appropriate.

  3. Review:The goal of the M-rating procedure To predict the worst-case level of in-use HA RFI (specified as IRIL: Input-Referred Interference Level) for a given combination of WD and HA. From the resultant assumed signal-to-noise ratio, an acceptability rating can be given for the combination. Implied M-rating summation calculation: IRIL (dB-SPL) = (2 x emission – AWF) – (2 x susceptibility) + 55 dB-SPL Where: emission = worst-case near-field WD emission in dB(V/m) or dB(A/m) susceptibility = level of dipole or GTEM field in dB(V/m) or dB(A/m) that results in 55 dB-SPL IRIL from hearing aid AWF = “Articulation Weighting Factor” in dB – to compensate for the varying subjective effects of different modulation protocols For example, for a WD emission of 38 dB(V/m) (high-band M3 rating) and HA susceptibility of also 38 dB(V/m) (M2 rating) and AWF = 0 dB, the predicted IRIL is 55 dB-SPL. With an assumed 80 dB-SPL speech level, the predicted S/N is 25 dB. The combined M5 rating predicts “normal use”.

  4. Measured susceptibility is the level of the unmodulated carrier that, when 80% modulated by a 1 kHz sine wave, results in 55 dB-SPL IRIL from the hearing aid. carrier level • Measured emission is presently the peak level of the WD modulated RF waveform, within a 20 kHz detection bandwidth. peak burst average bandwidth? average • Unresolved difficulties: • There is no established correlation between these two measurement methods. In fact, there is no consistent relationship across differing protocols between the strength and character of the detected audio and any direct measurement of the RF waveform. • There is no methodology described for determining the subjective effect of the demodulated WD audio for various protocols (AWF determination or equivalent). (Possible questions concerning the hearing aid’s response to a dipole or a GTEM cell vs. a WD near field, worst-case vs. typical, etc. will not be addressed here.)

  5. Essential WD RF Interference Level measurement requirements: • The measurement of WD emission must, at some point, involve a fast RF probe with a full audio response and square-law detection. • The detected audio must be subjectively weighted to predict its acceptability, which relates to its audibility and annoyance potential. weighting level measurement x2 fast probe (>10kHz) square-law detector spectral and temporal weighting (modified A-weighting) • Finally, the level of weighted recovered audio must be correlated to the HA susceptibility measurement method so that the IRIL for a WD/HA combination can be predicted. The weighting is described in the companion PowerPoint “New Subjective Weighting Function”. It is derived primarily from the results of the earlier telecoil coupling study, which included eight widely varying noise types. Seven of these correspond closely to expected RFI noise types. It is proposed that this weighting function additionally be substituted for A-weighting in the measurement of ABM2 Audio Band Magnetic Signal, Undesired.

  6. The definition of RF Interference Level: For a modulated RF signal that produces a given level measurement from the weighted output of a square-law detector, the rms level of a CW signal of a similar carrier frequency that, when amplitude-modulated to 80% by a 1 kHz sine wave, produces the same output level from the square-law detector. This is the relationship that must be established so that the implied M-rating calculation validly predicts the HA IRIL, and thus the S/N and user acceptability. Worst-case in-use weighted IRIL (dB-SPL) = 2(RF Interference Level – Susceptibility) + 55 dB-SPL RF Interference Level and Susceptibility both in dB(V/m) or dB(A/m). Note that there is no explicit AWF term included. Rather, the subjective weighting is part of the RF Interference Level measurement.

  7. RF Interference Level measurement (fast probe) and IRIL calculation: • 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, apply 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 rms field strength that the measurement probe just received. This is the RF Interference Level. • 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. weighting x2 weighting x2 reference Worst-case in-use weighted IRIL (dB-SPL) = 2(RF Interference Level – Susceptibility) + 55 dB-SPL The order of steps 2-4 can be reversed to enable pre-calibration of the fast probe.

  8. RF Interference Level measurement (slow probe) and IRIL calculation: • 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. (Step 4 of the fast probe procedure gives the RF Interference Level.) Slow probe Slow probe weighting x2 weighting x2 reference Worst-case in-use weighted IRIL (dB-SPL) = 2(RF Interference Level – Susceptibility) + 55 dB-SPL The order of steps 2-5 can be reversed to enable pre-calibration of the slow probe for an individual modulation characteristic.

  9. Effect of the generalized method on the studied modulation protocols: Using the available modeled test signals, results according to the new generalized method were compared to the results obtained according to the present standard’s test method. Although the AWF concept is not used in the new generalized method, it is possible to present the outcome as equivalent new “AWF” ratings for each test signal, for comparison to the present standard’s results. The standard’s WD emission measurement is presently of the waveform’s 20 kHz band-limited peak power, but comparisons based on average and burst average measurements are also given for reference. Compared to results based on the standard’s present peak power measurement, the new generalized method predicts 12.4 to 26.9 dB less hearing aid IRIL for the protocols studied, and a corresponding 6.2 to 13.4 dB increased allowance in WD RF emissions for a given category rating. These comparisons do not take into account the recently adopted 10 dB extra low band RF allowance, but could be considered to roughly justify it for both bands (relative to peak power RF measurements).

  10. Open questions/issues: • Pre-calibrated fast probes with integral square law detection could be made available, simplifying the testing. Would test equipment suppliers step up to provide these? Are there technical stumbling blocks? Will both E-field and H-field probes be needed? • Pre-calibrated slow probes could also be made available, but would need a calibration chart that covered each individual modulation type and sub-type (similar to probe modulation factors), with additions as new protocols appeared. • The weighting function (described in a separate PowerPoint) is straightforwardly mathematically defined and is readily implementable in hardware or software. Would test equipment suppliers step up to provide this function? • The effect of the generalized method relative to the present peak power measurement on any given modulation protocol is modelable and predictable. Is that sufficient to calm worries prior to actual testing? • An overall result of applying the generalized method would be a very significant relaxation of the WD emissions requirements, but not necessarily in addition to the recently adopted 10 dB low band relaxation. Would additional data on HA susceptibility vs. frequency be needed to reexamine the allowable RF level vs. frequency relationship? • Are there outstanding issues concerning a hearing aid’s response to a dipole or a GTEM cell in comparison to its response to a WD near field that need to be considered? • Does anyone think that a large Oklahoma-style study would be needed for verification?

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