A Proposal for RF Coupling Experiments ANSI C63.19 Working Group Submitted for discussion by Stephen Julstrom February 3, 2008.

A Proposal for RF Coupling Experiments

ANSI C63.19 Working Group

Submitted for discussion by Stephen Julstrom

February 3, 2008

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PINS item being specifically addressed:

4. To evaluate the correlation of hearing aid immunity test results using the dipole and WB TEM methods and consider the advisability of different field strength requirements for each of the methods to bring the test methods into closer agreement.

A further goal is to develop data to improve the correlation of the WD emission and HA susceptibility test methods and thus improve the predictive accuracy of the standard, in conjunction with the proposed Generalized RF Interference Level Method.

General considerations:

•It may be assumed that for far-field, free-space measurements

•RF sources (transmitting antennas) of various types producing the same nominal fields at the receiving location may be readily interchanged

•RF receivers of various types (probes, hearing aids) may be readily interchanged

•But for close source-receiver spacings (1cm),

•field strengths and orientations vary rapidly with relative source-receiver positioning

•source and receiver may interact to further modify the field

•In actual use, the user’s head and hand may also affect the coupling.

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Proposed Experiment:

Measure the susceptibility in three ways of several representative ITE and BTE hearing aids and the emissions of several representative, but varied style wireless devices. See how the predicted coupling compares to actual coupling, both in an open RF environment and under simulated in-use conditions on an RF phantom.

•The hearing aids should be pre-tested and preset (gain fixed) to have good (far from any output clipping) but measurable (not perfect) RF susceptibility with expected RF inputs, in both microphone and telecoil modes.

•For the simplest correlations with the most straightforward calculation, both susceptibility and emissions testing should use GSM modulation. For the susceptibility testing, this may be generated with a gated burst generator.

•The experiment should include both E and H-field measurements, which should help decide if keeping the H-field requirement is necessary. (IEC 60118-13 Annex A states their testing revealed, “In practically all cases, vertical E-polarisation of the RF-field, as used in the GSM system, gave rise to the highest interference levels.”)

•The tests should be performed over as wide a range of frequencies as practical.

Unanswered questions:

The standard assumes that the probe-measured RF field of a WD interacts with a HA in essentially the same manner as the probe-calibrated field of a dipole, presumably even with the complication of a user’s head and hand.

How true is this? Do GTEM measurements of HA susceptibility correlate? If so, how?

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Hearing aid measurements:

Measure HA susceptibility in three ways swept over a wide frequency range (> the available WD range), for both E and H-fields, in both microphone and telecoil modes.

1. Calibrate the source fields(measure burst or average power)

Measure in worst case orientation, per standard

3. Record (output referenced)

susceptibility vs. frequency

V = Vx + Vy + Vz

(due to square law detection, equivalent to V2 = Vx

2 + Vy2 + Vz

2)

Dipole

GTEM

Anechoicfree field

Measure in three orthogonal orientations(This differs from the methods of both C63.19, clause 5.4

and IEC 60118-13.)

V = V

2. Measure interference vs. frequency at the HA output

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Wireless device measurements:

Measure WD emissions according to the standard, swept over as a wide frequency range as can be made available, for both E and H-fields.

1. Scan and measure fields at preset WD power level(measure burst or average, same as HA measurement calibration)

3. Record complete scan data and sub-grid maximum data

2. Analyze data for maximum in each sub-grid

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Coupling measurements:

Measure WD-HA coupling over the available WD frequency range in both microphone and telecoil modes. Measure in likely WD-HA relative positionings that can be matched to WD sub-grids. Measure both in an open RF environment and in position on an RF phantom. (Tests conducted for IEC 60118-13 and reported in that standard’s annex resulted in a finding of “no significant difference” due to the presence of the head.)

1. In open RF environment 2. Positioned on RF phantom

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Experimental testing benefits of measuring relative quantities:

•Measuring both HA susceptibility and final WD-HA coupling with the same GSM pulse modulation that the cellphone emits simplifies correlating the results. The HA frequency response and the HA aid output measurement bandwidth and weighting become unimportant, as long as they remain constant for each HA during the course of testing.

•Referring HA measurements to their output, not input (IRIL), avoids the need for HA acoustic gain measurements. (For this experimental testing only! This is not a recommendation to change the standard.)

•GSM RF signal level (both WD output and HA test source calibration) can be conveniently measured as burst or average (again, for this experimental testing only!). If the same measurement probe is used for WD testing as is used for HA test source calibration, then the probe’s absolute calibration becomes unimportant.

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Insights to be gained from the experiment:

•The relationship between three methods of measuring HA susceptibility:

•with a dipole source, as presently specified

•in a GTEM chamber

•in a free field

•The correlation to actual WD-HA coupling of predictions based on those susceptibility measurements and the standard’s emissions measurements:

•with the WD and HA in an open RF environment

•with the WD and HA in position on an RF phantom

•Additional data on relative HA susceptibility vs. frequency

•Whether or not keeping the H-field requirement is important to the standard