Generalized Method for the Determination of Wireless Device RF Interference Level

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

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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 WDs 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.

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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.
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• 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 aids
response to a dipole or a GTEM cell vs. a WD near
field, worst-case vs. typical, etc. will not be
addressed here.)
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• 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 (gt10kHz) 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.
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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.
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RF Interference Level measurement (fast probe)
and IRIL calculation

1. 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.)
2. For the WD, measure the worst-case output level
   from the weighted, square-law detected, fast
   probe.
3. 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.)
4. 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.
5. 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 WDs 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.
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RF Interference Level measurement (slow probe)
and IRIL calculation

1. 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.)
2. For the WD, measure the worst-case output level
   from the slow probe.
3. 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.
4. 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.
5. 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.
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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 standards 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 standards
results. The standards WD emission measurement
is presently of the waveforms 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 standards
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).
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• 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
  aids 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?