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:
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.
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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
average
bandwidth?
Unresolved difficulties:
1)
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.
2)
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.)
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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.
x2
fast probe (>10kHz)
square-law detector
weighting
level measurement
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 A5
weighting in the measurement of ABM2 Audio Band Magnetic Signal, Undesired.
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:
x2
x2
weighting
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 WD’s RF
modulation.
weighting
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:
Slow probe
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
x2
x2
weighting
weighting
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
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modulation characteristic.
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.
CDMA
TDMA
GSM
UMTS
iDEN(2:6)
C63.19- General Method implied "AWF" change from present
2007
for various RF power measures "AWF"/peak power
AWF
average burst avg.
peak
"AWF"
RF level
0
3.9
9.7
12.4
12.4
6.2
0
4.6
14.0
26.9
26.9
13.4
-5
-6.5
11.7
14.2
19.2
9.6
0
-6.0
12.7
14.7
14.7
7.4
0
-5.1
4.8
23.6
23.6
11.8
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).
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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 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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