Wireless communication devices, when operating at their highest power levels, can generate significant electromagnetic interference (EMI) that may disrupt the functioning of nearby electronic equipment. Among these, the interference caused to hearing aids used by individuals with hearing impairments is particularly critical, as it directly impacts their ability to use these devices effectively. To address this issue, the ANSI C63.19 standard outlines specific test requirements for Hearing Aid Compatibility (HAC). Within this standard, the measurement of radio frequency (RF) electric field and RF magnetic field radiation from wireless devices plays a central role in evaluating their compatibility with hearing aids.
This section provides an overview of the methods used to measure RF electric field and magnetic field radiation in HAC testing. These tests are essential for ensuring that wireless devices do not cause harmful interference with hearing aids, allowing users to enjoy clear and uninterrupted communication.
The primary instruments used in HAC testing include:
- 1. Near-field electric field probe
- 2. Near-field magnetic field probe
- 3. Probe positioning device
- 4. Wireless Device (WD) support system
- 5. Additional equipment such as an RF shielding room
During the test, the wireless device must be set to its maximum rated output power, and the test should be conducted across high, medium, and low channels. It's crucial to ensure that all equipment, including the field strength probe and the test system, is functioning properly. The probe must be accurately positioned and calibrated before the test begins.
The HAC test is typically performed within a 5 cm area. The electric field probe is scanned across the surface to detect the maximum field strength. This is done over multiple pulses, using peak measurements to calculate the average field strength based on the duty cycle of the wireless device. For accurate coverage, the center of the probe must be scanned within the test area, maintaining a distance of 1.0 cm from the reference plane of the object being tested.
The general test procedure is illustrated in Figure 1 below:
Brief Test Flow
After the test, the result is recorded as the maximum peak reading and converted into equivalent peak values in volts per meter (V/m) or amperes per meter (A/m). These measured values are then compared against the limits specified in the ANSI C63.19 standard to determine the compatibility rating.
In addition to measuring field strengths, the ANSI C63.19 standard also defines procedures and classification methods for assessing the electromagnetic compatibility between hearing aids and digital mobile phones. Different models are evaluated and categorized into four levels—1, 2, 3, and 4—based on their performance. When both the hearing aid and the mobile phone are rated at level 4, the resolution index reaches 0.3, which is acceptable for basic communication. If the sharpness index exceeds 0.5, the call quality is considered normal, and if it reaches 0.7 or higher, the call quality is deemed very good.
Electromagnetic Compatibility Testing and Classification Methods
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