CSA.71 RF Interference Mitigation for Audio

What is the problem or concern we are addressing?

Taoglas can evaluate and solve issues that relate to the antenna and RF performance, receiver interference, audio interference and spurious transmitter emissions that cause performance and certification issues.

All wireless, and particularly cellular devices, face common performance and certification challenges, regardless of the types of radios being used. GSM audio & voice devices, in particular, are prone to self- interference causing audible “TDMA noise”. This service offering is focused on addressing this interference in order to meet customer or certification expectations.

TDMA noise is a common problem experienced by GSM audio and voice devices. The cause may not be obvious: the transmitted RF signal (from the mobile to the base station) turns on only for short “slots” of time. These “slots” reoccur at a rate of approximately 216 per second, or 216Hz. This RF signal is picked up by the audio circuitry, adding this to the sound the user hears. Because the rate (216Hz) is within the audible frequency range, filtering is often not an option. Therefore, other methods are required to mitigate the issue. To compound the issue, some certification bodies require test verification that the problem is not present.

After you’ve experienced this certification failure, or find the problem is beyond your customer’s expectations, Taoglas engineering can evaluate your device, the testing that has been done, and any additional testing that needs to be completed. Our engineers will then proceed with evaluating the device and reviewing the design files.

In evaluating the design files, our engineers will review all relevant schematics, PCB layout files, and mechanical design files to first understand the design intent and then analyze the pertinent portions of the design. This analysis guides the mitigation effort.

Taoglas engineering will then use samples of the device to reproduce the failure in our controlled lab environment. Once the issue has been reproduced, we will use the previous analysis to devise experiments to determine the underlying failure mode. Once this failure mode is understood, the next experiments will attempt to mitigate that failure mode on the device samples (to the extent that it is possible to do so). Some failure modes are closely tied to the PCB layout and, as such, it may not be possible to prove out a mitigation on the existing device samples. If a mitigation or short-term fix is found and functional device samples exist, these will be provided back to the customer for further verification and testing.

Following all of these efforts, a report is compiled which details the analysis completed and provide recommendations for design changes, both long- and short-term if appropriate.

Deliverables

The output from this effort will be presented in a written report which details the specific mitigations to be applied to each circuit in the system. To the extent it is possible to modify the existing device to demonstrate the effectiveness of the mitigations, a copy of the physical device will be provided. There will be an appendix with any comments from the post implementation review.

The Processes

Part 1

This service consists of the following:

• Analysis of the testing done to date and recommendations of additional testing as needed to determine exactly what aspect of performance or certification is insufficient.
• Reproduction of the failure mode in our controlled lab environment.
• Experimentation on the existing physical hardware to determine the failure mode and the root cause of the failure mode.
• Analysis of mechanical design, schematics, PCB layout and bill of materials for each board in the product, component by component, connection by connection to create a series of experiments to mitigate the root cause of the failure mode.
• Experimentation on the existing hardware of specific mitigation measures selected to address the root cause of the failure mode.
• A specific list of the mitigations to be applied to the device to address the failure mode.
• Iteration through these test steps as PCB changes are made to ensure the issues are resolved.

What does Taoglas need?

In all cases Taoglas will require the following:
• PDF format copies of your schematics for each board in the design. The best format is native Altium files.
• Design files for all PCBs in the design. The best format is native Altium files or Gerber files. Please include a document defining the PCB stackup, layer thicknesses, materials and finishes for the PCB.
• A spreadsheet of the bill of material for each PCB in the design.
• An eDrawing files for the mechanical assembly. The ability for Taoglas to hide parts, do cross sections and make measurements etc., an eDrawing is essential.
• In the situation where a product that has already had prototypes built, then testing on those prototypes is done to identify the baseline. If testing has been already been performed (e.g. prescans) Taoglas requires all the test data gathered including summary reports, raw data and pictures of the test setup.
• Four (4) fully functional copies of the existing device hardware. These devices may be consumed in the process.
• One complete set of any support devices such as spare battery packs, battery charger, programing interfaces or cables.
• Terminal access to the cell modem’s AT command interface is required. Instructions on how to connect the device, power on the device, and connect to the AT command interface must be provided. If the battery will need to be charged or replaced, include instructions on how to do so.
• The device must be configured so that the radio is automatically enabled when power is applied.
• Taoglas cannot make adjustments to device software in anyway. The device must be able to connect to a standard call box, such as a CMW500, and remain connected for at least 1 hour without interruption. Taoglas will spend no more than 1 hour attempting to achieve this connection before contacting the customer.
• If the customer directs Taoglas to “keep trying”, anytime spent beyond this initial hour or any customer directed/guided interactive effort will be billed at $500 USD per hour. If we cannot get the device to connect, we will send the device back to the customer for proper configuration.

Part 2

The output of the mitigation experiments will be a list of changes to your design that will mitigate the failure modes observed in testing. Your engineering team will implement the changes in a new set of design files.

All the changes are equally important and need to be executed. Most failing products have several factors causing the same failure mode and they all have to be addressed without compromise or the risk of failure remains.

Taoglas will review the changes to the design files. Output from this review will be appended to the initial design report and distributed to you and your engineering team. Output from this review will be appended to the initial design report and distributed to you and your engineering team. This is included as part of this service.

In the event it is not possible to apply physical mitigations to the existing hardware to prove all failure modes have been addressed, changes will be required to the design to apply additional mitigations as part of a full design revision including schematic, PCB & BOM changes.

In the event the provided changes are insufficient to address all failure modes, the process is repeated until all failure modes are addressed.

Service Delivery Objectives

A. Detailed analysis of implemented hardware and all design files
B. Report with recommended changes for long-term design fixes
C. Addition of RF shielding material on prototypes to attempt short term fix

OUR PROCESS DESIGNED, TEST & READY

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