Tackling RF Desense Challenges At The Source

A chip-package-system perspective for identifying the causes of desense and mitigating the problems.

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Just imagine you are stepping out of the electronics store with your brand new smart phone. You eagerly scroll down your contacts to dial your best friend and proudly tell them the great news, but as soon as they pick up, your reception is gone! What happened?

This problem is commonly described as desense, a degradation of the sensitivity of the receiver due to external noise sources. Desense isn’t new. Early radios and wireless phones experienced the same issue. Receiver sensitivity was suddenly compromised when other components like the display or memory interface became active. But it has become particularly troublesome for today’s wireless technologies, which are being integrated into smart devices and automotive infotainment systems.

So what causes this? There are two classical noise sources that can lead to desense—high-speed signal lines and supply networks both radiating power in the frequency range of the receiver.

Mobile Devices, Antennas and Flex I/O Cable
High-speed signals like memory buses or display channels are sending electrical pulses over transmission lines. An ideal transmission line by itself keeps this energy in its own structure. However, discontinuities like a connector or imperfect flex line allow some of this energy to be radiated into the surrounding to act as a noise source. In earlier times, during the time of feature phones, these signal buses were running at a lower frequency, barely 100MHz, and so only high order harmonics were able to interfere with radio frequencies. Nowadays, in the time of smart phones and automotive infotainment, mobile memory data rates have entered the multi-gigahertz range, directly interfering with the frequency band used for wireless communication.

Similar tendencies can be seen for the second noise source, the power network. Highly regular power networks that always pair supply and ground lines create very little interference, since the magnetic fields generated by power and ground currents largely cancel each other out. More recent mobile designs, in the area of smart phones as well as IoT and wearables, use much less regular power distribution lines, driven by the need to control stack height and tighter integration of package and chip technology. As a result we see modern electronic mobile, wearable, and infotainment systems susceptible to desense issues.

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Propagation in fields from cellular antenna to GPS Antenna

Detecting Desense Issues
A desense problem in an existing product can manifest itself in random crashes at particular modes of the operating hardware. The problem is complicated by the fact that many components in a system can contribute to these two types of noise–the integrated circuits used, the packages that the circuits are placed in, the printed circuit boards they are mounted on, and the system as a whole.

Problems that need to be identified include:

• modes of operation that trigger the failure
• noise sources leading to desense
• coupling paths to the receiver

Once identified, measures need to be taken to suppress them at the component or system level.

If desense issues can be identified during the design phase using simulations, this would lead to a much more cost-effective and efficient strategy to ensure a product achieves first-time success. Such an analysis would require the simulation of the receiver path, together with all noise sources that can cause densensing. In general this will be a system-level analysis, incorporating noise sources on the same SoC, as well as sources located on the package and overall system, i.e., a Chip-Package-System simulation.

Influence of Chip-Package-System Effects on Desense
The SoC (chip) itself has a lot of activity over a broad frequency range, and this alone creates a problem for the receiver. However, low power methodologies used in today’s mobile SoCs create even more drastic noise scenarios due to large transients during their power mode transitions. For example, a mobile SoC that suddenly switches on its graphics unit creates a huge transient on the system supply and the substrate. Packages can also be sources of noise or the coupling path by itself. In the never-ending quest for thinner devices, packages are becoming thinner, have less-perfect supply planes, and are more closely integrated to the chip itself, creating the potential for more interference. On the system side, connectors and flex wires create discontinuity in high-speed signal channels, acting as antennas and radiating energy. The system may also contain several intentional antennas each creating their own spectrum of energy, overlapping each other, as shown in the image, below.

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Spectral Energy Coupled to GPS Antenna

In this example, different sources are coupling to a GPS antenna: the cellular radio, the Bluetooth radio, and the digital I/O. Depending on the GPS receiver’s strength and filtering ability, just the digital I/O envelope alone could be sufficient to impact the GPS functionality, causing it to fail.

Radio Frequency Interference on the GPS Antenna
An integrated simulation environment that could analyze these interactions would greatly benefit designers of mobile systems, especially if a single design environment can support re-use of the same component models for SI, PI, and EMI simulations. At the chip level, a sample desense flow would allow visualization of chip power noise activity highlight low-to-high power transitions of the chip, which might create desense scenarios. At the system level, impedance and resonance analysis can help flag early design weaknesses and guide the selection and placement of PCB, package, and chip-level capacitance to reduce EMI effects. Finally, a desense simulation flow can help visualize antenna excitation and internal coupling to flex cabling and adjacent power and ground planes, to show where components are coupling to each other and guide placement of critical components affecting the design.

Full-System Transient and Frequency Analysis for RF Desense
With the design challenges of today’s mobile, wearable, and automotive infotainment electronics, simulation driven product development has become more important than ever. With exceptionally short design cycles and trends to smaller form factors and energy efficiency, the ability of the designer to converge quickly on an optimized design is critical. Solving potential problems in simulation rather than build and test saves time. Combining circuit simulation, field analysis, and chip power noise information can help visualize potential power noise, EMI, SI and antenna interference with the potential to create desense problems on mobile devices. ANSYS simulation tools can help designers address potential desense issues with an efficient integrated solution for Chip, Package, RF, and System-level components, allowing designers achieve first-time success on their IoT products.