RF Desense

Introduction: 

Have you ever wondered why the connectivity of your electronic product can sometimes be unreliable or flaky, even when it's designed with a wireless module from a reputable vendor using the latest standards? While it might be tempting to blame the wireless module vendor for slow internet connections, the reality is more complex. The short answer is no; the wireless module vendor may not be the main culprit. Different products with the same wireless module part number can exhibit varying wireless performance. 

In this article, we will introduce the term "radio frequency (RF) desensitization," also known as "RF desense" in the industry. This will be the central focus of our discussion.

• Note: RF desensitization is not solely limited to wireless receivers but can also affect other types of receivers, such as light detectors. For example, light detectors can experience desensitization due to light leakage or pollution. 

Definition:

Before we delve deeper into the concept of RF desensitization, let's clarify some key terms used in this article:

• Radio Frequency (RF) Desensitization: Radio Frequency (RF) desensitization, also known as RF desense, is a phenomenon that affects electronic products, particularly wireless receivers. It refers to a situation where the sensitivity of a wireless receiver to incoming signals is reduced due to electromagnetic interference. This interference can originate from external sources or stronger self-generated interference within the electronic system of the product. RF desense can lead to degraded wireless performance, resulting in connectivity issues and reduced range coverage.

• Receiver Sensitivity: Receiver sensitivity is a critical parameter for digital radio receivers used in wireless communication systems such as WiFi or Bluetooth. It is defined as the minimum detectable receiving signal power required by the receiver to achieve a specific bit error rate (BER). A higher receiver sensitivity indicates better performance, as it allows the receiver to detect weaker signals and maintain a stable communication link over longer distances.

• Electromagnetic Interference (EMI): Electromagnetic interference (EMI) is the unwanted electromagnetic radiation or noise that can disrupt the operation of electronic devices. EMI can be generated by various sources, including other electronic devices, power supplies, and high-speed interfaces. When EMI couples with wireless receivers, it can cause RF desensitization, leading to performance degradation and connectivity issues.

Goal: 

The purpose of this article is to demystify, identify the root cause of, and mitigate RF desense problems. By the end of this article, an intelligent designer should be able to diagnose wireless issues, pinpoint the root cause, and apply appropriate desense mitigation techniques.

Overview: 

Receiver sensitivity refers to the minimum detectable receiving signal power of a digital radio receiver required to achieve a specific bit error rate (BER) in accordance with wireless communication standards like WiFi or Bluetooth.

Theory of Operation: 

A desensitized wireless receiver experiences an increased noise floor due to electromagnetic interference, resulting in a reduced received signal-to-noise ratio. This leads to degraded receiver performance in terms of throughput vs. range. To maintain the same receiver performance in the presence of RF interference, a higher receiving signal power is required. However, the transmit power from the access point or wireless router remains fixed. As a result, the effective communication link range between the access point and the radio receiver must be reduced, leading to a reduction in wireless range coverage.

The cause of desensitization is electromagnetic interference, which can originate from external sources (e.g., nearby electronics like TVs) or, more commonly, stronger self-generated interference within the electronic system of the product. Self-generated interference often causes poor wireless performance. A good analogy for desense is shooting oneself in the foot!

Detailed Analysis: 

Electromagnetic interference (EMI) always has a source and can couple to wireless receivers in various ways. In an embedded system, conducted coupling paths can be noise from the power supply for the RF receiver IC, while radiated paths can arise from unshielded DDR memory banks radiating electromagnetic energy during read or write operations. This interference is picked up by the radio receiver antenna, resulting in desensitization.

By examining each coupling path and identifying the sources of EMI, one can apply early mitigation measures to prevent poor wireless performance.

Root Cause Methods: 

Testing: 

One should create a Desense Matrix containing the following items to test the level of radio receiver desensitization. For example:

• Identify all possible aggressors

• Identify all receiver operating bands and channels

• Set an acceptance criteria, e.g., minimum data throughput in bits per second

• Generally, test conditions are set at a very low sensitivity, with a 3dB desense level being used

• Note: Low receiver sensitivity test conditions allow for good detection of very low power noise/interference and are generally accompanied by a low MCS index.

Simulation:

To understand the exact noise spectrum of a well-known noise source, such as a HDMI interface, one can perform PCB level simulations and analyze power spectral density (PSD) plots to evaluate how signal transmission affects victim radio operating bands.

Mitigation: 

Several mitigation techniques can be employed:

• Antenna Isolation: Move the antenna away from high noise areas and use an antenna with good isolated ground. Use dedicated antennas for each receiver band, such as separated 2.4 GHz antennas for WiFi and Bluetooth.

• Good Grounding: Provide a low impedance ground plane for impedance-controlled signals and switching mode power supplies and IC ground pins. Stitch ground planes together with vias to achieve low impedance.

• Good Stackup: Use multilayered stackups with dedicated ground and signal layers. Sandwich signal layers with reference ground planes to provide low impedance.

• Add Filter Elements: Use ferrite beads/PI filters for power rails, common mode chokes for high-speed differential pairs, and shunt caps (e.g., 0201 COG xx pF capacitors) for all digital signal connector pins.

• Software Optimization: Change aggressor operating frequencies to shift harmonics noise out of receivers' operating bands and channels. Interleave the operation between transmitter and receiver to achieve time division isolation. Use spread spectrum clocking on high-speed interfaces like PCIe to reduce and spread EMI associated with high-speed signals.

• Good Layout: Bury high-speed digital interfaces between ground planes, add sufficient separation between high-speed signals and other signals, use star connections to route power to RF IC power pins, and implement guard rings for all RF paths. Stitch around the printed circuit board perimeter edge with ground stitching vias to form a Faraday cage.

Example problem

Summary

In this article, we've learned that RF desense is a radiated/conducted immunity problem specifically for wireless receivers. Often these self-generated noise/interference can be greatly suppressed by following good placement, layout, filtering, and shielding guidelines. It's very easy to point finger to bad hardware as a cause of poor connectivity; as a good designer, we now know that it's not the RF IC that is the problem but rather the system around it. 

As an intelligent designer, we need to understand the system first to order to find root cause and can improve wireless performance and ensure reliable connectivity in electronic products.