RF Desense

Created; April/2020Last updated: 5/15/2020

Introduction

Have you wonder why the connectivity of your electronic product is flaky? Even though the product is designed with a known wireless vendor with latest standards. Is it really the wireless module vendor the culprit of the slow internet connection? The short answer is no. Different products with same wireless module part number can have very different wireless performance.

We are going to introduce the term radio frequency (RF) desensitization also known as RF desense in industry, and this will be the focus of this article

    • Note desensitization is not limited to wireless receivers. Other receivers such as light detector can also be desensitized due to light leakage or pollution.

GOAL

The purpose of this article to demystify, root cause, and mitigate RF desense problems. After finishing this article, an intelligent designer shall able to be able to diagnose wireless problems, pin point the root cause, and apply appropriate desense mitigation changes.

Overview

Definition

Receiver sensitivity: it is the minimum detectable receiving signal power of the receiver in order to achieve the a specific bit error rate (BER)

Desensitization of the wireless receiver increases the receiver noise floor, lowering receiver's signal to noise ratio, resulting in degradation of the receiver performance. In order to maintain the same receiver performance (i.e the same packet error rate ) at presence of interference, a higher receiving signal power is needed to main the same signal to noise ratio, hence, receiver sensitivity is increased. A minimum receiver sensitivity is needed to maintaining a high throughput connectivity.

The cause of desensitization is due to electromagnetic interference; this interference can be from an external source (such as a nearby electronics) or self-generated. Most often than not, it is the self-generated interference that causes poor wireless performance. A good analogy to desense in shooting your self in the foot!

Detailed Analysis

EMI always have a source and can be coupled in many ways to the wireless receivers. In an embedded system, a conducted coupling path can be a noise power supply for RF receiver IC and a radiated path can be an unshielded DDR memory banks that radiates electromagnetic energy into the air (during a read or write operatio), and that energy is picked up by the receiver antenna.

By looking at each coupling path and identifying the sources of EMI, one is able to provide mitigation early on to prevent poor wireless performance.

A common source of interference also referred aggressors is any component that operates at high speed such as clock source, power supply, digital interfaces, memories, etc.

Root Cause Methods

Testing

One shall create a Desense Matrix containing following items to test the level of desense.

  • Identify all possible aggressors
  • identify all receiver operating bands and channels
  • Create an acceptance criteria
    • could be a minimum of throughput
      • Generally test condition is set at a very low sensitivity.
    • Couple a maximum sensitivity increase
      • Generally a 3dB desense level is used
      • Generally test condition is set at a very low sensitivity.

Note: A low receiver sensitivity test conditions allows for good detection of very low power noise/interference. Generally, low receiver sensitivity is also accompanied with low MCS index.

Simulation

To understand the exact noise spectrum of a well know noise source such as a HDMI interface, one can do a PCB level simulation and analysis and generate a power spectral density (PSD) plot to evaluate how the signal transmission affects victim radio operating bands.

Mitigation

Antenna Isolation

    • Move antenna away from high noise areas such as digital ICs, power electronics, high speed interfaces.
    • Use antenna that has good isolated ground.
    • Use dedicated antenna for each receiver band (e.g separated 2.4 GHz antenna for wifi vs. bt)

Good grounding

    • provide low impedance ground plane for impedance controlled signals
    • provide low impedance ground plane for switching mode power supply and IC ground pins.
    • stitching ground planes together with vias to achieve low impedance

Good stackup

    • Use multilayered stackup with dedicated ground and signal layers
    • Sandwich signal layer with reference ground planes to provide low impedance

Add filters elements

    • ferrite beads/ PI filter for power rails
    • common mode chokes for high speed differential pairs
    • Shunt caps (i.e 0201 COG xx pF capacitors) placed for all digital signal connector pins.
      • See how to choose desense capacitor for more design details.

Software Optimization

    • Change aggressor operating frequency to shift harmonics noise out of receivers' operating band and channels
    • Interleave the operation between transmitter and receiver to achieve time division isolation
    • Use spread spectrum clocking on high speed interface such as PCIE to reduce and spread EMI associated with high speed signals.

Good layout

  • Bury high speed digital interface between ground planes
  • Add sufficient separation between high speeds signal to other signals; recommend at least 2-3x trace width separation
  • Use star connection to route power to RF IC power pins
  • Guard ring for all RF paths.
  • Stitch the board perimeters with vias

Example problem

RF Desense

This slide goes over a real problem that actually happened during a product development. A detailed root cause analysis is carried out; several mitigation changes are applied to the design, and lastly the changes are validated through a list of desense tests.

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.