天线设计和RF Layout指导

www.cypress.com Document No. 001-91445 Rev. *D 1

Contents

1 Introduction ............................................................... 2

2 Antenna Basics ......................................................... 3

3 Antenna Types ......................................................... 4

8 Chip Antennas ........................................................ 17

9 Wire Antennas ........................................................ 19

10 Antenna Comparison .............................................. 20

11 Effect of Enclosure and Ground Plane on

12 Guidelines for Antenna Placement, Enclosure,

15 Antenna Tuning ...................................................... 35

16 RF Transmission Lines ........................................... 43

16.1 Microstrip Line ............................................... 43

16.2 CPWG (with Bottom Ground) ........................ 44

16.3 RF Trace Layout Considerations ................... 44

19.1 Power Supply Decoupling

Layout Considerations ................................... 48

20 Vias ........................................................................ 48

21 Capacitors and Inductors........................................ 49

21.1 Capacitors ..................................................... 49

21.2 Inductors ........................................................ 51

22 Design for Testability .............................................. 52

23 Support for External Power Amplifier/

B Appendix B: References ......................................... 58

Document History ............................................................ 59

Worldwide Sales and Design Support ............................. 60

www.cypress.com Document No. 001-91445 Rev. *D 2

1 Introduction

Antenna design and RF layout are critical in a wireless system that transmits and receives electromagnetic radiation

in free space. The wireless range that an end-customer gets out of an RF product with a current-limited power source

such as a coin-cell battery depends greatly on the antenna design, the enclosure, and a good PCB layout.

It is not uncommon to have a wide variation in RF ranges for designs that use the same silicon and the same power

but a different layout and antenna-design practice. This application note describes the best practices, layout

guidelines, and an antenna-tuning procedure to get the widest range with a given amount of power. Other important

general layout considerations for RF trace, power supply decoupling, via holes, PCB stackup, and antenna and

from the radio, the larger the distance it can cover for a given packet error rate (PER) and receiver sensitivity.

Similarly, a well-tuned radio at the receiver side can work with minimal radiation incident at the antenna. The RF

layout together with the radio matching network needs to be properly designed to ensure that most of the power from

the radio reaches the antenna.

www.cypress.com Document No. 001-91445 Rev. *D 3

2 Antenna Basics

An antenna is basically a conductor exposed in space. If the length of the conductor is a certain ratio or multiple of

the wavelength of the signal

fed to antenna is radiated into free space.

current standing waves are formed across the length of the conductor, as shown in Figure 2.

The electrical energy input to the antenna is radiated in the form of electromagnetic radiation of that frequency to free

space. The antenna is fed by an antenna feed that has an impedance of, say, 50 Ω, and transmits to the free space,

which has an impendence of 377 Ω

Thus, the antenna geometry has two most important considerations:

1. Antenna length

2. Antenna feed

The /2-length antenna shown in Figure 2 is called a dipole antenna. However, most antennas in printed circuit

boards achieve the same performance by having a /4-length conductor in a particular way. See Figure 3.

By having a ground at some distance below the conductor, an image is created of the same length (/4). When

www.cypress.com Document No. 001-91445 Rev. *D 4

Figure 3. Quarter-Wave Antenna

For a quarter-wave antenna that is used in most PCBs, the important considerations are:

1. Antenna length

2. Antenna feed

ground plane. This is generally fed by a 50-Ω

transmission line. The wire antenna gives the best performance

and RF range because of its dimensions and three-dimensional exposure. The wire can be a straight wire, helix,

or loop. This is a three-dimensional (3D) structure, with the antenna over a height of 4-5 mm over the PCB plane,

protruding into space.

Figure 4: Wire Antenna

The feed is generally of 50 ohm in most RF PCB catering to low-power wireless applications. However, other impedance values

are possible.

www.cypress.com Document No. 001-91445 Rev. *D 5

3. Chip Antenna: This is an antenna in a small form-factor IC that has a conductor packed inside. This is useful

when there is limited space to print a PCB antenna or support a 3D wire antenna. Refer to Figure 6 for a

Bluetooth module containing a chip antenna. The size of the antenna and the module in comparison with a one-

cent is coin is given below.

4 Choosing an Antenna