As a continuation of our previous discussion about selection criteria of BLE SoC for building BLE based IoT devices, we will discuss in detail about the most important considerations for BLE RF layout design and antenna selection from various types available. The communication range of a wireless device with a current limited power source depends mainly on the RF layout, antenna design, and enclosure. Increased operating distance can be achieved with the type of antenna chosen together with carefully designed RF layout with few matching components to ensure most of the power from the BLE SoC reaches the antenna. The more power an antenna can transmit from the SoC, the larger the distance it can cover.

RF layout

RF layout involves routing the transmission lines from BLE SoC to antenna with few matching components in between. RF transmission lines acts as a medium that carry RF power from a BLE SoC to antenna, hence they need to be routed with many constraints to get maximum RF power delivery to antenna. There are several types of transmission lines, the two most popular types are:

1. Microstrip Line
2. Coplanar wave guide (CPWG)

Both of these are PCB traces differing in how they are constructed for maintaining the 50-ohm characteristic impedance. There are online calculators available which help us to calculate the impedance of the transmission based on our parameter input.

Microstrip Line – This type of transmission line has a signal trace on top of a substrate with a ground plane beneath the substrate. A microstrip line is simple to construct, simulate, and fabricate. The characteristic impedance of a Microstrip line depends on the following factors,

1. Substrate height (H)
2. Dielectric constant of the substrate (εr)
3. Width of the trace (W)
4. Thickness of the RF trace (T)

MicroStrip Line

CPWG – This is similar to the microstrip, but it has a copper pouring on either side of the RF trace with a gap between them. It provides better isolation for RF traces and a better EMI performance and makes it easier to support the grounding of matching components on an RF trace. The characteristic impedance of a CPWG depends on the following factors:

1. Substrate height (H)
2. Dielectric constant of the substrate (εr)
3. Width of the trace (W)
4. The gap between the trace and the adjacent ground fill (G)
5. Thickness of the RF trace (T)

Coplanar Wave Guide

Nowadays the SoC manufacturers provide reference designs from which we get the guidelines for routing the RF transmission lines and the values (typically capacitance and inductance values) of the matching components. The designer in addition to the routing guidelines must also note the recommended PCB stackups for desired performances, since the impedance of the RF lines will change depending on the PCB layers stackups. In most of the cases, the PCB fabricator may not match the exact stackup as recommended and at these conditions there will be a need for slight changes in the RF trace width, gaps or thickness to ensure the correct impedance value.

Antenna Types

Antenna is a critical part of any wireless devices that transmits and receives electromagnetic radiation in free space. Antenna is nothing but a conductor exposed in space. When the length of a conductor is a certain multiple or ratio of the wavelength of the signal (λ) it behaves like an antenna and radiates the electrical energy into free space in the form of electromagnetic radiation of that frequency to free space. BLE device range requirement, costs and form factor are the main factors to be considered for choosing the antenna. For BLE applications (2.4GHz), most common types of PCB antennas are as follows,

1. Wire antenna
2. PCB Trace Antenna
3. Chip antenna

• Wire antenna: It is a piece of wire rise from PCB plane and protrudes to free space over a ground plane. Wire antenna produces best performance and RF range due to its dimension and better exposure. They can be in different forms such as straight wire, helix, loop, etc. A through-hole pad is sufficient to solder the wire antenna, thus saving the board dimension and hence low PCB cost.

Wire Antenna – Straight and Helix Type

• PCB antenna: This is a copper trace drawn on the PCB. These antennas are inexpensive and easy to design, because they are a part of the PCB and provide good performance. Meandered trace, inverted F-trace is the most popular PCB antenna’s used in many designs. Meandered trace antenna is recommended for applications that require a minimum PCB area and Inverted F antenna is better compared to meandered antenna for radiation, but it requires space higher than meandered antenna. Main drawback of PCB antenna is that, it may require two or more revision to get expected range performance. This can be avoided by using the antenna design application notes and stack up recommended by the chip vendors carefully.

PCB Antenna Types

• Chip antenna: For applications where the PCB size is to be extremely small, chip antenna is a good choice. They are commercially off the shelf antennas that occupy very small PCB area and offers reasonable performance. But the disadvantage of chip antenna is the increased BoM and assembly cost since they are external components that need to be purchased and assembled. Also the chip antennas are very sensitive to RF ground clearance and the manufacturers RF ground clearance recommendations must be followed strictly.

Chip Antenna

There are some applications which need antenna’s to be placed on or outside the enclosure for better reach. In such conditions, there are options for providing antenna connectors on board and extending the connection to the external antenna or other mating connector through shielded wire.

Antenna Connectors

• U.FL connector: There are miniaturized RF connectors for high frequency signals. The male connectors are generally surface mounted and soldered directly to the PCB. The female connectors are crimped at one end of the shielded wire and the other end may be a PCB antenna or a mating connector such as SMA, MMCX, etc.

U.FL Connectors

• MMCX connector: These are Micro-Miniature Coaxial connector. They have a lock-snap mechanism which allows 360 degree rotation. It is comparatively better than U.FL in terms of insertion and removal lifetime and has over 10 times that of U.FL connectors. They are also available in surface mount package.

MMCX Connector

About Embien

Embien Technologies is a leading provider of embedded design services for the Semi-conductor, Industrial, Consumer and Health Care segments. Embien has successfully executed many projects like based on IoT such as healthcare Wearables, Gateways, and Data Analytics etc. Embien also offers a set of wearable design collections complete with electronics, firmware and Cloud that can be used to shorten product development costs and time significantly.