Building a low cost reference platform-based wireless Zigbee design

Examples include environmental measurement and control applications such as thermostats, condition monitoring such as vibration and temperature sensing to detect premature motor failure, and automatic meter reading. The explosive growth expected in this area is being driven by the emergence of new open standards like IEEE 802.15.4 and Zigbee.

Cost, of course, plays an important role in the viability of any system. The lowest-cost way to implement wireless functionality is to add the wireless sensor network subsystem—typically a single-chip wireless microcontroller— to the board of the existing product. This is has driven the development of this low-cost reference design.

Figure 1: Predicted radiation patterns for the dipole antenna are shown.

Integrating the subsystem
The objective of this reference design is to achieve the lowest possible cost for adding wireless sensor network functionality to a typical product, so the requirements for this design are driven by the need to integrate it into existing board designs.

The first requirement is the use of a two-layer circuit board. Many RF module designs use a four-layer board to give the opportunity to minimize the size—this is cost-effective for a small module. However, typical products that will use this technology are large compared with a module. Furthermore, in a larger product, the use of two layers rather than four significantly reduces the board cost by 30 to 40 percent.

As this design is for high-volume products, all components need to be surface-mounted. There are three other components that contribute significant cost to the system—the crystal, antenna and RF balun.

Without a size constraint, a larger crystal can be used—an HC49/U surface-mounted component can be half the cost of the equivalent miniature surfacemounted product. Furthermore, the extra space makes it possible to develop a printed antenna design that can interface directly to the RF port of the chip.

Hence, we can address the antenna cost by integrating it into the board and remove the need for the balun completely. The target size for the overall module was 50mm x 30mm x 8mm maximum thickness. The BOM target for the module was $5.

Selecting the antenna
To fit within the total target size, there were three different antenna types considered. Each has different advantages and disadvantages.

Slot antenna.This is a radiating structure formed by making a slot in a ground plane on the board. It can be easily formed by realizing a small slot or notch in ground planes pre-existing on the module circuit board. In principle, the slot could be placed virtually anywhere on the board, subject to

RF interference issues. While it can be designed to include a balanced feed, maintaining phase balance and reducing feed losses is difficult at the relatively high 200 ohm impedance of the chip interface. The input impedance is also difficult to control since it can be very high, unless the slot is extremely narrow with consequent tolerance issues.

Loop antenna. This can be easily formed using a small loop of track connecting together each side of the balanced RF port. It is a magnetic type antenna with good performance in handhelds where it is used close to a human user as it is less easily detuned in such situations.

Design issues for this antenna type include the difficulty of providing matching at the 200 ohm impedance and removing any spurious responses. This antenna's major disadvantage is the difficulty of fitting it into required dimensions.

Dipole antenna. This is a textbook balanced antenna. It is easily matched to the 200 ohm chip impedance by using a folded dipole derivation. The impedance and resonance center frequency are easily adjustable by small changes to geometry.

The structure is broadband and can easily be fitted within the required form factor. It offers the possibility to meander the lines if reduced size is required. However, the radiation pattern can be perturbed by nearby structures—this requires characterization using wire grid simulation.

For this development, it was decided to use a variation of a folded dipole antenna. The printed arms of the antenna are varied in their width to achieve the required step up impedance transformation, approximately 75 ohm to 200 ohm or about 1:2.6.

The driven element is narrow compared with the undriven (parasitic) element and this, combined with the correct element spacing, performs the impedance transformation.

Simulation
Starting from the basic circuit structure, the design was simulated using method of moments simulation. This allows modeling to a first approximation to verify that the design goals on impedance and radiation pattern have been achieved.

The antenna elements are modeled as printed sheets of thin, flat copper on a glass-fiber dielectric. Additional elements were added to take account of board non idealities. The wire model also included a large ground plane that is typical of many actual products.

Using the model, the expected radiation pattern can be predicted. This shows dipole-like performance, with radiation intensity reduced in the direction of the ground plane.

This is typical of many wall-mounted products where the main requirement for radio communication will typically be in either an upwards or downwards direction - for example a light switch communicating to a light fitting in a room.

It should also be noted that smaller ground planes would produce less attenuation in the rearwards direction. Modeled radiation patterns in 3D and in the main axis of the antenna are shown in Figure 1 above .

A ground strap is added on the lower layer that is via-hole connected to the center of the undriven dipole element. This has little influence on the wanted balanced mode of operation in the 2.4-2.5GHz ISM band, but it provides some suppression of the unwanted 4.8-5.0GHz unbalanced mode to help compensate for the removal of the balun that provided some attenuation of these signals.

Figure 2: The reference design layout shown incorporates low-cost features for a BOM under $5.

Ground plane
The design of the module ground plane is probably the most important design factor contributing to the RF performance and any spurious emissions. For good RF performance, it is important to ensure that there are no features that can interrupt the RF current flows in the ground plane.

In this device, the critical area is mainly between the balanced RF output and the antenna central grounding point. The rule is to keep all possible signal paths as short as possible. This means placing decoupling components as close as possible to their chip pin and ensuring a direct ground plane path to the RF ground of the chip.

So, for example, a track that interrupts the ground plane between the chip and the decoupling for the RF supply would lead to large ground loops and would result in high levels of spurious emissions.

For similar reasons, the centertapped RF ground on the antenna must have as short a path as possible to the chip ground. The antenna was implemented on a test board to allow evaluation without IC effects and tuning of the antenna dimensions.

The input match and through loss were measured to verify that it was on tune. After tuning the dipole length, the through loss was measured, showing that the design was correctly tuned to the required 2.4- 2.5GHz band.

Having established the best dimensions for the design, it was then included on a reference module design incorporating all of the low cost features. The layout and configuration of the board is shown in Figure 2 above.

In this configuration it is difficult to estimate the exact antenna performance due to variabilities in chip performance, but qualitative range performance tests showed that the module design performed similarly to one using a commercially available dipole antenna with a gain of around 2dB.

Likewise, spurious emissions were within regulatory requirements. The completed reference design has a total BOM of under $5 and provides a design that can easily be incorporated into low-cost products.

Colin Faulkner is Product Marketing Manager and Lee Neal is an RF consultant at Jennic Ltd.