Adding wireless connectivity to any product has never been easy. However, even when a wireless solution doesn’t seem to make sense, the potential exists. The cost is reasonable, and you add unexpected value and flexibility to the product. But what if you aren’t a wireless engineer? Don’t worry, because in many cases, the wireless chip and module companies have made such connectivity a snap.
SELECTING A TECHNOLOGY
The table lists a marvelous collection of wireless options. These technologies are all proven and readily available in chip or module form. No license is required since most operate in the unlicensed spectrum. They also operate under the rules and regulations in Part 15 of U.S. CFR 47. When considering wireless for your design, you should have a copy of Part 15 handy. You can find it at www.fcc.gov.
The table only provides the main options and enough information to get you started. For a more in-depth look, check out the organizations and trade associations associated with each standard.
Some of the wireless standards have relatively complex protocols to fit special applications. For example, Wi-Fi 802.11 is designed for local-area-network (LAN) connections and is relatively easy to interface to Ethernet. It also is the fastest, except for Ultra-Wideband (UWB) and the 60-GHz standard. It’s widely available in chip or module form, but it’s complex and may consume too much power.
ZigBee is great for industrial and commercial monitoring and control, and its mesh-networking option makes it a good choice if a large network of nodes must be monitored or controlled. It’s a complex protocol that can handle some sophisticated operations. Its underlying base is the IEEE 802.15.4 standard, which doesn’t include the mesh or other features, making it a good option for less complex projects.
If you’re looking for something simple, try industrial, scientific and medical (ISM) band products using 433- or 915-MHz chips or modules. Many products require you to invent your own protocol. Some vendors supply the software tools for that task. It’s a good way to go, because you can optimize the design to your needs rather than adapt to some existing overly complex protocol.
For very long-haul applications that require reliability, consider a machine-tomachine (M2M) option. These cell-phone modules use available cellular network data services like GRPS or EDGE in GSM networks (AT&T and T-Mobile) or 1xRTT and EV-DO in cdma2000 networks (Sprint and Verizon). You will need to do the interfacing yourself and sign up with a carrier or an intermediary company that lines up and administers cellular connections. Though more expensive, this option offers greater reliability and longer range.
Cypress Semiconductor’s proprietary WirelessUSB option operates in the 2.4-GHz band and targets human interface devices (HIDs) like keyboards and mice. It offers a data rate of 62.5 kbits/s and has a range of 10 to 50 m.
The Z-Wave proprietary standard from Sigma Design Zensys, used in home automation, operates on 908.42 MHz in the U.S. and 868.42 MHz in Europe. It offers a range of up to about 30 m with data-rate options of 9600 bits/s or 40 kbits/s. Mesh capability is in the mix, too (see “Wireless In The Works”).
BUILD VS. BUY
Deciding whether to build or buy is a crucial step when it comes to adding wireless. It’s generally a matter of experience. With less experience, it’s probably better to buy existing modules or boards. With solid high-frequency or RF experience, consider doing the design on your own. Almost always, you’ll start with an available chip. The tricky part is the layout.
When self-designing, grab any reference designs available from your chip supplier to save time, money, and aggravation. Primary design issues will include antenna selection, impedance matching with the antenna, the transmit/receive switch, the battery or other power, and packaging. Most modules will take care of these elements.
Factoring in the testing time and cost is another essential design step. Any product you design will have to be tested to conform to the FCC Part 15 standards. Arm yourself with the right equipment, especially the spectrum analyzer, RF power meters, field strength meter, and electromagnetic interference/electromagnetic compliance (EMI/EMC) test gear with antennas and probes. An outside firm also could perform the testing, but that’s expensive and takes time. Factor in some rework time if you fail the tests. Most modules are pretested, so it pretty much comes down to the packaging and interfacing with the rest of the product.
CONSIDERATIONS AND RECOMMENDATIONS
If longer range and reliability are top priorities, stay with the lower frequencies— 915 MHz is far better than 2.4 GHz, and 433 MHz is even better. This is strictly physics. The only downside is antenna size, which will be considerably greater at lower frequencies. Still, you won’t be sorry when you need to transmit a few kilometers or miles. Though not impossible at 2.4 GHz, it will require higher power and the highest possible directional gain antennas.
As for data rates, think slow. Lower data rates will typically result in a more reliable link. You can gain distance by dropping the data rate. Lower data rates also survive better in high-noise environments.
Your analysis of the radiowave path is essential for a solid and reliable link. So, the first step should be to estimate your path loss. Some basic rules of thumb will give you a good approximate figure to use. Once you know your path loss, you can play around with things like transmitter power output, antenna gains, receiver sensitivity, and cable losses to zero in on hardware needs. To estimate the path loss between the transmitter and receiver, try:
dB loss = 37 dB + 20log(f) + 20log(d)
The frequency of operation (f) is in megahertz, and the range or distance (d) is in miles. Another formula is:
dB loss = 20log(4π/λ) + 20log(d)
Continue to page 2
Reprints
