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Don't Be Intimidated By Low-Power RF System Design

Oct. 8, 2009
Take advantage of the multiple short-range wireless technologies available to maximize your

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-Works21847|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)

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Wavelength (λ) and range or distance are both in meters. Both formulas deliver approximately the same figures. Remember, this is free space loss without obstructions. The loss increases about 6 dB for each doubling of the distance.

If obstructions are involved, some corrective figures must be added in. Average loss figures are 3 dB for walls, 2 dB for windows, and 10 dB for exterior structure walls.

When finalizing a path loss, add the fade margin. This “fudge factor” helps ensure good link reliability under severe weather, solar events, or unusual noise and interference. As a result, transmitter power and receiver sensitivity will be sufficient to overcome these temporary conditions.

A fade margin figure is just a guess. Some conservative designers say it should be 15 dB, while others say 10 dB is acceptable. If unusual weather or other conditions aren’t expected, you may get away with less, perhaps 5 dB. Add that to your path loss and adjust everything else accordingly.

Another handy formula to help estimate your needs is the Friis formula:

PR = PTGRGTλ2 /(16π2d2) PR is the received power in watts, PT is the transmit power in watts, GR is the receive antenna gain, GT is the transmit antenna gain, λ is the wavelength in meters, and d is the distance in meters. The transmit and receive gains are power ratios. This is 1.64 for a dipole or ground plane antenna. Any directional antenna like a Yagi or patch will have directional gain. It is usually given in dB, but it must be converted to a power ratio. The formula also indicates why lower frequency (longer wavelength) provides greater range (λ = 300/fMHz).

Transmitter output power, another key figure, is usually given in dBm. Some common figures are 0 dBm (1 mW), 10 dBm (10 mW), 20 dBm (100 mW), and 30 dBm (1 W). Receiver sensitivity also is usually quoted in dBm. This is the smallest signal that the receiver can resolve and demodulate. Typical figures are in the –70- to –120-dBm range.

One last thing to factor in is cable loss. In most installations, you will use coax cable to connect the transmitter and receiver to the antennas. The cable loss at UHF and microwave frequencies is surprisingly high. It can be several dB per foot at 2.4 GHz or more. So, be sure to minimize the cable length.

Also, seek out special lowerloss cable. It costs a bit more, but coax cable with a loss of less than 1 dB per foot is available if you shop around. This is especially critical when using antennas on towers where the cable run could be long. You can offset the loss with a gain antenna, but it’s still optimal to minimize the length and use the best cable.

With all of this information, compute the final calculation:

Transmit power (dBm) + transmit antenna gain (dB) + receive antenna gain (dB) – path loss (dB) – cable loss (dB) – fade margin (dB)

This figure should be greater than the receiver sensitivity. Now play with all of the factors to zero in on the final specifications for everything. Two design issues remain— the antenna and its impedance matching.

The antenna requires a separate discussion beyond this article. There are many sources for antennas. A wireless module most likely will come with an antenna and/ or antenna suggestions. The most common is quarter-wave or half-wave vertical. When building an antenna into the product, the ceramic type is popular, as is a simple copper loop on the printed circuit board (PCB). Follow the manufacturer’s recommendations for the best results. If it’s a single-chip design, you may need to design the impedance matching network between the transceiver and the antenna. Most chip companies will offer some recommendations that deliver proven results. Otherwise, design your own standard L, T, or π LC network to do the job.

One final hint about testing: Part 15 uses field strength to indicate radiated power measured in microvolts per meter (µV/m). A field strength meter makes the measurement at specified distances. The result can be converted to watts to ensure the transmitter is within the rules. The following formula, which is a close approximation, lets you convert between power and field strength:

V2/120π ≈ PG/4πd2

where P is transmitter power in watts, G is the antenna gain, V is the field strength in µV/m, and d is the distance in meters from the transmit antenna to the field strength meter antenna. A simplified approximation at a common FCC testing distance of 3 m with a transmit antenna gain of one is P ≈ 0.3 V2.

SOME EXAMPLE PRODUCTS FreeWave Technologies has a line of reliable, high-performance spread-spectrum and licensed radios for critical data transmissions. The high-speed MM2- HS-T (TTL interface) and MM2-HS-P (Ethernet interface) come ready to embed in OEM products like sensors, remote terminal units (RTUs), programmable logic controllers (PLCs), and robots and unmanned vehicles. They operate in the 900-MHz band and use direct-sequence spread spectrum (DSSS).

Thanks to the radios’ over-the-air speed of 1.23 Mbits/s, users can send significantly more data in a shorter period of time. The MM2-HS-T is ideal for embedded applications that require high data rates, such as video and long distances (up to 60 miles). Both radios fit many industry, government, and military applications where it’s necessary to transmit large amounts of data, including multiple high-resolution images and video along with data.

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The MM2-HS-T measures 50.8 by 36 by 9.6 mm and weighs 14 g (Fig. 1). The MM2-HS-P shares a similarly small footprint. Both radios offer RISC-based signal demodulation with a matched filter and a gallium-arsenide (GaAs) FET RF front end incorporating multi-stage surface-acoustic-wave (SAW) filters. The combination delivers unmatched overload immunity and sensitivity.

The MM2-HS-P includes industrialgrade high-speed Ethernet that supports TCP, industrial-grade wireless security, and serial communications. Each unit can be used in a security network as a master, slave, repeater, or master/slave unit, depending on its programming. Free- Wave’s proprietary spread-spectrum technology prevents detection and unauthorized access, and 256-bit AES encryption is available.

The ADF7022 and ADF7023 lowpower transceivers from Analog Devices fit well in smart-grid and other applications operating on the short-range ISM band for remote data measurement. Smart-grid technology not only measures how much power is consumed, it also determines what time and price are best to save energy, reduce costs, and increase reliability for the delivery of electricity from utility companies to consumers. RF transceivers are needed for the secure and robust transmission of this information over short distances, for storing measurement data, and for communicating with utility computers over wireless networks.

Applications for the ADF7022 and ADF7023 include industrial monitoring and control, wireless networks and telemetry systems, security systems, medical devices, and remote controls. Analog Devices’ free, dowloadable ADIsimSRD Design Studio supports both devices.

One particular hot area for RF transceivers involves utilities that are building advanced metering infrastructures, including automatic meter reading, to monitor and control energy usage. Analysts expect more than 150 million smart meters to be installed worldwide. The ADF7022 and ADF7023 target these smart-grid and home/building automation applications.

The ADF7022 is a highly integrated frequency-shift-keying/Gaussian frequency- shift-keying (FSK/GFSK) transceiver designed for operation at the three iohomecontrol channels of 868.25, 868.95, and 869.85 MHz in the license-free ISM band. It fully complies with ETSI-300-200 and has enhanced digital baseband features specifically designed for the io-homecontrol wireless communications protocol.

As a result, the device can assume complex tasks typically performed by a microprocessor, such as media access, packet management/validation, and packet retrieval to and from data buffer memory. This allows the host microprocessor to remain in power-down mode. Also, it significantly lowers power consumption and eases both the computational and memory requirements of the host microprocessor.

The ADF7023 low-IF transceiver operates in the license-free ISM bands at 433, 868, and 915 MHz. It offers a low transmit- and-receive current, as well as data rates in 2FSK/GFSK up to 250 kbits/s. Its power-supply range is 1.8 to 3.6 V, and it consumes less power in both transmit and receive modes, enabling longer battery life.

Other on-chip features include an extremely low-power, 8-bit RISC communications processor; patent-pending, fully integrated image rejection scheme; a voltage-controlled oscillator (VCO); a fractional-N phase-locked loop (PLL); a 10-bit analog-to-digital converter (ADC); digital received signal-strength indication (RSSI); temperature sensors; an automatic frequency control (AFC) loop; and a battery- voltage monitor.

The CC2530 from Texas Instruments is a true system-on-a-chip solution (SoC) tailored for IEEE 802.15.4, ZigBee, Zig- Bee RF4CE, and Smart Energy applications. (RF4CE is the forthcoming wireless remote-control standard for consumer electronics equipment.) Its 64-kbyte and up versions support the new RemoTI stack for ZigBee RF4CE, which is the industry’s first ZigBee RF4CE-compliant protocol stack.

Larger memory sizes will allow for on-chip, over-the-air download to support in-system reprogramming. In addition, the CC2530 combines a fully integrated, high-performance RF transceiver with an 8051 MCU, 8 kbytes of RAM, 32/64/128/256 kbytes of flash memory, and other powerful supporting features and peripherals (Fig. 3).

The TI CC430 wireless platform consists of TI radio chips. Also, the company’s MSP430 16-bit embedded controller can implement the IETF standard 6LoWPAN, which is the software that enables 802.15.4 radios to carry IPv6 packets. Thus, low-power wireless devices and networks can access the Internet. Furthermore, the platform can implement Europe’s Wireless MBus technology for the remote reading of gas and electric meters.

See figure 2

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