Wireless Short Range Devices and Narrowband Communications
By Conor O’Mahony, Applications Engineer, Analog Devices

Introduction
The term “Short Range Device” (SRD) covers radio transmitters which provide either unidirectional or bi-directional communication and which have a low capability of causing interference to other radio equipment. Due to the many different services provided by these devices, there is no exhaustive list covering the SRD application space, however, the following categories are amongst those covered:

• Telecontrol for Home/Building Automation Systems
• Automatic Meter Reading
• Wireless Sensor Applications
• Medical Telemetry
• Alarms
• Automotive, including Remote Keyless Entry & Remote Car Start Applications

When designing a SRD wireless system, careful consideration needs to be paid to the choice of frequency on which your radio will be communicating over and also the specified frequency plan required by the regulations in that particular region. Many of the worldwide regulations governing SRDs stipulate narrow communication channels, which significantly limits the transmit bandwidth (to 25kHz or less) and thus, the maximum data throughput.

Historically, for such applications, discrete transmitters and receivers would have been used to meet the difficult performance requirements needed in such narrowband applications. However, in recent years, fully integrated radio solutions like the ADF7021 are providing similar or better performance than discrete solutions at a fraction of the cost and size.

Narrowband Communications
Why Choose Narrowband?
In many cases, the radio regulations in a particular region will stipulate that communication channels have to be narrowband. A narrowband channel is generally defined as a channel bandwidth = 25 kHz. Such narrow channelizations are needed in many cases due to a lack of bandwidth and so, there is a need to fit as many channels into a given bandwidth as possible. For example, in Japan many of the ISM bands are less than 1MHz. Table 1 lists the four main worldwide regulations covering narrowband short range communications and describes the various RF operating frequencies and channel spacings.

Narrowband Performance Specifications
Due to the narrowband nature of the channels, there are various difficult specifications to meet for both transmit and receive. A description of some of these specifications is given below:

Transmitter Specifications
• Adjacent Channel Power (ACP): The amount of spectral energy leaking into adjacent channels
• Frequency Drift: The amount the transmitter will drift from its nominal operating frequency
  under extreme test conditions (voltage and temperature)
• Spurious Emissions: Emissions at frequencies other than those of the wanted carrier
  frequency and its sidebands

Receiver Specifications
• Adjacent Channel Rejection (ACR): The ability of the receiver to reject an interferer located
  in an adjacent channel
• Blocking: The ability of the receiver to reject out of band interferers (usually at 1- 10 MHz)

ADF7021:
The Narrowband Solution
The ADF7021 is the only available solution that covers all of the various sub 1 GHz worldwide narrowband operating frequencies. It is a low power, highly integrated BFSK/3FSK/4FSK transceiver designed for operation in the narrowband license-free ISM bands and licensed bands in the frequency ranges 80-650 MHz and 862–940 MHz. It has both Gaussian and Raised Cosine data filtering options to improve spectral efficiency for narrowband applications. It is suitable for circuit applications targeted at European ETSI-EN300-220, the Japanese ARIB-T67, the Chinese Short Range Device regulations and the North American FCC Part 15, Part 90 and Part 95 regulatory standards.

It provides excellent sensitivity, as low as -125 dBm at 1kbps, which adds several dB to the link margin and as a result, increases communication range. A low IF architecture is used in the receiver (100 kHz), minimizing power consumption and the external component count while avoiding interference problems at low frequencies. The IF filter has programmable bandwidths of 12.5 kHz, 18.75 kHz and 25 kHz. The ADF7021 supports a wide variety of programmable features, including Rx linearity, sensitivity, and IF bandwidth, allowing the user to trade off receiver sensitivity and selectivity against current consumption, depending on the application. A detailed block diagram of the ADF7021 is shown in Figure 1.

The ADF7021 has several features which aid in meeting various regulatory standards. To mitigate against spectral splatter when turning the Power Amplifier (PA) on and off, there is an automatic PA ramp feature, which has eight programmable settings. There is an automatic frequency control (AFC) feature, which allows real time tracking of a frequency error in the received signal for a maximum ±50 kHz error, thus allowing the use of low cost crystals. A summary of the ADF7021 specifications is given in Table 2.

To aid in meeting the difficult transmission masks and ACP specifications of narrowband regulations, the ADF7021 supports Gaussian and Raised Cosine data filtering and advanced modulation schemes like 3-Level FSK and 4-Level FSK. These features greatly increase the amount of data throughput that can be achieved in the narrow transmission bandwidths. Figure 2 highlights the use of Gaussian and Raised Cosine filtering in meeting a particular transmission mask in a narrow band application.

ANALOG DEVICES
www.analog.com
TXTLINX.COM101
Email this article to a friend!