Wireless connectivity is essential in smart meters for electricity, water, gas, and community heating distribution networks, but designing a wireless transceiver from scratch is challenging and time-consuming. Smart meter applications demand high-performance wireless solutions that meet a variety of international standards, including FCC part 15 and part 90 in the U.S., ETSI EN 300 220, ETSI EN 303 131 in Europe, ARIB STD T67, T108 in Japan, and SRRC in China. They need to support data rates up to 500 kilobits per second (kbps). They must include secure encryption and authentication, be compact, and operate in challenging environments up to +85°C. Many applications require a battery life of several years.
To meet those challenges, designers can choose from RF transceiver ICs or complete RF transceiver modules, depending on the needs of the smart meter application. RF transceiver ICs are available that guarantee an RF link budget in excess of 140 dB with output power up to +16 dBm and that support SIGFOX™, Wireless M-Bus, 6LowPAN, and IEEE 802.15.4g networking connectivity. RF modules are available that support the Wireless M-Bus protocol stack or multiple radio modulations such as; LoRa, (G)FSK, (G)MSK, and BPSK; with options for adaptive bandwidth, spreading factor, transmission power, and coding rate to meet various application needs and are compliant with a breadth of international regulations including ETSI EN 300 220, EN 300 113, EN 301 166, FCC CFR 47 part 15, 24, 90, 101 and the ARIB STD-T30, T-67, and T-108. These modules are complete RF systems, needing only an antenna, and include secure encryption and authentication, and ultra-low power modes for extended battery life.
This article reviews the connectivity challenges confronted by designers of wireless smart meters and looks at possible solutions. It then presents a range of options, including RF transceiver ICs and RF modules from STMicroelectronics, Move-X, and Radiocrafts, along with design considerations when integrating the antenna.
One of the first decisions faced by designers is the selection of a communications protocol. Common choices include near field communications (NFC), Bluetooth, Bluetooth Smart, Wi-Fi for the Internet of Things (Wi-Fi for IoT), and Sub Gigahertz (SubGHz). There are four important factors to consider:
- Required data throughput
- Low power modes
- Required transmission range
- Need for web access
Wi-Fi for IoT can be the best choice for applications that need the maximum data transfer, but it also has the highest power requirements. While SubGHz requires only moderate power and delivers the maximum transmission range, other communications protocols offer varying sets of performance tradeoffs (Figure 1).
Figure 1: Wi-Fi for IoT has the greatest throughput and power consumption, while SubGHz offers the most extended range with moderate power demands. (Image source: STMicroelectronics)
Many smart meter applications demand multiyear battery lives, making it challenging to use a technology like Wi-Fi for IoT. Fortunately, these applications also have relatively limited data throughput requirements and can benefit from using NFC, Bluetooth Smart, Bluetooth, or SubGHz technologies. While NFC has attractively low power consumption, its equally low data throughput and range can eliminate it from consideration in smart meter applications.
In addition, the overall design of the smart meter is vital in determining power consumption. Keeping the device in a low power state as long as possible and entering an active state for the shortest needed time is a key factor in extending battery life in wireless smart meters. The choice between using a module-based or discrete radio frequency (RF) communications implementation is another factor in the design’s success. When making that decision, consider performance, solution size, footprint flexibility, certifications, time-to-market, and cost requirements.
Benefits of using an RF module
An RF module is a complete communications subsystem. It can include an RF IC, oscillator, filters, power amplifier, and various passive components. No RF expertise is needed to use a module solution, enabling designers to focus on other aspects of the smart meter design. A typical RF module arrives calibrated and certified to the required standard(s). In addition, the module will include the network matching circuitry to ease the integration of the antenna and minimise any signal loss. The antenna can be internal or external with module solutions.
Modules are simple to integrate into the design. The simplicity of design integration extends to the manufacturing process flows since there are no complex discrete RF devices to handle, just a standard printed circuit board (PCB)-based module. The module maker has already handled all the nuances of integrating RF systems. Using a module reduces the risk associated with a discrete RF design, such as gaining certifications, achieving required efficiency and overall performance levels, and speeding time-to-market.
Benefits of discrete IC implementations
Although they are more complex, discrete IC designs can offer important benefits in terms of cost, solution size, and form factor. A module will be more expensive in most instances than an IC-based solution. In instances where the RF subsystem design is used in high volumes, the added cost of designing the IC-based solution is compensated by lower manufacturing costs. It’s also possible to use a common RF subsystem across multiple wireless smart meter platforms, increasing overall production volumes and further reducing long-term costs.
A discrete IC-based design is almost always smaller than a module-based solution. That can be an important consideration in space-constrained applications. In addition to occupying a smaller footprint, a discrete IC design can be more readily shaped to fit in the available space.
Sub GHz RF transceiver IC
Designers needing a discrete IC-based solution in the SubGHz band can turn to the S2-LP, a high-performance ultra-low power RF transceiver IC with an operating temperature range of -40°C to +105°C, in a 4 x 4 mm QFN24 package (Figure 2). The basic design operates in the industrial scientific and medical (ISM) license-free bands and the short range device (SRD) bands at 433, 512, 868, and 920 Megahertz (MHz). Optionally, the S2-LP can be programmed to operate at other frequency bands such as 413-479, 452-527, 826-958, and 904-1055 MHz. A variety of modulation schemes can be implemented, including 2(G)FSK, 4(G)FSK, OOK, and ASK. The S2-LP has an RF link budget > 140 dB for long communication ranges and meets regulatory requirements in the United States, Europe, Japan, and China.
Figure 2: This RF IC is specified for operation to +105°C and is packaged in a 4 x 4 mm QFN24. (Image source: STMicroelectronics)
To simplify the integration process when using the S2-LP, designers can use the BALF-SPI2-01D3 ultra-miniature balun with a 50 Ω nominal input that is conjugate matched to the S2-LP for 860 – 930 MHz frequency operation. It integrates a matching network and harmonics filter and uses integrated passive device (IPD) technology on a non-conductive glass substrate to provide optimised RF performance.
Designs using the S2-LP and operating in the 868 MHz ISM band can be developed using the X-NUCLEO-S2868A2 expansion board (Figure 3). The X-NUCLEO-S2868A2 connects to the STM32 Nucleo microcontroller using serial peripheral interface (SPI) connections and general purpose input-output (GPIO) pins. Adding or removing resistors from the board can change some GPIOs. Additionally, the board is compatible with Arduino UNO R3 and ST morpho connectors.
Figure 3: The X-NUCLEO-S2868A2 expansion board can speed the development of designs using the 868 MHz ISM band. (Image source: DigiKey)
RF module simplifies integration
For applications that require fast time-to-market and low power consumption, the MAMWLE-00 module can simplify system integration. It uses a 50 Ohm U.FL connector for the RF output and has a 48 MHz Arm Cortex M4 32-bit RISC core in a 16.5 x 15.5 x 2 mm package. This RF module has several choices of low-power operation states. It implements multiple radio modulations, including LoRa, (G)FSK, (G)MSK, and BPSK, with different options for bandwidth, spreading factor (SF), power, and coding rate (CR) (Figure 4). An embedded hardware encryption/decryption accelerator can implement various standards such as advanced encryption standard (AES, both 128 and 256 bits) and public key accelerator (PKA) for PKA for Rivest-Shamir-Adleman (RSA), Diffie-Hellmann, or Elliptic Curve Cryptography (ECC) over Galois fields.
Figure 4: The MAMWLE-00 module gives designers choices for power-saving modes and various RF modulation standards. (Image source: DigiKey)
M-Bus RF module
Using the M-Bus wireless protocol, designers can turn to Radiocrafts’ RC1180-MBUS RF transceiver module that measures 12.7 x 25.4 x 3.7 mm in a shielded surface-mounted package (Figure 5). This RF module has a one-pin antenna connection and a UART interface for configuration and serial communications. It meets the Wireless M-Bus specification S, T, and R2 modes, operates in 12 channels in the 868 MHz frequency band, and is pre-certified for operation under the European radio regulations for license-free use.
Figure 5: The M-Bus wireless protocol can be implemented using Radiocrafts’ RC1180-MBUS RF transceiver module (Image source: DigiKey
The RC1180-MBUS3-DK sensor board with an M-Bus radio module development kit makes it simple for designers to quickly evaluate the onboard sensor module, tune the application, and build prototypes. It includes two 50 Ω quarter-wave monopole antennas with SMA male connectors, two USB cables, and a USB power supply (Figure 6). This development kit can be a concentrator, gateway, and/or receiver for the sensor board.
Figure 6: This M-Bus development kit includes two 50 Ω quarter-wave monopole antennas with SMA male connectors, two USB cables, and a USB power supply (not shown). (Image source: DigiKey)
Antenna integration
When connecting an antenna to an RF module, Radiocrafts recommends that the antenna be connected directly to the RF pin, which is matched to 50 Ohms (Ω). If it’s not possible to connect the antenna to the RF pin, the PCB trace between the RF pin and the antenna connector should be a 50 Ω transmission line. In the case of a two-layer FR4 PCB with a dielectric constant of 4.8, the microstrip transmission line width should be 1.8 times the thickness of the board. The transmission line should be on the top side of the PCB with a ground plane on the bottom of the PCB. For example, when using a standard 1.6 mm thick, two-layer FR4 PCB, the width of the microstrip transmission line should be 2.88 mm (1.8 x 1.6 mm).
A quarter-wave whip antenna is the most straightforward implementation and has a 37 Ω impedance when used above a ground plane, and a 50 Ω matching circuit is not usually needed. Alternatively, a PCB antenna can be fabricated using a copper trace with the ground plane removed from the PCB back side. There should be a ground plane on the rest of the PCB, optimally as large as the antenna to act as a counterweight. If the PCB antenna is shorter than a quarter wave, a 50 Ω matching network should be added.
Summary
When selecting between various wireless protocols for use in wireless smart meters, designers need to consider several factors, including the data throughput, power consumption, transmission range, and the need for web access. In addition, the choice between RF ICs and modules involves tradeoffs between solution size, cost, flexibility, time-to-market, regulatory compliance, and other factors. Once the appropriate RF protocol has been identified, the choice made between ICs and modules, and the basic RF system designed, antenna integration is critical to developing a successful wireless smart meter.
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