Meeting the Challenges of Wireless Metering

August 13, 2019 Arwyn Roberts, CML Microcircuits

The word ‘smart’ is perhaps overused these days, but nevertheless, it now occupies a space in the common lexicon that is synonymous with modern, advanced and efficient. The development of Smart Meters meets this definition and they have evolved over the last two decades or so to become an integral part of utility delivery. Smart meters provide live updates on the use of utilities such as gas, water and most commonly electricity, which gives the home owner or business user a better idea of their own personal consumption, as well as enabling the utility provider to better manage their services.

That’s the public face of smart metering, but behind the scenes it involves much more than simply removing a ‘dumb’ meter with a mechanical dial and replacing it with a ‘smart’ meter featuring an LCD/LED readout. The smarts involved have seen metrology become first a differentiator and then a standard feature for providers. Early adopters had a competitive advantage that conveyed some level of lock-in with a provider, something that governments were keen to avoid. Legislation or other schemes were put in place that ensured consumers were once again free to move suppliers without the fear of losing their smart meter benefits.

Relatively quickly, the consumer’s option to have a smart meter has turned into a supplier’s obligation to provide one, often with some deadlines in place through legislation. The cost of supplying a smart meter for a utility supplier must be shouldered, even with the knowledge that the consumer may switch provider before they see a return on that investment. For this reason, smart meters need to be designed with both cost and longevity in mind, which means new meters need to be even more robust and reliable than their predecessors.

For many reasons, wireless connectivity is the preferred form of local communications. This may take many shapes, including cellular and unlicensed (ISM) solutions. As 5G rolls out across nations it may become a viable solution for smart metering, however, today the coverage and cost would prohibit this. While GSM/3G/4G is viable, the demand for bandwidth coupled with the density of devices operating in both licensed and unlicensed bands is putting pressure on many kinds of radio devices already in the field. As the spectrum becomes more congested it introduces problems that, although the protocols are often equipped to address, can reduce the efficacy of the solution. For this reason, meter manufacturers and component suppliers are working together to combat the rising challenge presented by two factors; the reallocation of GSM bandwidth and the continued congestion of ISM bands.

Metering and the IoT

Wireless metering networks could be viewed as a pre-runner to the wider Internet of Things (IoT); a neighborhood may contain many smart meters that connect to a local aggregator, or gateway, using a variety of wireless protocols. In turn, the aggregator would communicate back to the network over some other form of connection, which may be wired or wireless. Maintaining what can already be seen as legacy technology, particularly as the local area becomes more congested in RF terms, can be difficult.

Typically, the meter would use some form of system-on-chip based on a microcontroller with integrated RF functionality operating in the 902 – 928MHz band. While these types of devices meet the technical requirements for smart metering, their output power is often limited to between 10mW and 100mW. In order to operate in a local area network, the output power of the smart meter may have initially been relatively low, but as the RF spectrum becomes more congested, the link budget can suffer, causing disruptions to communications. In some IoT applications, this may be tolerable, but in the energy sector where real-time data is used to calculate the actual cost and network demand, loss of data or communications becomes mission-critical. One simple but effective way of combating this problem is to increase the output power of the smart meter, while staying within the allowable power budget for a given wireless standard.

In any design, the output of RF SoC would feed into a power amplifier (PA), where the gain would boost the signal to a power level in the region of 1W, or 30dBm. While the output power of ISM bands is limited in some regions, this approach gives a much higher link budget to cover a wider area and also overcome losses associated with hard to reach places such as basements. It is good practice to limit the amount of gain in a single amplification stage in order to maintain stability. For a gain of 30dB or so in the UHF range, that would typically correspond to a 3-stage PA. Adding an integrated 3-stage power amplifier to the output of an RF SoC will extend the range of the radio-link without incurring significant design expense.

Selecting the Right PA

While replacing the output power amplifier stage may be a simple workaround to extending the useful range of an existing smart meter design, it is also good practice when developing a new design to build in some flexibility and extensibility. The key to both of these approaches is selecting the right power amplifier.

The CMX901 is a broadband 3-stage power amplifier able to operate over a frequency range of 130MHz to 950MHz, with an output power of up to 2.5W at 160MHz and 1.5W at 915MHz. Operating from a single supply voltage of between 2.5V and 6V, it can deliver 40dB of gain for VHF/UHF signals using a range of modulation schemes, including FSK, FFSK/MSK, GFSK/GMSK and multi-level FSK. This makes it suitable for a number of RF applications, including extending the range of a smart meter.

It employs different amplifier modes across each stage; the first stage is biased as Class A, while the second stage is Class AB. The third stage is a Class C amplifier and together, the three stages deliver optimum efficiency. The device features dedicated power rails for each of the stages, and the overall power output can be modulated by adjusting the voltage, VPARAMP, which in turn adjusts VGS1 and VGS2; these are effectively a product of using the recommended circuit layout to set the supply pins for stages 1 and 2, which are connected together via resistors to create VPARAMP.

The small outline and power requirements of the CMX901 make it an ideal choice for old and new designs.

Conclusion

Smart metrology is still an active and growing market segment, as the IoT continues to expand, it brings benefits through automation. Automated meter reading is an early example of how centralized monitoring can be a benefit, but as the useful wireless spectrum becomes more crowded, it will become more necessary to adjust design practices to overcome the challenges that success presents.

By adding a flexible power amplifier to the output stage of an RF SoC, manufacturers can extend the lifetime of their proven designs and build-in some future-proofing to new designs. The CMX901 is a good example of how a multi-stage power amplifier can deliver a simple solution to a growing problem.

Arwyn has 20 years of experience in the RF semiconductor industry and honed his extensive broad knowledge through Field Applications support around the world. As RF Product Manager, Arwyn is responsible for all aspects of CML’s RF Building Block product range. In his spare time (of which there is little) Arwyn likes to keep fit with a variety of activities including kayaking on the river Wye.

Previous Article
How to Choose an Embedded Operating System

Almost all modern embedded systems are built using an operating system (OS) of some sort. This means that t...

Next Article
Introduction to Real-Time Operating Systems (RTOS)

One of the most important components that go into today’s embedded systems is the “RTOS” or “real time oper...