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As North American electric, water and gas utility companies work on modernizing their grids, they often choose the unlicensed 900MHz ISM frequency band to deploy their wireless advanced meter Infrastructure (AMI) networks. Using this frequency band is attractive because utilities do not need to purchase, or even license, their own spectrum. The FCC allows up to 1W (30dBm) of RF Tx power in the 900MHz ISM band. This enables designers to widen their system’s coverage area reducing installation density and lowering total system cost, which is critical for utility suppliers.

One of the challenges engineers face when designing a wireless AMI network is how to power the embedded wireless meter transmitter unit (MTU) co-located at the smart meter. When transmitting a message burst, the terminal demands a supply of current that could approach 1A, depending on the supply voltage and the transmitter’s RF Power Amplifier (RF PA) efficiency.

One would think that powering the MTU would not be an issue in electricity meters where power is readily available. Electric utility companies, however, aren’t always willing to share their network infrastructure or strictly limit how much energy an MTU can draw from the electric grid. System designers have come up with a scheme where small amounts of current are drawn from the electric grid and stored in a super capacitor (i.e. supercap) until enough energy has been accumulated to power the MTU’s RF PA for the duration of one or multiple transmission bursts, depending on its capacity and system requirements.

A single-cell super capacitor (SSC) is nominally rated at 2.7V, but its voltage will eventually fall below 2V as its stored energy is depleted by the transmitter. Most RF PA’s are designed to operate from voltage supplies ranging from 2.5V – 5V or even narrower, in some cases. So, when the system designer decides to use these RF PA’s in an SSC powered meter reading application, the designer has to: a) Use the Power Amplifier outside its specified supply range or b) use a series supercap module to support the RF PA supply voltage, but this adds cost and complexity to the solution.

Table 1 below shows several popular 1W 900MHz Power Amplifiers and front-end modules. It also shows 1W Power Amplifiers are normally specified to deliver ~1W from voltage sources higher than 3.6V supplies, and their output power falls well below the targeted 1W when running from a 2.7V supply.

Given this, engineers are forced to accept a lower output power or use higher power devices such as the RF5110G or GRF5509, shown in table 2, and run them from a 2.7V supply to obtain a full Watt (or more) of output power. Of course, these PA devices are considerably more expensive.

Table 2
 

Finally, PA manufacturers often design their devices to support multiple frequency bands (i.e., VHF, UHF, 900MHz, etc.) requiring external input/output matching networks which increases external component count, size, and component and manufacturing costs.

Unlike electric meters, water meters are normally installed far away from any source of electricity, so they must be powered from permanently installed batteries that are never re-charged. In addition, utilities demand batteries to last for at least 10 years to avoid sending crews to periodically replace them. Lithium-thionyl-chloride (LiSOCl2) batteries can store enough energy to last 10+ years and can sustain their voltage throughout their entire discharge life, but have two significant weaknesses for powering an RF PA:

1. A LiSOCl2 battery’s low source impedance limits the maximum current it can deliver and therefore it can’t supply the high current required to feed the MTU.
2. Increasing the current drawn from the battery significantly reduces its operating life. So, engineers must balance current draw vs. the desired operating life of the battery/system.

To meet the long operating life and high current requirements of an AMI transmitter, a LiSOCl2 battery is paired with a supercap which can store and then deliver the current surge needed by the transmitter’s RF PA. The battery is then used to slowly recharge the supercap for its next transmission. Because LiSoCL2 batteries are rated at 3.6V, a buck charging circuit is often employed to recharge the supercap. Alternatively, a series-connected supercap module can be used to eliminate the need for a buck charging circuit.

CML recently introduced the CMX90A003 Power Amplifier, which is a member of the company’s new SµRF family of products. Designed to operate from 1.9V – 3.0V, the device can deliver over 1W when powered by only 2.7V. In addition, the CMX90A003 integrates input and output matching network, dc blocking cap and power supply decoupling inductor, making it the lowest external part count design. This in turn translates into the smallest and lowest cost solution, which is very important in meter reading applications where saving every penny is important. For single cell Li-Ion battery powered applications, like the water meter case described earlier, or for applications that demand higher output power, CML also introduced the CMX90A004 Power Amplifier which is very similar to the CMX90A003 but can operate from 2.7V – 4.5V supplies and can deliver 32.5 dBm (~1.8W) from a 3.6V supply.

Tables 1 and 2 show that CML’s new 900MHz Power Amplifier’s compare favorably against other solutions in meter reading cost-sensitive applications where high levels of integration, low-voltage operation, and high output power are needed.