An efficient and state-of-the-art building and energy management system includes the automatic recording of consumer data for electricity, gas and water. It reduces costs and limits the susceptibility to errors while obviating the time-consuming manual on-site meter reading.

Advanced metering infrastructure (AMI) systems are used to record consumer data. They transfer this data by radio to the networks of the utility companies, where they are analyzed by energy management systems. The sensitivity and selectivity of the transceivers are crucial for ensuring a reliable radio link between the AMI and the network.

There are two kinds of AMI systems: simple transmit-only systems and more complex transceiver systems. Whereas transmitters send the data with dedicated timing, transceivers do so only after being polled by a unit that also confirms proper reception (see Fig. 1).

	FIGURE 1: TRANSMITTER AND TRANSCEIVER CIRCUITS
AMI transmitter with power amplifier and SAW filter. AMI transceiver with power amplifier and two SAW filters.

Modulation procedures

Various modulation schemes are used to assure reliable data transmission. As a rule, multi-channel applications employ either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) techniques for this purpose. In contrast, single-channel applications use amplitude shift keying (ASK) or frequency shift keying (FSK). The AMI systems must be able to handle these modulation procedures. But the RF front ends must also be highly sensitive as well as largely immune to interference. In practice, such interference may be generated by other RF applications such as mobile phones or amateur radio.

Existing AMI systems use SAW filters to suppress the harmonics and interfering emissions generated by the IC. They additionally ensure high selectivity, for example when the filter is located in the receive section of the AMI receiver directly behind or in front of the antenna.

A typical SAW filter for AMI uses.

Although some semiconductor manufacturers offer ICs that they claim need no SAW filters, the latter offer considerable advantages over the discrete filter solutions proposed to replace them. Even broadband SAW filters have greater selectivity at a lower insertion loss compared with LC filters, for example, which results in higher sensitivity (Fig. 2). Another advantage of SAW filters is their lower temperature coefficient.

	FIGURE 2: COMPARING SAW FILTERS WITH LC FILTERS
The selectivity of a broadband SAW filter (red) compared with a Chebyshev filter of third order (blue) based on LC components.

The role of the circuit topology

Higher selectivity always means increased insertion loss and thus poorer sensitivity – for both LC and SAW filters. The circuit topology plays a key role for the sensitivity of AMI systems. This is illustrated on the basis of four calculations (Examples 1 to 4). In each calculation, the noise factor F or the more usual logarithmic noise figure NF are used as a measure of sensitivity. The noise factor describes the signal-to-noise voltage ratio at the input of a four-poled element (quadripole) in relation to this ratio at its output:

or

where: S_i respects signal at the input S_o the signal at the output, N_i the noise at the input, and N_o the noise at the output of the quadripole. In the case of more than two quadripoles cascaded as in Fig. 1, the following applies for the overall noise factor F_1-n:

where:

n	number of quadripoles
Fn	noise factor
Gn	gain factor of the nth quadripole

The first quadripole, which is located directly behind the antenna, then plays a critical role. In principle, its noise figure already defines the range in which the overall noise figure is located.

For the sake of simplicity, the calculations of the overall noise figure in the corresponding example of switching topologies were carried out without transmit/receive (Tx/Rx) switches or baluns. The insertion loss IL of the SAW filters, the gain G, the noise figure NF and the noise factor F of the LNA as well as of the receiver IC all have the same values in these examples. It is also assumed that the gain of a SAW filter corresponds to its insertion loss, whereas the noise figure of a SAW filter assumes the negative value of its IL.

The preconditions for all four examples are:

L_SAW = G_SAW  =  -2.9 dB

G_{SAW (Linear)} = 0.513

NF_SAW = 2.9 dB → F_SAW = 1.95

G_LNA = 15 dB

G_{LNA (Linear)} = 31.62

NF_LNA = 1.5 dB → F_LNA = 1.41

NF_RxIC = 8 dB, NF_{RxIC (Linear)} = 6.31

All these examples have advantages and drawbacks. The configuration of Example 4 turns out to be the best solution for RF-based AMI systems with an overall noise figure of 5.37 dB. It is characterized by greater sensitivity and selectivity in combination with the improved common-mode rejection ratio resulting from the symmetrical operation of the second SAW filter.

EXAMPLE 1: SAW FILTER – RECEIVE IC

F1-2 = 12.3 => NF = 10.9 dB   The noise figure of the receiver IC has a great impact on the noise. The overall noise figure is 10.9 dB, the sensitivity of the receiver is significantly reduced. This structure has the advantage of blocking the interference signals outside the SAW band, which in turn protects the internal LNA of the receiver IC from going to saturation. In order to reduce the noise figure dramatically, a stage with a high gain factor and low noise figure must be placed in front of the SAW filter.

EXAMPLE 2: LNA – SAW – RECEIVE IC

F1-3 = 1.77 => NF = 2.48 dB   The overall noise figure NF is significantly lower, as the LNA with its very low noise figure is the first stage directly behind the antenna. Another advantage of this configuration is that the SAW filter simultaneously provides the functions of a balun. This allows both the common mode rejection ratio and also the selectivity to be increased. A drawback is that the receiver is more susceptible to strong interference, for instance from mobile phone signals. Especially when the AMI system transmits the consumer data to a polling base station via mobile phone, the strong signal can push the LNA into saturation during the transmission and prevent the receiver from recognizing the consumer data sent by another system in an meshed network.

	EXAMPLE 3: SAW – LNA – RECEIVE IC
F1-3 = 3.08 => NF = 4.88 dB   Switching to topology 3 strikes a compromise between lower noise figures and improved insensitivity to interference signals. The SAW filter placed directly behind the antenna protects the LNA and reduces the probability of it going to saturation.

	EXAMPLE 4: SAW – LNA – SAW – RECEIVE IC
F1-4 = 3.45 => NF = 5.37 dB   The shown extended version of a receiver front end operates with a noise figure of 5.37 dB. This is a significantly lower value compared with Example 1. Major advantages: high sensitivity and selectivity with simultaneously improved common mode rejection.

TYPICAL EPCOS SAW FILTERS FOR AMI APPLICATIONS

Reliable in all respects

AMI systems must not only be sensitive and selective, but must also operate with absolute reliability because they are often deployed in harsh environments. All their components must consequently be sufficiently rugged to withstand periodic changes in temperature and humidity as well as being resistant to shocks and vibrations. The SAW filters from EPCOS are qualified to AEC-Q200 and satisfy all these conditions. The standard published by the Automotive Electronic Council (AEC) for applications in the automobile sector is generally regarded as very rigorous. It is consequently well suited for the qualification of components to be used in harsh environments.

To satisfy this standard, AEC-Q200-qualified SAW filters are accommodated in a ceramic package on whose underside the quartz or lithium tantalate chip is cemented. The input and output pins (including the ground pins) are connected to the package with bonding wires. This structure assures that the active SAW structure is protected from most of the mechanical stresses affecting the package. The filter itself is hermetically sealed to protect it from humidity, i.e. the metal lid enclosing the package is welded to the upper side of the package. In addition to their great ruggedness, the electrostatic discharge (ESD) sensitivity of the SAW filter also plays a critical role for the reliability of the AMI system.

Author: Günther Schlegel, Application engineer SAW Wireless Terminals