Low Power Strategies
This chapter presents architectural considerations for the proposed transceiver. Four main aspects are identified as essential for ultra-low power consumption. First, the definition of the receiver architecture is a fundamental issue as it affects the str
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Low Power Strategies
3.1 Zero-IF RX Architecture The definition of the receiver architecture is a fundamental design decision which drastically impacts the achievable power consumption. To select the appropriate receiver architecture, the requirements of the receiver in terms of spectral selectivity have to be taken into account. The Bluetooth low energy standard operates in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band, which is shared with many other services. Therefore, in-band interferers have to be suppressed sufficiently to allow for correct demodulation of the GFSK input signal. In the BLE standard, interferers with an input power level 17 and 27 dB higher than the desired signal power must be tolerated at an offset frequency of 2 and 3 MHz or more, respectively [3]. Assuming a signal-to-noise ratio (SNR) of the demodulator of 15 dB, the interference suppression at these frequency offsets has to be at least 32 and 42 dB, respectively. This requirement already disqualifies the super-regenerative receiver architecture for the targeted application. Narrow-band receivers for low power applications usually employ either a low intermediate frequency (IF) [57, 119] or a zero-IF down-conversion receiver [24, 109]. The low-IF architecture, shown in Fig. 3.1a, down converts the received signal to an IF which is usually on the order of the signal bandwidth, i.e., a few MHz. Then the adjacent channel interferers are filtered out by means of a complex band-pass filter, which not only removes the interferers at directly adjacent channels but also interference at the negative IF, also referred to as image frequency. The principal advantage of the low-IF architecture is that at no point signals around DC (0 Hz) are processed and so DC offset and flicker-noise problems are circumvented. On the other hand, to suppress the interferers at the image frequency accurate quadrature signals from the local oscillator are needed [48]. The achievable image rejection ratio (IRR) for a given quadrature accuracy can be calculated from (ΔA/A)2 + Δψ 2 1 = IRR 4 J. Masuch and M. Delgado-Restituto, Ultra Low Power Transceiver for Wireless Body Area Networks, Analog Circuits and Signal Processing, DOI: 10.1007/978-3-319-00098-5_3, © Springer International Publishing Switzerland 2013
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3 Low Power Strategies
where ΔA/A is the relative amplitude error and Δψ is the phase error of the quadrature LO signals [106]. Hence, to obtain an image rejection of 42 dB for example, only a phase error of Δψ = 0.9◦ can be tolerated if perfectly matched amplitudes are assumed. If amplitude errors were taken into account, the tolerable phase error would be even lower. To relax this stringent accuracy requirement for the LO, the BLE standard defines an exception of the interference rejection at the image frequency which permits interferers that are only 9 dB above the desired signal level instead of 27 dB [3]. This reduces the required IRR to 24 dB, which translates to a tolerable phase error of 7.2◦ (assuming equal amplitudes again)
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