Low-Power 60-GHz CMOS Radios for Miniature Wireless Sensor Network Applications - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 187

Fig. 5 Micrograph of the Cymbet microbattery and its discharge curves
is typically used to accurately set the center frequency of the radio [ 19 ]. Reducing the
RF synchronization power consumption can greatly improve the WSN node lifetime
and alleviate the burden of cubic-mm-scale microbatteries.
mm-Scale Frequency References
A frequency reference provides a stable frequency over process, voltage, and temper-
ature (PVT) variations for synchronization of a communication system. The accuracy
requirement depends on the system specifications (e.g., frequency allocations, mod-
ulation type, etc.) [ 18 ], and can be achieved through different ways. A quartz crystal
is the most common source for a frequency reference. It provides excellent stability
with PVT variations. However, its size has not scaled down significantly with pro-
cesses or frequency. Furthermore, crystals require a piezoelectric process and special
packaging, which are incompatible with monolithic integration for ICs. In order to
achieve stable oscillation out of crystals, a certain amount of driving power is still
necessary [ 20 ]. Figure 6 shows a COTS WSN node, the TelosB [ 24 ]. The 32 kHz
crystal is highlighted with a red square, illustrating the volume and packaging of
these references. Therefore, bulky size and power become the bottlenecks for using
crystals at the mm scale. Recently, microelectromechanical systems (MEMS) and
complementary metal-oxide semiconductor (CMOS) references have been demon-
strated with near-crystal accuracies [ 20 - 23 ]. Table 1 shows a comparison of compact
frequency references made of crystal, MEMS and CMOS. These frequency accura-
cies are acceptable for typical WSN node specifications; however, powers are high
and none of their sizes are below 1 mm 3 . A compromise must therefore be made on
the frequency accuracy, in order to allow a stable frequency reference to be integrated
in CMOS at the mm scale, along with other WSN circuits, with a power consumption
that better matches what can be supported by a microbattery.