Biomedical Engineering Reference
In-Depth Information
Fig. 8
Atmosphere
attenuation of
electromagnetic wave
versus frequency
GHz, the guided wave length
λ
in silicon is about 2.5 mm; which is on the same
scale as the active circuit blocks implemented in CMOS (e.g., processor, memory,
and radio circuits). Furthermore, the elimination of expensive off-chip antennas,
path loss, crystal references, and the total size reduction makes 60-GHz operation
more appealing for wireless system integration. Sixty gigahertz is also referred to
as the oxygen absorption band [
31
]. As shown in Fig.
8
, the oxygen molecule (O
2
)
absorbs electromagnetic energy due to resonance at different frequencies. The exces-
sive attenuation causes high path loss at 60 GHz. Despite this increased attenuation,
the 60-GHz band can still be beneficial for short-range applications like WSNs and
wireless personal area networks (WPAN).
60-GHz Antenna-Referenced Frequency-Locked Loop
In order to eliminate the bulky off-chip components and further reduce the size of
a fully integrated WSN node, we will discuss a solution of using a 60-GHz on-
chip patch antenna as both the radiator and frequency reference in this section. The
natural resonant frequency of the patch antenna serves as the frequency reference.
The resonant frequency is mainly determined by the physical dimensions of the patch
antenna, and the standard deviation due to process variation is around 1,100 ppm.
This accuracy level is suitable for, e.g., low-rate communication with a noncoherent
energy-detection receiver, commonly used in ultralow power WSN radios. With
the antenna as the reference, an FLL circuit to track the reference frequency for
RF synchronization is described and implemented. Furthermore, the area beneath
the patch antenna ground plane is investigated for containing additional circuits for
higher integration. Figure
9
depicts the concept of a complete mm-scale WSN node.
By integrating the antenna, frequency reference, RF front end, and digital circuitry
onto the same die, a form factor of 1 mm
3
is feasible.
Search WWH ::
Custom Search