Page 183 - System on Package_ Miniaturization of the Entire System

158 Cha pte r F o u r

components becomes less important than the value of each of these components. In
contrast, designs utilizing discrete passives are more concerned with the total number
of passives and not the individual values of each; the cost of assembly depends only on
the number of discrete elements to be soldered on board, and the packaged size for
different values of capacitance or inductance usually remains the same for commercially
available discrete devices. This requires novel designs for RF components using these
embedded passives, with priorities shifting from reducing the component count to
keeping the value of the passives low.
Noise coupling between digital and analog circuits remains a problematic issue
even in SOP-based implementations. Although coupling through the silicon substrate
of an SOC chip has been minimized, new noise sources, like electromagnetic
interference (EMI) from high-speed signal lines and fluctuations in the power plane,
arise and have to be dealt with through careful modeling and analysis.
Wireless radios have incorporated multiple standards and hence have become
multiband devices. Apart from noise sources, next-generation mixed-signal systems
present many architectural challenges as well. Multiband functionality in components
can be achieved in numerous ways. For example,
1. Implementation of devices with wide bandwidths capable of operating at
multiple frequency bands.
2. Implementation of multiple single-band devices with matching networks at the
input and output producing a single-input–single-output (SISO) component.
Each single-band device has a narrow operating bandwidth, and multiband
operation is achieved for the component by switching between the different
single-band circuits. In these implementations, the component outputs only a
single frequency at any given time [7].
3. Implementation of concurrent devices that achieve simultaneous multiband
functionality and require sampling one or more frequencies at a time depending
on the application [8].
Option 1 is difficult to implement due to technological concerns and the presence of
large blocker signals close to the frequency bands of interest. Option 2 is a component-
scale replica of the multiband architecture, and as such exhibits problems such as large
size and high power consumption. In contrast, option 3 (the use of true multiband
devices) leads to lower power consumption and a vastly reduced footprint.
The passive devices like antennas, filters, and baluns can be completely integrated
in the package. For the circuit components lying at the interface of the chip and package
domains (LNA, VCO, and PA), design partitioning and optimization is required, to take
into account the parasitics involved in a chip-package signal transition. Examples of
PAs with the output matching networks implemented using embedded passives and
VCOs with high-Q embedded inductors for improvement in phase noise has been
reported in the literature [9–11].
It is possible to achieve complete integration by integrating passives in the package.
However, this approach neglects the fact that compared to on-chip inductors with low
Q values and discrete passives with fixed Q values, the use of embedded passives
leads to the development of the passive Q as a new variable in circuit design. With Q
values ranging from 20 to 200 [12], designers now have a choice in the Q value they
need for a particular component. Higher Q values result in new tradeoffs, particularly
with respect to device size, and a design partitioning and optimization strategy is thus