Page 207 - System on Package_ Miniaturization of the Entire System

Mixed-Signal (SOP) Design 181

it enables lower noise, thereby leading to higher performance as measured by the BER.
With the trend toward integration, chip-package codesign is becoming a requirement
for RF circuits since the RF performance can degrade due to the semiconducting
properties of silicon ICs. For example, the integration of inductors using standard
silicon processes can generate eddy currents in the silicon substrate, thereby reducing
its Q. High Q inductor integration, however, is achievable in the package, and hence
placement of these inductors in the package can increase performance. In general, active
RF circuits contain a large number of passive components, consisting of inductors,
capacitors, and resistors. Resistors are mainly used for biasing, while the inductors and
capacitors are necessary for the circuit’s high-frequency operation. Though in general,
low-loss passive components are required for RF circuits, not every passive component
requires a high Q. Hence, during system partitioning, only the devices that require a
high Q can be integrated in the package. The passive components embedded in the
package can be concurrently designed with the circuit components on silicon, to meet
the performance specifications. Another exercise in chip-package codesign is the real
estate required for implementing the passive components. If the area occupied by the
passives is a large fraction of the transistor area, then it is not economical to integrate
these components in silicon and hence can be moved into the package.
In this section, two classes of RF circuits are designed using the chip-package
codesign methodology. The first example is a low-noise amplifier, which is the first
active stage in any receiver architecture. Here, the gate inductor is embedded in the
package for achieving a lower noise figure. This implementation is also more economical
since 70 percent of the real estate on silicon can be occupied just by the gate inductor.
The second example is a voltage-controlled oscillator (VCO). The phase noise of a VCO
is related to the power and Q of the passive components as [45]:

L{Δω ∝ F (4.9)
}
Q ⋅ P
2
o
For a VCO, low phase noise is required. This can be achieved either by increasing P ,
0
which is related to the direct current (dc) power or by increasing the Q of the passive
components. For a mobile application, achieving low power is necessary. Because of the
square law dependence, it is therefore necessary to use passive components with a high
Q in the VCO circuit. Hence, chip-package codesign of the VCO with passive components
integrated in the package is necessary.

4.3.1 Low Noise Amplifier Design
The low-noise amplifier (LNA) is the first active device of any RF front-end architecture.
Essential requirements of this amplifier circuit are reasonable gain, a good input
impedance match, linearity, and the lowest possible noise figure (NF). If the device is to
be used in a portable device, the need for low power consumption also becomes
important. The noise factor (F) of an LNA is a measure of the amount of noise added by
the circuit to the incoming signal and is defined as the ratio of signal-to-noise ratio
(SNR) at the input of the device to the SNR at the output.
The cascode LNA architecture of Figure 4.31 has also been used widely for its low
NF and high input-output isolation, particularly for single-chip solutions where the
transistor parameters can be strictly controlled. The design process for the inductively
degenerated LNA consists of sweeping the NF with respect to the transistor (M1) gate