Digital Signal Processing Reference
In-Depth Information
Chapter 6 ultimately puts the theoretical basis of the previous sections into
practice and evaluates the prototype implementation of a pulse-based receive
unit in 0
.
18
m cmos. The reader should keep in mind that the presented
hardware is not a full-scale implementation of a stand-alone pulse-based ra-
dio receiver. The presented prototype contains the analog front-end of a single
receive unit and is part of a larger, full-blown radio system, where multi-
ple of these units work together in parallel. Chapter 6 goes through the gen-
eral floorplan of the prototype chip, briefly discussing topics such as the rf
input stage (which includes the pulse-based extension layer) and the base-
band signal-processing circuits in the back-end. One of the major themes run-
ning through this chapter is the issue of spurious injection into the extremely
sensitive signal-chain. This undesirable side-effect is caused by the presence
of switching transient signals in the pulse-to-baseband conversion process.
Chapter 6 also includes measurements results of the prototype chip, such as
the aperture width of the pulse-based input stage, some test results showing
the demodulation of a qpsk signal and a
noise figure
measurement on the
complete front-end.
Chapter 7 elaborates in greater detail on a particular building block of the
prototype receiver chip. The
variable gain amplifier
(vga) in the baseband
section of the pulse-based receiver employs an open-loop amplifier, which is
loaded with a nonlinear output section. In a wide variety of applications, am-
plifiers are arranged in a feedback structure in order to suppress the second-
and third-order distortion components caused by the nonlinear behaviour of
the active gain element. Distortion suppression however, strongly depends on
the amount of
excess gain
that is available in the feedback loop. Exactly the
lack of excess gain at higher frequencies makes feedback amplifiers unsuitable
for incorporation in a wideband signal chain. In contrast, the open-loop cmos
amplifier described here employs a nonlinear load, which allows to combine a
good distortion suppression with a wide operating bandwidth. Chapter 7 pro-
vides both a rigorous theoretical background of the working principles and an
in-depth mismatch sensitivity analysis of the nonlinear loaded open-loop am-
plifier. Again, the theoretical framework is supported by the implementation of
an amplifier in 0
.
13
μ
m cmos. Measurements show a flat frequency response
in the 20-850 MHz frequency band and a 30 dB voltage gain. Under these
conditions, the oip
3
is better than 13 dBm over the aforementioned frequency
band.
Some extra background on the frequency-dependency of distortion suppres-
sion in feedback amplifiers has been included in Appendix 7. The overview
starts with the derivation of the well-known second- and third-order harmonic
distortion formulas for a generalized amplifier in feedback configuration. After
this, the expressions are extended in order to include the frequency-dependent
μ
