This work has been supported by the Science and Technology Development Fund (STDF), Egypt
An Integrated SAWless Narrowband RF Frontend
Mohamed Abouzied, Hattem Osman, Ahmed N. Mohieldin, Ahmed Emira, Ahmed Soliman
Department of Electrical Engineering, Cairo University, Giza 12613, Egypt
Email: aemira@ieee.org
Abstract—
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I. INTRODUCTION
High frequency RF bandpass filters with narrow fractional
bandwidths are required in wireless communication receivers
for band selection and image rejection. RF surface acoustic
wave (SAW) filters have been widely used for RF band
selection due to their accuracy and high selectivity. However,
SAW filters are expensive, bulky, and lossy elements.
Furthermore, the fact that SAW filters are not programmable
makes them unattractive in software defined radios (SDRs)
[1]. Several attempts can be found in the literature to eliminate
the offchip passive filters and obtain a fully integrated
solution which will reduce power consumption, area, cost, and
need of impedance matching. This becomes more important
for today’s multistandard and multiband receivers.
Monolithic implementations of active LC filters have been
proposed in low cost CMOS technology [2]. Due to the
limited quality factor of onchip inductors, Qenhancement
has been used to design narrowband RF filters. However, this
has been plagued with high power consumption, poor
selectivity, linearity and noise performance.
The concept of blocker filtering using a receiver
translational loop was introduced in [3]. It employs a feed
forward frequency translational loop driven by the receiver
local oscillator (RX LO). This technique suffers from limited
blocker rejection due to mismatches between two inherently
different RF paths which do not track across process and
temperature variations. Another blocker filtering technique
that uses either a feedforward or a feedback frequency
translational path driven by the transmitter local oscillator (TX
LO) was introduced in [4]. This technique is only useful for
rejecting blockers generated by self transmitter signals in
singlechip full duplex systems such as WCDMA. A negative
feedback blocker filtering technique using a frequency
translational path driven by the receiver local oscillator was
introduced in [5]. The rejection in the feedback technique does
not depend on gain matching. It has been shown [6] that the
feedback technique is more robust to I/Q mismatch effects
than its feedforward counterpart. In this paper, the system of
the negative feedback blocker cancellation loop is analyzed. It
will be analytically proved that the presence of a signal at 2f
LO
after downconversion results in limited blocker rejection and
more seriously, in a significant frequency shift in the closed
loop filter characteristic. It will be shown that tradeoffs
between blocker rejection, frequency shift, and stability render
this implementation unpractical.
The need for an alternative architecture to implement a
monolithic tunable RF filter with high blocker rejection
remains as the key towards the realization of a monolithic
universal SDR receiver. A new architecture will be presented
to decouple the blocker rejection, frequency shift, and stability
by placing a notch filter at 2f
LO
at the IF node.
II. SYSTEM ANALYSIS
A. Architecture
The proposed narrowband direct conversion frontend
with 2f
LO
notch is shown in Fig. 1. The incoming desired
signal is at frequency f
LO
associated with the blocker signal.
Both are amplified by the LNA and then downconverted to
baseband by the receiver LO frequency f
LO
. The LNA is
composed of a transconductance g
m
and an output LC tank
Z(s). The output of the downconversion mixer contains two
frequency components; one at baseband and the other at 2f
LO
.
A highpass filter HPF is then used to remove the desired
baseband signal component while the blocker passes through.
The blocker is then upconverted and subtracted at the output
of g
m
of the LNA.
Figure 1 Proposed narrowband RF frontend architecture with 2fLO notch
978-1-4244-7773-9/10/$26.00 ©2010 IEEE 664