research activities in 2007 - CSEM

Quasi-Harmonic Quadrature CMOS Relaxation Oscillator
J. Chabloz, D. Ruffieux
In this paper, a solution to realize a local oscillator (LO) for a low power super-heterodyne receiver is presented. The complete local frequency
synthesis solution comprises a fixed-frequency bulk-acoustic wave (BAW) RF oscillator with low phase noise and low tuning range.
A quasi-harmonic quadrature relaxation oscillator with a large tuning range is realized and can be used to compensate for variations in the
RF oscillator as well as to cover the entire bandwidth for multiple channel selection.
The receiver for which the presented oscillator is specifically
designed is based on a super-heterodyne architecture and its
block schematic is described in Figure 1. The receiver frontend
and baseband parts have already been integrated and
characterized with external oscillators [1] . A BAW oscillator
embedded within a digital frequency synthesis [2] is used as a
first local source (BAW RF LO) and runs at a fixed frequency,
determined by the physical dimensions of the BAW resonator.
The second downconversion uses a quadrature relaxation
oscillator (IF LO) with a large tuning range [3] that is presented
below.
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Passband
Balun
BAW
Filter
2400.0 - 2483.5 MHz
On-chip
LNA
2330.0 MHz
fixed
BAW
LO
IF Amplifier
Quadrature
IF Oscill.
Frequency Synthesis
Figure 1: Super-heterodyne receiver architecture
70.0 - 153.5 MHz
tunable
Since the second downconversion is direct, the selected
channel frequency is determined by the sum or the difference
of both oscillator frequencies. Therefore the allowed frequency
excursion on the second oscillator allows to compensate
entirely for the lack of tunability of the first one.
The proposed quasi-harmonic quadrature relaxation oscillator
architecture is described in Figure 2. Coupling transistors are
used to couple two classical relaxation oscillator cores.
Coupling
transistors
M3
M1
M2
Figure 2: Quadrature oscillator architecture
The wide variation of bias current needed to tune the
oscillation frequency over the required range also creates
wide variations in the oscillator operating point, more
specifically in the steady-state oscillation amplitude. In order
to increase the tuning range, linearize the current-to-frequency
characteristic and allow the oscillator to stay in the quasiharmonic
mode over the entire range, an amplitude control
has been designed.
The presented circuit has been fabricated in a standard
0.18 µm CMOS process. Figure 3 shows the actual receiver
test chip photograph. The BAW resonator is bonded together
with the chip.
Figure 3: Receiver test chip photograph
The oscillation frequency has been measured with different
amplitude settings and the results are shown in Figure 4. It
can be seen that the tuning range covers the wanted
frequencies which range from 70 MHz to 150 MHz.
Oscillation frequency [MHz]
150
140
130
120
110
100
90
80
70
60 0
120
140
1
160
2
180
3
200
Relaxation oscillator current consumption (wo buffer) [μA]
4
220
5
240
6
K = 1.35
K = 1.2
Figure 4: Measured oscillator frequency tuning range with two
different amplitude settings
The quadrature coupling principle is verified by measuring an
average phase difference of 89° between the I and Q signals.
Spot phase noises of -104 dBc/Hz at 1 MHz offset and
-111 dBc/Hz at 3 MHz offset have been measured.
[1] J. Chabloz, et al., “A Low-Power 2.4GHz CMOS Front-End using
BAW Resonators”, ISSCC’06 proceedings, 1244-1253
[2] D. Ruffieux, et al., “An Agile 2.4GHz MEMS-Based Digital
Frequency Synthesizer”, CSEM Scientific and Technical Report
2006, page 24
[3] J. Chabloz, et al., “Frequency Synthesis for a Low-Power
2.4GHz Receiver using a BAW Oscillator and a Relaxation
Oscillator”, ESSCIRC’07 proceedings, 492-495
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