Automatic Tuning of Integrated Q-Enhanced LC Filters
Shaorui Li and Yannis Tsividis
Columbia Integrated Systems Laboratory, Columbia University, New York
Introduction
High-Q, high-frequency, low-power, fully-integrated filters are
essential for highly integrated wireless transceivers.
However, the poor quality of on-chip passive devices,
especially inductors, cannot meet the requirements for highselectivity filters. Active devices have been employed to
enhance the quality of the passive devices, but the
performance of the resulting Q-enhanced LC filters relies
heavily on the quality of the passive devices. Because the
quality of passive devices is process-dependent and difficult
to predict in prefabrication simulations, automatic tuning
systems are essential to maintaining the desired filter
specifications.
In this work, two types of automatic tuning systems,
voltage-controlled-oscillator (VCO) based and voltagecontrolled-filter (VCF) based, are presented.
Principle of Automatic Tuning for
Integrated Q-enhanced LC Filter
For Q-enhanced LC filters, an inaccurate quality factor is
mainly caused by the low and process-dependent quality
factor of on-chip inductors. The process tolerance of the
losses from the on-chip inductors is large, and the loss does
not scale with inductance. Thus, it is desirable to have all the
inductors in the filter be identical to simplify the control over
the loss compensation, so the filter's structure is limited to
certain types. Among them, the coupled-resonator filter with
identical high-Q LC branches, is the most popular.
Q-enhanced LC resonators
A VCO Q-Tuning System Using
Conductance Reference
and phase response at the resonant frequency. This principle
is applied in the VCF tuning scheme [5], as shown in Figure 4,
to control the effective loss of the Q-enhanced LC resonator.
Figure 7: Comparison of measured filter responses of four
chips: without VCO Q-tuning (left) and with VCO Q-tuning
(right), with tuned center frequency.
Figure 2: Conventional Q-tuning scheme using VCO with
magnitude-locked loop.
Conventional Q-tuning scheme uses a VCO with a magnitudelocked loop [2], shown in Figure 2. Analysis in [3] shows that
the achieved Q of the resonators in the filter is not infinity,
but rather a finite negative number depending on the
nonlinearity and the amplitude used in the reference
resonator.
For the filter to operate correctly, it is desirable to control
the magnitude of resonator Q to a predictable value
independent of the process parameters. So we choose to use
a conductance reference, to which the linear part of the
equivalent loss of the resonator is forced to be equal,
achieving a filter resonator Q independent of process
parameters. A system implementing this scheme [4] is shown
in Figure 3. In this scheme, the tuned filter resonator Q is not
process-dependent and the magnitude can be easily
controlled by setting the reference conductance accordingly.
Figure 4: A VCF Q-tuning scheme.
Figure 5: The circuit implementing the envelope detectors,
the difference circuit, and the integrator.
Unlike the case in VCO Q-tuning schemes, the accuracy of
the frequency tuning circuitry affects the tuned Q of the
resonator in this VCF Q-tuning scheme. The Q deviation
caused by inaccurate frequency tuning is severe, compared
to that introduced by mismatches of the VCF and the
reference circuitry, and by loop integrator offset.
Figure 1: A 4th-order coupled-resonator filter.
Since direct tuning of the filter Q is so far an unsolved
problem, we apply the method used for Q-enhanced LC filter
in [1]: use a loss control loop to tune the Q-enhanced LC
resonator into an ideal lossless LC tank, and the expected
filter Q is thus achieved when the filter is embedded between
proper terminations. Since the LC tank has a high Q only at a
specific frequency, this method is suitable for narrow-band
applications. Many of the integrated continuous-time filters
reported in the literature use tuning systems based on a VCO
or a VCF. A summary comparing the two tuning systems is
given in Table 1.
VCO tuning system
Pros
• Heavy leakage.
• Need of oscillation
Cons initializing.
• Effects of nonlinearity.
This work was supported in part by a gift from Intel
Corporation.
References
[1] D. Li and Y. Tsividis, “A 1.9 GHz Si active LC filter with onchip automatic tuning,” in IEEE Int. Solid-State Circuits
Conference (ISSCC), Digest of Technical Papers, 2001, pp.
368-369, 466.
[2] R. Schaumann and M. A. Tan, “The problem of on-chip
automatic tuning in continuous-time integrated filters,” in
Proc. IEEE Int. Symp. on Circuits and Systems (ISCAS), 1989,
pp. 106-109.
VCF tuning system
• Need of input signal.
• Serious interference
of freq. and Q tuning.
Acknowledgement
Measurement Results
Table 1: Comparison of VCO and VCF tuning systems.
• No need of input signal. • Light leakage.
• Little interference of
• Linear system.
freq. and Q tuning.
Figure 8: Comparison of measured filter responses of four
chips: without VCF Q-tuning (left) and with VCF Q-tuning
(right), at fixed frequency tuning voltage.
Figure 3:
reference.
[3] S. Li and Y. Tsividis, “Analysis of oscillator amplitude
control, and its application to automatic tuning of quality
factor for active LC filters,” in Proc. IEEE Int. Symp. on
Circuits and Systems (ISCAS), 2004, pp. 141-144.
VCO Q-tuning scheme using conductance
A VCF Q-Tuning System
An important characteristic of a lossless parallel LC tank is
that the impedance of the tank is infinite at the resonant
frequency. Thus, having a lossless parallel LC tank in parallel
with other loads in a circuit has no effect on the magnitude
Figure 6: The die photo of the prototype chip.
[4] S. Li, N. Stanic, K. Soumyanath, and Y. Tsividis, “An
integrated 1.5 V 6 GHz Q-enhanced LC CMOS filter with
automatic quality factor tuning using conductance
reference,” in IEEE Radio Frequency Integrated Circuits
(RFIC) Symposium, 2005.
[5] S. Li, N. Stanic, and Y. Tsividis, “A VCF loss control loop
for Q-enhanced LC filters,” to be published.