340 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 2, FEBRUARY 2007
An Agile VCO Frequency Calibration Technique
for a 10-GHz CMOS PLL
Tsung-Hsien Lin, Member, IEEE, and Yu-Jen Lai
Abstract—This paper reports an agile VCO frequency calibra-
tion technique and its application on a 10-GHz CMOS integer-N
phase-locked loop. The proposed calibration method accomplishes
efficient search for an optimum VCO discrete tuning curve among
a group of frequency sub-bands. The agility is attributed to a pro-
posed frequency comparison technique which is based on mea-
suring the period difference between two signals. Other mixed-
signal circuits are also developed to facilitate this approach. The
PLL incorporating the proposed calibration technique is imple-
mented in a 0.18-
m CMOS process. The measured PLL phase
noise at 10 GHz is
102 dBc/Hz at 1-MHz offset frequency and
the reference spurs are lower than
48 dBc. The PLL consumes
44 mW in the low-current mode. The calibration time is less than
4
s.
Index Terms—Calibration, CMOS integrated circuits, frequency
synthesizer, period-based frequency comparison, phase detector,
phase-locked loop (PLL), voltage-controlled oscillator (VCO).
I. INTRODUCTION
P
HASE-LOCKED loops (PLLs) are important building
blocks for communication systems. In many applications,
they are implemented with wide tuning ranges in order to
cover desired operating frequency bands and to accommodate
process, voltage, and temperature (PVT) variations. As the PLL
operating frequencies increase while the supply voltages scale
down with advanced CMOS technologies, the voltage-con-
trolled oscillator (VCO) tuning gain (
, expressed in
MHz/V) increases considerably. A large
can degrade
the PLL phase noise and spur performance severely. This issue
can be addressed by employing both discrete and continuous
tuning mechanisms in the VCO design. The implementation is
conceptually illustrated in Fig. 1 [1]–[3]. It employs multiple
overlapped tuning sub-bands to cover the desired frequency
range, instead of using just a single tuning curve. Such topology
achieves low VCO gain while still covering a wide frequency
tuning range. However, this method requires a VCO calibra-
tion (also called coarse tuning) circuit to select the optimum
frequency sub-band that covers the desired frequency. The
time spent on the calibration is in addition to the intrinsic
PLL settling or locking time, and presents an overhead to a
Manuscript received January 11, 2006; revised August 23, 2006. This work
was supported in part by the National Science Council, Taiwan, R.O.C. under
contract number NSC93-2215-E-002-031.
T.-H. Lin is with Graduate Institute of Electronics Engineering and Depart-
ment of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan,
R.O.C. (e-mail: thlin@cc.ee.ntu.edu.tw).
Y.-J. Lai is with the Taiwan Semiconductor Manufacturing Company,
Hsinchu 300, Taiwan, R.O.C.
Digital Object Identifier 10.1109/JSSC.2006.889360
Fig. 1. Technique for reducing the VCO tuning gain
(
K
)
.
communication system. This may result in a reduced data
throughput. For many communication applications, especially
for frequency hopping systems, such overhead must be kept to
a minimum. To address this issue, an agile VCO calibration
(coarse tuning) method is proposed in the paper.
This paper is organized as follows. In Section II, the existing
VCO calibration techniques are briefly reviewed. The concept
of the time-domain calibration method and the proposed ap-
proaches are detailed in Section III. The circuit implementa-
tion of the 10-GHz integer-
N PLL is described in Section IV,
and the chip results are presented in Section V. Section VI dis-
cusses the advantages of the proposed technique and compares
with other conventional methods. Finally, a conclusion is given
in Section VII.
II. VCO C
ALIBRATION TECHNIQUES
The purpose of performing VCO calibration is to determine
an optimum VCO sub-band under which the PLL can acquire
locking to a reference signal properly. During calibration, com-
parison between the VCO (or its derivative signal) frequency
and the reference frequency is involved. The process of fre-
quency comparison can be carried out when the PLL loop is
either closed or opened. Therefore, the VCO calibration tech-
niques can be categorized into two types according to the PLL
status during calibration.
A. Closed-Loop VCO Calibration Technique
The closed-loop VCO calibration approach is depicted in
Fig. 2(a), where the PLL remains closed during calibration [1],
[2]. For a VCO embedded in a PLL, the loop works to lock
the VCO to a desired frequency under a given VCO frequency
sub-band setting. When the loop settles, the
voltage is
compared against a predefined voltage range (between
and ). If the settled value of falls outside this range,
the PLL is considered unable to lock the VCO properly. This
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