A Low-Power Calibration-Free Fractional-N Digital
PLL with High Linear Phase Interpolator
Fan Yang, Hangyan Guo, Runhua Wang, Zherui Zhang, Junhua Liu* and Huailin Liao*
Key Laboratory of Microelectronic Devices and Circuits (MOE)
Institute of Microelectronics, Peking University,
Beijing, China, 100871
E-mail: junhualiu@pku.edu.cn, liaohl@pku.edu.cn
Abstract—This paper presents a low-complexity calibration-
free digital PLL architecture. The PLL adopts a fractional
frequency divider with a harmonic rejection current steering
phase interpolator which is free from pre- and background-
calibration. The harmonic rejection technology could improve
linearity of interpolator. A simplified glitch-free control logic for
fractional operation is proposed to lower architecture complexity
and minimal design effort. A high frequency resolution digitally-
controlled oscillator with an equivalent variable inductor is also
utilized. A 2.2-GHz digital PLL has been implemented in a 55-nm
CMOS technology. The frequency resolution of DCO is 1.58 kHz,
and in-band phase noise of PLL is -104.4 dBc/Hz. The PLL
consumes 2.43 mA from a 1.2-V supply voltage and occupies an
active area of 0.216 mm
2
.
Keywords—Calibration-free; Digital PLL; Fractional-N;
Glitch-free Harmonic Rejection; Phase Interpolator;
I. INTRODUCTION
Digital Phase Locked Loop (DPLL) has been widely
studied and utilized, for its advantages on chip area, power
consumption and loop stability, especially its benefits from
process scaling down. In radio frequency transceivers, a
fractional-N PLL is necessary for channel selection, frequency
modulation and so on, but meets some difficulties when
switching PLL from analog to digital implementations.
An Ȉ-ǻ modulator-based frequency divider is a key
building block in mostly fractional-N PLLs, but would face
new challenges in digital PLLs. Conventional digital PLLs
adopt a time-to-digital converter (TDC) as a phase frequency
detector to quantize the phase difference, so that the resolution
of TDC influences quantizing noise. On the other hand, when
applying an Ȉ-ǻ modulator in digital PLLs, the dithering of
divider output would increase maximal phase error and the full
scale range of TDC. The high resolution and scale range
increase the difficulty of TDC design. TDC also suffers from
device mismatch and variation of process, voltage and
temperature and needs pre-calibration or back-ground
calibration. Bang-bang phase detectors (BBPDs) are employed
to replace TDC for their low jitter, low complexity and high
linearity. However, BBPDs are limited to integer-N PLLs due
to their tiny locking range.
Digital-to-time converters (DTCs) are introduced to digital
PLLs to overcome the limitation [1-3]. For a specific frequency
dividing ration, a fractional divider usually consists of an
Integer-N divider for integer part and a DTC for fractional part.
Generally, a DTC can be implemented using either a digitally
controlled delay line (DCDL) [1] or a phase interpolator (PI)
[2]. But complex digital calibration techniques are required to
correct for variation of delay-lines and non-linearity of PIs. In
addition, DCDL adjusts signal in time domain and a digital
mapping is required to transfer control signal from phase
domain to time domain for different operation frequencies. R.
Nonis introduces a current steering PI-based fractional-N
digital PLL in [3]. The prime advantage of current steering PI
is a full scale range of 2ʌ by nature, which needs no
background calibration. However, a non-linearity of PI would
introduce spurs and a glitch may happen when changing output
phase of PI by switching current of each signal, which would
lead to a critical error in counter-based frequency divider.
In this paper, a low-complexity calibration-free digital PLL
architecture is presented, as shown in Fig. 1. A current steering
PI with harmonic rejection (HR-PI) is presented to improve the
linearity of PI. A simplified glitch-free control logic is
proposed to lower complexity and minimal design effort. The
PLL also has a high frequency resolution digitally-controlled
oscillator (DCO) with a differential tapped inductor, which is
introduced in our previous work [4]. A 2.2-GHz prototype is
implemented in a 55-nm CMOS technology. Experimental
results show that the resolution of DCO is 1.58 kHz and the in-
band noise of PLL is -104.4 dBc/Hz. The PLL consumes 2.43
mA from a 1.2-V supply voltage and occupies an active area of
0.216 mm
2
.
This w