JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.2, APRIL, 2015 ISSN(Print) 1598-1657
http://dx.doi.org/10.5573/JSTS.2015.15.2.202 ISSN(Online) 2233-4866
Manuscript received Sep. 7, 2014; accepted Jan. 21, 2015
1
School of Information and Electronics, Beijing Institute of Technology
2
Institute of China Electronic Equipment System Engineering
Company
E-mail : zhoubo07@bit.edu.cn
High-Robust Relaxation Oscillator with Frequency
Synthesis Feature for FM-UWB Transmitters
Bo Zhou
1
and Jingchao Wang
2
Abstract—A CMOS relaxation oscillator, with high
robustness over process, voltage and temperature
(PVT) variations, is designed in 0.18 μm CMOS. The
proposed oscillator, consisting of full-differential
charge-discharge timing circuit and switched-
capacitor based voltage-to-current conversion, could
be expanded to a simple open-loop frequency
synthesizer (FS) with output frequency digitally tuned.
Experimental results show that the proposed
oscillator conducts subcarrier generation for
frequency-modulated ultra-wideband (FM-UWB)
transmitters with triangular amplitude distortion less
than 1%, and achieves frequency deviation less than
8% under PVT and phase noise of –112 dBc/Hz at
1 MHz offset frequency. Under oscillation frequency
of 10.5 MHz, the presented design has the relative FS
error less than 2% for subcarrier generation and the
power dissipation of 0.6 mW from a 1.8 V supply.
Index Terms—Relaxation oscillator, high robustness,
frequency synthesis, switched-capacitor, FM-UWB,
subcarrier generation
I. INTRODUCTION
Frequency-modulated ultra-wideband (FM-UWB)
transmitters employ dual frequency modulation
technique: FSK-modulated triangular subcarrier
generation where triangular frequency f
1
represents data
“0” and f
2
represents data “1”; followed by RF frequency
modulation (FM) where the triangular amplitude voltage
converted to UWB FM signal by using an RF voltage-
controlled oscillator (VCO) with carrier frequency f
C
calibrated [1, 2]. Hence, an oscillator having 2-FSK
modulated triangular waveform output is indispensable.
The relaxation oscillators [3, 4] based on a slew-rate
control by a timing capacitor and a charge-discharge
current are good configurations to generate triangular
waveform, with low complexity, power, noise and high
linearity. However, oscillation frequency f
OSC
encounters
poor robustness over process, voltage and temperature
(PVT) variations. Thus these oscillators need to work in a
closed loop to conduct FSK-modulated subcarrier
generation, which complicates the system design. Fig. 1
shows the block diagram of FM-UWB transmitters
employing fractional-N phase-locked loop (PLL) with
relaxation VCO for subcarrier generation [5, 6].
The existing oscillators [7-10] with temperature
immunity lack enough consideration of, or are sensitive
to process deviations. The existing oscillators [11-13]
with good PVT robustness have large power dissipation
in comparison to low oscillation frequency, which causes
poor figure of merit (FOM) values. The existing
Fig. 1. Block diagram of FM-UWB transmitters.