Download A Free-space Optically-Locked VCO with Picosecond Timing Jitter in 0.18µm CMOS

A Free-space Optically-Locked VCO with Picosecond
Timing Jitter in 0.18µm CMOS
Xuebei Yang, Xuyang Lu and Aydin Babakhani
Department of Electrical and Computer Engineering, Rice University, Houston, TX, 77005
Wireless Synchronization
Microwave Radiation
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Low directivity
Multi-path propagation
Low-quality synchronization signal at the receiver
Low-quality synchronization achieved
Large phase noise
Large timing jitter
Free-Space Optics
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High directivity
Line-of-sight (LOS) propagation
High-quality synchronization signal at the receiver
High-quality synchronization achieved
Low phase noise
Low timing jitter
Combine Free-Space Optics with CMOS?
CMOS technology is one of the most important factors that lead to the blossom of the
electronic industry. By achieving wireless synchronization using free-space optics on a
CMOS platform, we could have:
1. A compact system
2. Reduced system cost
3. Mass production with high yield
In this work we have used IBM 7RFSOI process. Here is the highlight for this process:
• 180nm technology node
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• 4 metal layers
• 1µm thick buried oxide
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• 1.5V core VDD
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During the design, in order to fulfill design rules while
improve component performance, we have blocked
metal fillings at critical positions and carefully added
dummy metal structures.
Illustration of multi-path propagation
Illustration of LOS propagation
Proposed Optically-Locked Voltage Controlled Oscillator
In this work, for the first time, we report a fully-integrated CMOS-based Optically-Locked Voltage Controlled
Oscillator (OL-VCO) that operates based on the concept of spatial injection locking. The chip is implemented
using a commercial CMOS process technology without any post-processing and can be readily integrated with
complex analog and digital circuits. The proposed OL-VCO has a free-running frequency around 1.3GHz. It can
be tightly synchronized through a free-space optical beam at 850nm wavelength. The reported link achieves a
picosecond timing jitter with distance of larger than one meter.
The right figure shows the schematic of the OL-VCO, which
adopts a differential cross-coupled structure. The OL-VCO
can operate in both free-running and locked modes. In the
free-running mode, Vtune is set to be larger than VDD so that
the custom-designed diodes D1 and D2 are in the reverse
bias region. The inductor L1 resonates with capacitance of
D1 and D2, while transistors M1 and M2 provide negative
resistance required for oscillation.
The micrograph of the fabricated chip.
Both photodiodes are illustrated in
the figure. The dimension of the chip
is 550µm by 500µm.
PC
Active silicon
Oxide
Substrate
1µm
Cross-section for IBM
7RFSOI process
No post-processing is used!
Combine Free-Space Optics with CMOS?
The right figure shows the tuning range of the OLVCO in the free-running mode versus Vtune. As Vtune
increases from 2V to 6V, the capacitance of the
diode reduces by 23%, and the free-running
oscillation frequency of the OL-VCO increases from
1295MHz to 1381MHz. The output power is -8dBm
across the frequency range.
The tuning range for the
OL-VCO in the freerunning mode
The left figure shows the measured power
spectrum of the OL-VCO in both free-running
and locked modes. It is clear that the OLVCO in the locked mode generates a sharp
tone at the injected frequency.
Schematic of the proposed OL-VCO
Setup for the wireless synchronization of the OL-VCO
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A block diagram of the measurement setup for
the OL-VCO in the locked mode is presented
in the left figure. A directly-modulated Vertical
Cavity Surface Emitting Laser (VCSEL)
operating at 850nm with 11GHz bandwidth is
used as the optical source. The amplitude of
the optical signal is modulated by an Anritsu
68369B RF signal generator. The modulation
frequency is close to the free-running
frequency of the OL-VCO. In this work, the
laser beam is collimated and focused onto the
photodiode of the OL-VCO through two
discrete lenses. The lenses can be potentially
removed if a laser source with higher power is
used. It may also be integrated onto the same
CMOS chip, as CMOS processes with
integrated microlenses have recently been
reported. The distance between the VCSEL
and the OL-VCO chip is 1.5 meters.
The measured power spectrum
of the OL-VCO in both freerunning and locked modes.
The right figure shows the measured phase noise
of the OL-VCO in both free-running and locked
modes. Based on these measured values, the
phase noise improves by about 25dB at 100Hz and
1000Hz frequency offsets.
The measured phase noise of
the OL-VCO in both freerunning and locked modes.
The left figure shows the measured locking
range of the OL-VCO. It is clear that the
locking range increases with the reverse bias
of photodidoes and modulated laser power.
The measured locking range.