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Phase and Gain stability of optical
fibre link used in MeerKAT
Author : Roufurd Julie
Supervised by : Prof Michael Inggs
SKA HCD bursary conference Dec 2008
Outline of Presentation
• Background / Introduction to project
• Project Objective
• Relevance of Project to SKA/KAT
• Methodology & Results
• Conclusions
Background / introduction to project
• The meerKAT project consists of 80 antennas configured
as an interferometer
• Optical fibre is used to transport the RF signal
• Interferometry requires a GAIN and PHASE stable RF
path
Why Optical fibre?
Very high bandwidth -> greater sensitivity
Low attenuation -> signal travel further
Why analogue optical modulation?
Advantages
• Simpler and cheaper
• Geographically separate the noisy digital backend from
sensitive RF front end
Disadvantages
• Needs much better S/N than digital
• Limited dynamic range (at this stage!)
• Signal travels further before being digitised, and is
susceptible to amplitude and phase effects
Optical terminal equipment manufacturers
Background / introduction to project
• The meerKAT project consists of 50+ antennas
configured as an interferometer
• Optical fibre is used as the signal medium
• Interferometry requires a GAIN and PHASE stable RF
path
Defining PHASE and GAIN stability
• Stability in this research, means some parameter change
in terms of time
• GAIN stability is GAIN change over time
• Similarly PHASE stability is PHASE change over time.
• Ideally, GAIN and PHASE should be constant with time
(all other things being equal)
Small note about phase and propagation delay
Relevance of project to KAT / SKA / other
• Phase instability has a deleterious effect on the GAIN of
the synthesised beam i.e. % decorrelation
• Not good for Imaging dynamic range
• GAIN stability has an influence on the sensitivity of the
radio telescope
• These effects need to quantified as it influences how
often system needs to be calibrated
Project objectives
• Investigate properties of the optical fibre path that affects
phase and gain stability
• Measure and report on performance of optical link using
a commercial Tx and Rx pair.
Some quick facts
• Ambient temperature around cable affects phase stability
• Loose-tube cabled optical fibre has been shown to have
the lower of the delay/kelvin coefficients
• Bending of fibre causes attenuation which affects gain
stability
• Laser Tx power is affected by temperature, thus these
units need to be kept at constant temperature
• Units used were Miteq & Photonics, 1550nm, DFB laser
with external modulation
Methodology
• Find out phase requirements from the meerKAT Imaging
team. (Not complete success yet)
• Study physical properties of components in the Optical
fibre chain that contribute to phase and amplitude
instabilities
• Create engineering tools to measure the stability
• Quantify VNA and optical terminal equipment stabilities
• Study the HartRAO optical fibre link installation
Tools 1 – Measuring system
Tools 2 - Labview
Tools 3 – Optical Tx and Rx
Tools 4 – Overall setup
Results 1 – Group delay changes @ SUNRISE
Results 2 – Phase changes @ 1.5GHz during SUNRISE
Results 3 – Gain changes @ 1.5GHz during SUNRISE
Phase @ 1.5GHz vs temperature.
2 hr snapshot during SUNSET
Results 4 – Phase @ 1.5GHz vs Azimuth position
(mean removed)
Results 5 – Phase vs Elevation position
(mean removed)
Results 6 – GAIN @ 1.5GHz vs Elevation position
Results 7 – GAIN vs Azimuth position
Results 8 - Gain [dB] vs Azimuth
Conclusions
• This optical path will have an effect on the amplitude and
phase performance of the system
• I measured the performance of the HartRAO links.
• Movement of the dish affects GAIN more, while
Temperature affects PHASE more
• Data analysis still in progress
Acknowledgements
• SKA HCD for bursary funding
• Mike Inggs and Venkat for their daily inputs
• Mike Gaylard for his many hours of trying to explain interferometry to
me
• KAT project office for usage of measuring equipment
• KAT and HartRAO staff for their various bits and pieces along the
way.