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Lecture 10- Radiometers (Random Noise (Natural signals all generated by…
Lecture 10- Radiometers
Random Noise
Natural signals all generated by random processes and have same basic form when incident on the receiver
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Signal voltages indistinguishable from therm agitation voltages in resistant components of the receiver
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Only stable/measurable quantity at a single point in space is the average power or variance of the random variable
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Radio astronomy signals are very weak, so need huge amplification <100dB before they can be measured and quantified
Band-limited noise
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Typical zero crossing times separated by 1/vrf and the amplitude modultion "envelope" exhibits a charcteristc "coherance time"
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Extension to sum of random sine waves of the concept of "beats" ie. modulation resulting when two pure sine waves are added together
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Key concepts
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Easiest way to measure power is to put amplified voltage into a semiconductor "square law detector" for small input signals
Square law detection
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input has gaussian stats, output has non gaussian stats
Central limit theorum: any large sample of independent random variables will exhibit a gaussian distribution
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Detecting a weak source
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Schematic radio telescope record of the output power of a receiver after final averaging over a timescale seconds as the beam moves over a week source
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because telescope is pointed a the source for time longer than tau avg the rise in mean temperature due to the source can be detected
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