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Lecture 12- Noise and attenuation (Noise into the receiver(descriptive)…
Lecture 12- Noise and attenuation
Noise into the receiver(descriptive)
In diagram
From the atmosphere, CMB and other background sources within the beam
Stray radiation from grounded rest of the sky
Noise powers add-not voltages - so we can express ourselves in terms of temperatures
Adds noise and attenuated TA
Adds noise from electonics as if an additional matched resistor at temperature TLNA was connected to the LNA input
Noise into the receiver (where)
Antenna
Target source
Attenuating transmission line
Low noise amplifier
Rest of receiver
Noise into the receiver
Radiation from target ad other directions each produce independent noise voltages in the receiver. These voltages have no phase relationships between them, they are uncorrelated, so the products after the square law detectore average to 0 (over many coherance time)
Effects of attenuation
Attenuator at physical temperatrue Tatt w/ transmission factor nu reduces the input signal and also emits its own random noise
as nu falls the transmitted signal gets smaller and the noise generated increases a lose-lose situation
attenuators loss usually in dB
Seek to avoid loss before the 1st low noise amplifier, the noise from the irst amplifier swamps the rest
Example and some maths
Lossy components
Power at output frequency never better than 0.5 since always split between upper and lower frequency bands
power is lost in other frequency harmonic (often many) and in resistive losses in diodes
typically <25% of input power translated to IF stages, > factor of 4 or 6dB loss.
Typical noise budgets table
Receiver power gain fluctuations
flicker or pink or 1/f noise
more fluctuating power on longer timescales i.e. variations at lower output frequencies. The statistical excursions from a short-term mean value increase with time (inversly with frequency)
come to dominate the receiver output fluctuation for long integration times
Not actually noise, gain variations or fluctuations
Receiver gain variations can mimic real sources or obscure them
1/f noise power spectrum
the power spectrum of thermal noise fluctuations is constant as a function of time or frequency in the output called
The power spectrum of gain fluctuations rises at lower frequencies so these frequencies will be the dominant source of output fluctuations on time scales longer than 1.fknee where fknee is the so called knee frequency
As the thermal noise level risesw the knee frequency falls so white noise is dominant for longer periods over 1/f noise
white"