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Understanding Noise Figure Measurement Using Natural Noise Sources

Learn about measuring noise figure using thermal noise sources, receiver sensitivity, and converting noise figure to noise temperature. Discover how to utilize natural noise sources such as the sky and ground for comparison, including insights on solar noise and antenna performance evaluation.

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Understanding Noise Figure Measurement Using Natural Noise Sources

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  1. Noise Figure Measurement using Natural Noise Sources

  2. Receiver sensitivity • Generally want best possible sensitivity • Important at UHF/microwave frequencies where background noise levels are low • Weak signals such as beacons can be useful

  3. Noise figure definition

  4. Thermal Noise N = KTB N = Noise power K = Boltzmann’s constant T = Absolute temperature B = Bandwidth

  5. Thermal noise • All bodies at a finite temperature emit noise • Noise due to random motion of electrons • As temperature increases random motion increases. • As temperature increases noise increases

  6. Noise power • Noise power from a matched load at the input of a receiver = KTB Watts • Noise is proportional to temperature.

  7. Noise temperature • Noise temperature is defined as the temperature of an input termination to a “perfect noiseless amplifier” that would give the same noise output power as with the real amplifier.

  8. Conversion from noise figure to noise temperature.

  9. Conversion from NF to Te • RSGB

  10. Measurement of Noise Temperature

  11. Hot and Cold load method • Can make use of this relationship • Use resistors at different temperatures • PROBLEM: • Need to cool to low temperatures for accurate results – liquid nitrogen!

  12. Natural noise sources • Ground is at about 300 kelvin • Sky is electrically “cold” at about 20-30 kelvin • Point antenna at sky and ground and observe difference at receiver output. • The Sun?????

  13. Ground – Sky Comparison • Ground at Room temp – approx 290 K • Sky is electrically cold – approx 0 K

  14. Ground – Sky noise Comparison • RSGB

  15. Solar Noise • The sun varies but at “quiet times” it is a useful point source allowing the performance of antennas to be checked. • If the NF of the system is known then it is possible to get a good estimate of an antenna gain from the sun noise.

  16. Sun Noise • RSGB

  17. Sun Noise for Antenna Checks • RSGB

  18. Noise measurement • Can simple measure the audio noise output from a receiver. • Make sure AGC is off • PROBLEM – Fluctuation of readings due to narrow bandwidth. • SOLUTION – Use a wideband receiver and a power meter.

  19. Noise Voltage • RSGB

  20. Demonstration of NF Measurement

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