Noise Figure Measurement using Natural Noise Sources Receiver

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Noise Figure Measurement using Natural Noise Sources

Noise Figure Measurement using Natural Noise Sources

Receiver sensitivity • Generally want best possible sensitivity • Important at UHF/microwave frequencies where

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

Noise figure definition

Noise figure definition

Thermal Noise N = KTB N = Noise power K = Boltzmann’s constant T

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

Thermal noise • All bodies at a finite temperature emit noise • Noise due

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

Noise power • Noise power from a matched load at the input of a

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

Noise temperature • Noise temperature is defined as the temperature of an input termination

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.

Conversion from noise figure to noise temperature.

Conversion from noise figure to noise temperature.

Conversion from NF to Te • RSGB

Conversion from NF to Te • RSGB

Measurement of Noise Temperature

Measurement of Noise Temperature

Hot and Cold load method • • Can make use of this relationship Use

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!

Natural noise sources • Ground is at about 300 kelvin • Sky is electrically

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? ? ?

Ground – Sky Comparison • Ground at Room temp – approx 290 K •

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

Ground – Sky noise Comparison • RSGB

Ground – Sky noise Comparison • RSGB

Solar Noise • The sun varies but at “quiet times” it is a useful

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.

Sun Noise • RSGB

Sun Noise • RSGB

Sun Noise for Antenna Checks • RSGB

Sun Noise for Antenna Checks • RSGB

Noise measurement • Can simple measure the audio noise output from a receiver. •

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.

Noise Voltage • RSGB

Noise Voltage • RSGB

Demonstration of NF Measurement

Demonstration of NF Measurement

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