Noise Coupling in Focal Plane ArraysJacki van der MerweMScEngStellenbosch University

OutlineFocal Plane ArraysReceiver NoiseLow Noise AmplifiersIntroduction to Noise CouplingCalculating Noise CouplingActive Noise Figure and Noise TemperatureExamples

Focal Plane ArraysLarge, dense phased arrays of antennasPlaced in focal plane of reflectorIn theory, any number of beams can be formedEnables complete collection of the entire electric field in the focal plane of the reflectorBecause they are densely packed, there exists a high level of mutual coupling between the element

Receiver NoiseNoise Temperature and Gain of the first element have the greatest effect on overall noise temperature of the systemUsually Low Noise Amplifier (LNA)

Low Noise Amplifiers (LNA)Designed to add very little noise to systemVery Little NoneMost of the noise travel toward the loadA small portion travels back toward the antenna and is radiated

Noise CouplingNoise signals from LNA travel towards antennas and are radiatedRadiated noise are received by other elements and amplifiedThis is referred to as Noise Coupling

Calculating Noise CouplingRepresent coupled, noisy network as equivalent circuit diagramDirection of noise coupling

LNA Noise ModelOptimum Source Admittance (Ys=Ym)Specific source admittance allowing for optimum noise matchMinimum noise Figure (Fm )Noise Figure resulting from optimum noise matchEquivalent Noise Resistance (Rn)Representation in Ohm of the spectral density of a noise-voltage generator

Coupled Antennas2 Coupled Antennas at a timeAll elements are identical, only coupling terms differUse superposition to calculate total coupling

Calculating Noise CouplingEquivalent Circuit diagram nowDirection of noise coupling

Calculating Noise CouplingReduce Circuit diagramCalculate noise power coupled to each element

Active Noise Figure and TemperatureProblem: If coupled noise is considered input noise, the input SNR is degraded, this results in false impression that the Noise Figure of the LNA improved.All LNAs are seen as one multiport LNA bankAdd coupled noise from other LNAs in the bank to the noise power generated by the isolated LNA

Verification Example

Active Noise Temperature of 8x8 Vivaldi ArrayAdded noise temperature of 14 Kelvin 9% of Isolated Noise TemperatureFor SKA 1.3 K !!!

SKA Tsys Budget

Estimate T Table

Any Questions?

Simplify problem-> use only two elements at a time-> assume one element is the noise generator and the other the noise receiver-> direction of coupling

Want to have generic expressions to calculate noise coupling for any configuration-> equivalent circuit diagram in terms of LNA parameters and coupling terms of array

2 partially correlated noise sources