Noise Coupling in Focal Noise Coupling in Focal Plane ArraysPlane Arrays

Jacki van der MerweJacki van der Merwe

MScEngMScEng

Stellenbosch UniversityStellenbosch University

OutlineOutline

Focal Plane ArraysFocal Plane Arrays Receiver NoiseReceiver Noise Low Noise AmplifiersLow Noise Amplifiers Introduction to Noise CouplingIntroduction to Noise Coupling Calculating Noise CouplingCalculating Noise Coupling Active Noise Figure and Noise Active Noise Figure and Noise

TemperatureTemperature ExamplesExamples

Focal Plane ArraysFocal Plane Arrays Large, dense phased Large, dense phased

arrays of antennasarrays of antennas

Placed in focal plane of Placed in focal plane of reflectorreflector

In theory, any number of In theory, any number of beams can be formedbeams can be formed

Enables complete Enables complete collection of the entire collection of the entire electric field in the focal electric field in the focal plane of the reflectorplane of the reflector

Because they are densely Because they are densely packed, there exists a high packed, there exists a high level of mutual coupling level of mutual coupling between the elementbetween the element

Receiver NoiseReceiver Noise

Noise Temperature Noise Temperature and Gain of the and Gain of the first element have first element have the greatest effect the greatest effect on overall noise on overall noise temperature of the temperature of the systemsystem

Usually Low Noise Usually Low Noise Amplifier (LNA)Amplifier (LNA)

Low Noise Amplifiers (LNA)Low Noise Amplifiers (LNA)

Designed to add very Designed to add very little noise to systemlittle noise to system

Very Little ≠ NoneVery Little ≠ None

Most of the noise Most of the noise travel toward the loadtravel toward the load

A small portion travels A small portion travels back toward the back toward the antenna and is antenna and is radiatedradiated

Noise CouplingNoise Coupling

Noise signals from Noise signals from LNA travel towards LNA travel towards antennas and are antennas and are radiatedradiated

Radiated noise are Radiated noise are received by other received by other elements and elements and amplifiedamplified

This is referred to This is referred to as as Noise CouplingNoise Coupling

Calculating Noise CouplingCalculating Noise Coupling

Represent coupled, noisy network as Represent coupled, noisy network as equivalent circuit diagramequivalent circuit diagram

Direction of noise coupling

LNA Noise ModelLNA Noise Model Optimum Source Optimum Source

Admittance (YAdmittance (Yss=Y=Ymm))

• Specific source admittance Specific source admittance allowing for optimum allowing for optimum noise matchnoise match

Minimum noise Figure (FMinimum noise Figure (Fm m ))

• Noise Figure resulting Noise Figure resulting from optimum noise from optimum noise matchmatch

Equivalent Noise Equivalent Noise Resistance (RResistance (Rnn))

• Representation in Ohm of Representation in Ohm of the spectral density of a the spectral density of a noise-voltage generatornoise-voltage generator

Coupled AntennasCoupled Antennas

2 Coupled Antennas 2 Coupled Antennas at a timeat a time

All elements are All elements are identical, only identical, only coupling terms coupling terms differdiffer

Use superposition Use superposition to calculate total to calculate total couplingcoupling

Calculating Noise CouplingCalculating Noise Coupling

Equivalent Circuit diagram now…Equivalent Circuit diagram now…

Direction of noise coupling

Calculating Noise CouplingCalculating Noise Coupling

Reduce Circuit diagramReduce Circuit diagram

Calculate noise power coupled to Calculate noise power coupled to each elementeach element

Active Noise Figure and Active Noise Figure and TemperatureTemperature

Problem:Problem: If coupled noise is considered input noise, the If coupled noise is considered input noise, the input SNR is degraded, this results in false impression that input SNR is degraded, this results in false impression that the Noise Figure of the LNA improved.the Noise Figure of the LNA improved.

All LNAs are seen as one “multiport LNA bank”All LNAs are seen as one “multiport LNA bank”

Add coupled noise from other LNAs in the bank to the noise Add coupled noise from other LNAs in the bank to the noise power generated by the isolated LNApower generated by the isolated LNA

Verification ExampleVerification Example

Active Noise Temperature of 8x8 Active Noise Temperature of 8x8 Vivaldi ArrayVivaldi Array

Added noise Added noise temperature of 14 temperature of 14 KelvinKelvin

≈ ≈ 9% of Isolated 9% of Isolated Noise TemperatureNoise Temperature

For SKA ≈ 1.3 K !!!For SKA ≈ 1.3 K !!!

SKA TSKA Tsyssys Budget Budget

Estimate Estimate ΔΔT TableT Table

Any Questions?Any Questions?