1. Noise Coupling in Focal Plane Arrays
Jacki van der Merwe
MScEng
Stellenbosch University

2. Outline Focal Plane Arrays Receiver Noise Low Noise Amplifiers
Introduction to Noise Coupling
Calculating Noise Coupling
Active Noise Figure and Noise Temperature
Examples

3. Focal Plane Arrays Large, dense phased arrays of antennas
Placed in focal plane of reflector
In theory, any number of beams can be formed
Enables complete collection of the entire electric field in the focal plane of the reflector
Because they are densely packed, there exists a high level of mutual coupling between the element

4. Receiver Noise
Noise Temperature and Gain of the first element have the greatest effect on overall noise temperature of the system
Usually Low Noise Amplifier (LNA)

5. Low Noise Amplifiers (LNA)
Designed to add very little noise to system
Very Little ≠ None
Most of the noise travel toward the load
A small portion travels back toward the antenna and is radiated

6. Noise Coupling
Noise signals from LNA travel towards antennas and are radiated
Radiated noise are received by other elements and amplified
This is referred to as Noise Coupling

7. Calculating Noise Coupling
Represent coupled, noisy network as equivalent circuit diagram
Direction of noise coupling
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

8. LNA Noise Model Optimum Source Admittance (Ys=Ym)
Specific source admittance allowing for optimum noise match
Minimum noise Figure (Fm )
Noise Figure resulting from optimum noise match
Equivalent Noise Resistance (Rn)
Representation in Ohm of the spectral density of a noise-voltage generator
2 partially correlated noise sources

9. Coupled Antennas 2 Coupled Antennas at a time
All elements are identical, only coupling terms differ
Use superposition to calculate total coupling

10. Calculating Noise Coupling
Equivalent Circuit diagram now…
Direction of noise coupling

11. Calculating Noise Coupling
Reduce Circuit diagram
Calculate noise power coupled to each element

12. Active Noise Figure and Temperature
Problem: 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 bank”
Add coupled noise from other LNAs in the bank to the noise power generated by the isolated LNA

13. Verification Example

14. Active Noise Temperature of 8x8 Vivaldi Array
Added noise temperature of 14 Kelvin
≈ 9% of Isolated Noise Temperature
For SKA ≈ 1.3 K !!!

15. SKA Tsys Budget

16. Estimate ΔT Table

17. Any Questions?