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Design Notes

VSS Phased Array Signal Splitter for Receivers (PHARRAY_RXSIG)

This project demonstrates:

1.   The use of the VSS PHARRAY_RXSIG block

2.   The use of VSS RFB measurements

3.   How to measure Gain-to-System-Temperature-Ratio

The built in G/T measurement computes the ratio of isotropic antenna gain to system equivalent noise temperature.  The Gain is measured directly from output port of antenna, and the noise at output of system is referred back to this antenna output port.

To measure G_T of entire Phased array, the noise and Gain must be measured separately and G_T computed using equations since we cannot directly access gain of single antenna.

Because every noise contribution is multiplied by the amplifier gains g > 1 and the attenuation coefficient e, and the signal contribution likewise, then the S/N (and antenna G/T) is constant throughout the cascaded graphs.  This means we can measure G_T from any reference point.

In the system diagram Test,

which consists of single cascade of Antenna and Amp, the built in G_T measurement referenced to output of antenna, is compared to budget measurement of SPWR_node and T_node at output

G_T calculated as:

(SPWR_node(output)-SPWR_node(input))-10*log(T_node(output)

G_T agrees with the built-in measurement.

This method was then applied to complete array.  The input source was split using PHARRAY_RXSIG.  The input signal is passed as-is to all the channels.  This is akin to defining a multipath channel to all the arrays.  This ensures that the gains of the antennas add correctly

The source and the PHARRAY_RXSIG are made noiseless and an antenna temperature is added to each antenna.  This means that the antenna temperatures Tant are uncorrelated.  If the noise were instead added to the source (making each antenna noiseless and due account taken of antenna gain by scaling source temp  by factor 10^(-antgain/10)) to give same noise temp at output of each antenna, then in this case the  antenna noise would be fully correlated.  This is because of the action of the PHARRAY_RXSIG - noise generated outside of the PHARRAY_RSSIG emerges correlated.  Noise generated internal to the element emerges uncorrelated.

Correlation of the Antenna noise gives a higher Tsys and worse G_T

Some references used in this project:

- Memo 98.  Definition of Array Receiver Gain and Noise Temperature.  SKA Bert Woestenburg

- www.skatelescope.org/PDF/memos/98_Memo_Woestenburg.pdf

- www.skatelescope.org/SKAmeeting_Paris/pdf/ArrayGTDef.pdf

The references discuss ways antenna gain and noise values may be modified to take into account coupling between array elements, using conventional 2 port analysis.  This has not been taken into account in this project