RF front end for a phased array satellite antenna systemMobile satellite communications on planes, boats or buses require low-profile antenna systems. The antenna systems used for such applications are usually electronically steered phased arrays. A phased array antenna system is an array of antennas in which the relative phases of the respective signals received by each single antenna are delayed in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired direction.
Such systems usually provide reception of part of the Ku-band (10.7 - 12.75 GHz) for vertical as well as horizontal polarized signals.
Bruco has made two ASICs and the pcb for a phased array antenna system, capable of receiving the full Ku-band at once. The buildup of the antenna is modular, and consists of multiple tiles. A tile (roughly 10x10cm) is built up out of 64 antenna elements (an 8x8-array). The total required delay is rather difficult to achieve electronically. In this antenna system the solution is found in making use of an optical beam former (OBFN). An antenna tile consists of 2 different ASICs and an OBFN.
Receiving the full Ku-band instantaneously requires a system bandwidth of 2 GHz, while the group delay is constant.
The noise figure (NF) of the total system should be NF=1.5dB, therefore requiring an NF≈3dB for the ASICs.
The satellite is transmitting horizontal and vertical polarized signals, hereafter called V and H. The antenna is placed on a moving object, so the polarization of the receiving signals with respect to the antenna is arbitrary. The antenna puts out an X and Y component. Before a satellite receiver can process these signals they should be translated back in V- and H-polarization. This position processing is done by the ASICs.
The complete system for 64 antenna elements should fit on the back of the antenna tile of 10x10cm.
The ASICs are made in a fT≈180GHz SiGe BiCMOS technology providing us with the best components to develop our RF radio.
To realize the delays on the two ASICs real true-time delays (TTdelay) are used instead of phase shifters. Phase shifters as delay element will have a group delay which is not constant over frequency of the 2GHz bandwidth.
The first ASIC (ASIC1) processes the signals of four antenna elements. The signal processing starts with a LNA, and through an I/Q mixer the signal is transferred to lower frequencies (2 GHz, IF). Each of the 4 signals gets a dedicated delay through a TTdelay circuit. After combining the 4 input signals the V and H polarization is calculated from the X and Y polarized signals.
The second ASIC (ASIC2) combines the outputs of four ASIC1 signals; again by adding a dedicated delay via a TTdelay circuitry. The output of ASIC2 is the combined signal of sixteen antenna elements. The OBFN combines the outputs of four ASIC2 , and provides the larger delay.
A dedicated rf board is designed (13 metal layers) with an integrated antenna patch, RF routing, and on the back five ASICS.
The project is realized in a Dutch program called Satrax, subsidized by the Dutch government. The consortium consists of the following Dutch parties: Satrax, NLR, University of Twente, VPS, Lionix, Phoenix and Astron.