The WSRT telescope does have the property to run as a Tied array. A specific dedicated signal processing Unit, the Tied Array Adder Unit takes the output of each selected antenna, apply a proper time delay and add the signal in phase. Each antenna signal is sampled by a 2 bits analog to digital converter and the phasing, addition and scaling is realized in the Tied Adder Array Module (TAAM). It ends up with 12bits output signal. We have to consider 64 12 bits signals at the output of the TAAM, 8 subbands, 2 polarizations and 2 sub-arrays with 20MHz bandwidth (sampled 40MHz).
The Tied Array Distributing Unit (TADU) is a new unit giving the possibility to route and apply specific signal processing on the output signals from the TAAM.
TADU distributes the TAAM output signals to 5 different output clients. The VLBI (Very Long Baseline Interferometer), PUMA (Pulsar...), spare digital 1 (32 digital signal) and spare digital 2 (16 digital signals) output and spare analog output constitute these different clients. The signal characteristics and requirement are different for each output client. Therefore, specific embedded processing should be available on the TADU board.
The main client for the signal processing point of view is the VLBI. The 12 bits input signal 20MHz bandwidth should be processed to end up with a scalable bandwidth and quantization output. In fact, the European astronomical community proposes to normalize the signal output of the telescopes to enhance the VLBI quality and data exchange.
A presentation given during the IVS General Meeting (Ottawa) proposes an architecture called the Digital Base Band Converter (DBBC). This architecture should deliver different quantization and bandwidth outputs for a single antenna telescope. The TAAM gives the possibility to emulate the WSRT as a single antenna telescope by combining the signal from the 14 25 meter dishes. Therefore to be compatible with the next generation of base band converter and to realize a possible interfacing with other telescopes, the requirement of TADU should match the DBBC proposal by delivering a scalable quantization output range with 2,4,8 output bits (1 bit sign, 1, 3, 7 magnitude bits).