|Hajee Pepping & Daniel van der Schuur
| Recently, great progress has been made in the implementation of the Apertif correlator and in the transpose functionality. This progress is visualized in the block diagram. In this update, we elaborate on the verification method of the correlator cell, in the dashed blue rectangle of the block diagram.
To verify the correlator firmware module, we generated (12 telescopes * 2 polarizations) 24 input signals. Each input was the same simple sine wave, but shifted by one degree in phase w.r.t. its neighbour. We then let the correlator firmware cross-correlate these 24 signals, and plotted the resulting phase differences. The first (single channel) plot shows 0 degrees of phase shift between input 0 and itself (red), up to 23 degrees of phase shift between input 0 and input 23 (blue). This shows that the correlator correctly processes a single channel.
As the correlator processed 64 channels, we included the channel filterbank firmware, which produces 64 channels for each input. Instead of a single sine wave per input, we now fed the sum of 13 sine waves (again shifted in phase w.r.t. each other) to each of the 24 inputs of the channel filterbank. The second graph shows plots of all 64 output channels of the correlator: 41 channels that do not carry a proper signal, and 13 channels that do.
The final graph highlights (color) the 13 channels that should carry a signal, and again we see the correct phase differences - the channel-filterbank-correlator subsystem is working as expected.
Overall, great progress has been made and a lot of uncertainties have been taken away. There is still work to do, but we are still on track of having a correlator system that can generate the first Apertif fringes in August 2015.