Bulk and fine delays - orders of magnitude and practical values.



The bulk delay is obtained by shifting the write pointer of one antenna relatively to a center value. The read pointers sequence has a fixed timing which is  the same for all antennas. The fractional part of the delay is performed in the software by linear ponderation of the phase after the FT and prior to long term integration.

One increment in address pointer has the effect of delaying the data presented to the correlators by 16 samples, or 4ns. For +/- 2km the geometrical delay has to vary +/-6.6 us, or  +/- 1650 integer values . The demultiplexing scheme allows to embed the bulk delay as long as its excursion is  smaller than 1/16th the sector playing time, or 33us in our case. This is sufficient for PdB and its future needs.

For +/- 2km the geometrical delay changes at the maximal rate of 0.5 ns/s so at fastest the integer part will change every 8 seconds. The hardware is such that changes in bulk delay will be effective at the 32 pps event next to the time the delay card has received the new command word. The signal will be correlated during the following 1/32th sec period and the products read out one period after. The correlator can execute an integer delay change any time. However,if a coordinated phase compensation with the LO2 is needed, it might be a good idea to wait for the next 1pps event to apply the change.

The fractional part of the delay is applied on a whole signal segment of 1/32th sec. During that period the sky delay is assumed to be constant (residual<15ps).  Whenever the bulk delay value needs to be changed, the software should first change the integer part of the delay and later (3 to 4 periods of 1/32th sec) change the corresponding fractional part, in order to compensate for several latency  times  across the system .