Fig. 13 Pulsar System
The Pulsar Backend PUB86 is the central data taking facility of the pulsar observing system. The system also comprises pulse generators - controlled by the control computer (via CAMAC) - to provide the necessary timing, a detector unit, a 16 channel filter unit and a 240 channel digital dedisperser.
Fig. 13 shows the block diagram of the system. The unit named PUB86 is a fast data acquisition device with 4 input channels. Its timing is controlled by a signal synchronous with the pulsar period that defines the beginning of the measurement window. Pulsar periods from 1.024 ms to about 10000 s are possible, sampling intervals (phase bins) range from 1 microsec to 4096 microsec. 1024 samples can be stored per pulsar period and synchronously integrated over 1 to 65535 periods, before the data is transferred to the main computer system.
The timing device also generates a signal starting at the beginning of the measurement window with a duration of several samples. This signal switches the calibration noise source of the frontend in use.
Input to the PUB86 comes either as a frequency encoded intensity from a detector unit with fast (5/10 MHz) V/F-Converters or as parallel digital data. In the first case the integration over one sampling interval is done within the PUB86 by counting, in the second case the signal is read into the memory at the sampling rate of the individual phase bins.
The PUB86 is controlled via an IEEE-Interface from the Control Computer, the accumulated data are transferred to the computer system via the DMA Data Bus.
The dedisperser has 4 * 60 channels (see Fig. 14). Within one unit the IF signal is split at the input into a filter comb of 60 filters of 660 kHz bandwidth each, detected and converted to a frequency proportional to the signal power. This signal is counted over a time interval chosen in such a way, that the integration time is equivalent to the differential delay caused by the interstellar medium. The result of the integration of each channel is shifted as the preset value to the counter of the next filter and counting adds to this value for the same interval. The output of the last channel is the pulsar signal with the dispersion removed. The delay per channel can be chosen between 6.3 ms and 9.99 ms.
!!! The dedisperser needs the correct sideband (upper) to work properly !!!
A 16 channel filter unit has recently been added to be used for broadband dedispersion during e.g. pulsar search. It is constructed for fast sampling and uses a special data compression technique. See Fig. 15. A new multichannel data collecting backend to be used with these and other filterbanks is under construction in the digital lab.
As a result of a cooperation with the University of Berkeley (Prof. D. Backer) a new pulsar backend, the Effelsberg Berkeley Pulsar Processor (EBPP) has recently been added.
Observers should contact A. Jessner at the 100-m-telescope for further details concerning the use of pulsar measurement equipment.
