Fig. 7 Continuum System
The so called "Digital Backends" are used for continuum observations in Effelsberg. Fig. 7 shows the block diagram of a typical system including monitor equipment. The observation is divided into cycles consisting of a number (typically 4) of measurement intervals of equal length, typically 16 ms each. During one interval the status of the frontend switches is kept the same, the signal power is detected by a square law detector or multiplier and then converted to a pulse train having a frequency proportional to the signal power.
The integration of the signal over one measurement interval is now performed by just counting these pulses during this interval. By stopping the counter during blanking time it is also easy to exclude signal transitions from integration. Another advantage of this scheme is that detection and A/D-Conversion can already be done in the frontend at least in the case of broadband detectors and polarimeters. Sending digital pulses over 300 m of cables is less sensitive to instabilities than IF-Signals of e.g. 500 MHz bandwidth.
Fig. 8 shows how the numbers obtained during measurement cycles are treated in the Digital Backend and in the control computer prior to transfer to the data reduction programmes. There is a large flexibility of switching schemes and data handling, only one example is therefore given. The Digital Backend is completely controlled by the control computer and the user is free to choose his own way of observation, switching and integration.
The maximum number of channels to be recorded is 80. The observer can choose, which channels he wants to be measured. Only data from these channels are integrated and transferred to the control computer.
Most of the receivers at the 100 m telescope have two or more channels and may be used for polarisation measurements. We use IF-polarimeters to deal with these systems. Fig. 9 shows the block diagram of a polarimeter. Narrowband (up to 200 MHz bandwidth) polarimeters are connected to the Narrowband IF in the control building, broadband polarimeters (up to 2 GHz bandwidth) are built into the frontend boxes.
