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4.Front End |
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4.Front End |
4.2 Distributions
Usually the connections among the radiotelescope's foci involve three different kinds of signal :
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Local Oscillator : in order to cut down the expenses related to the construction of an high number of independent superheterodyne recevers, a very common solution is to share some local oscillators (at least for one conversion). A single local oscillator therefore can serve more receivers through a signal's distribution system. |
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IF : the RF signals, once received and converted by the Front End, must be sent to the Back End (installed in the BWG cabin or in the Control Room) for the processing. |
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Reference : reference's signals which are necessary for the correct antenna calibration |
All the signals will be distributed via coaxial cable, the connection between the antenna and the control room will be instead realized with a fiber optic link.
The OL distribution, necessary for the conversions, is sketched in the following :
Fig. 4.2 : Local oscillator signal's distribution, first conversions
The first conversions in gregorian focus and BWG will be served by the same two local oscillators (one per each channel), installed in gregorian focus. The signal's distribution systems are named OLD.
This choice allows the two channels to be placed in different position inside the receiver's useful band, enlarging the frequency range for spectral line studies. If there won't be the need of this solution, it will always be possible to have two equal channel (polarimetric usage or, in total power, more sensibility), using one local oscillator only.
The second conversions will be instead independent (each receiver will be equipped with its local oscillator)
In primary focus the conversions will be all independent, with fixed local oscillators : the receivers located at this focus have the useful band as wide as the istantaneus band, therefore the channels must be superimposed.
If in the future a higher frequency receiver will be installed in primary focus then an OLD system from the gregorian focus will be developed, for at least one conversion.
The IF signals will be distributed through a multiplexing system, which will allow the handling of an high IF's number. This system has been choosed because more multifeed receivers are likely to be installed at each focus.
The multiplexer are separate from the dewar systems, so that any future operation on the receiver (maintenance, raplacements, etc...) will be easier.
In its first configuration the systems will be able to handle 20 IF channels per each focus, it will be possibile to increase this number by placing more analogous systems in series.
Fig. 4.3 : IF signal's distribution
The usage of coaxial cables for long connections implies a disequalization of the signal which will be offset by ad hoc amplifiers (equalizated amplifiiers AmpEQ, a prototype is already under test at the Medicina antenna). Each focus will be equipped with its own AmpEQ system.
COMPENSATION OF DISEQUALIZATION DUE TO LONG COAXIAL CABLES PATHS (.pdf - Italian only)
Also the distibution of the reference signals will be realized with coaxial cables, as shown in the following :
Fig. 4.4 : Reference signals
