8. Back End


8.2 Continuum

8.2.1 Analog Total Power Backend (2-GHz bandwidth)

The backend is formed by 4 printed circuit boards, also including a digital part for board control and setup, and 4 voltage-to-frequency converters to digitise the detected signals. The digitised samples are sent to the acquisition computer by means of a LAN (Local Area Network). Each board is designed to have three inputs, so that it can be connected to different receivers.
The backend architecture is such that each input, before detection, is available at two backend outputs. This allows the system to simultaneously send every receiver output to another backend, for instance a FPGA backend (e.g. for spectroscopy or VLBI observations), or to a remote detector (this last chain is specifically optimised in dynamics for a fibre optic link).
Prior to the detection, the signal goes through:
 - a variable attenuator, to set the signal power level in a suitable range;
 - a filter bank, to reduce the bandwidth if necessary. 
All these characteristics make this system both a data acquisition machine and a signal distributor.
The control stage of the acquisition system, realised by a small Linux computer with an embedded FPGA board, allows the control system to remotely set up the equaliser, the attenuation values, the filter bandwidth and the sample rate of the acquisition.
The sampling signal can also control the switching of a noise source of known temperature, which is injected through the receiver chains, so that the gain variation of the receivers can be tracked and compensated during the post processing phase.
The main specifications of this backend can be summarised as follows:
14 x 3 IF inputs in the range 0.1 - 2.1 GHz;
2 IF outputs in the range 0.1 - 2.1 GHz;    
selectable bandwidths: 230, 725, 1200, 2000 MHz;
0 - 15 dB variable attenuators, configurable with 1 dB step;     
up to 21 bit resolution;
automatic cancellation of output offset;
noise source chopping frequency: 0.5 - 500 Hz.
The backend dynamical range varies with the data sampling interval. 
Rule-of-thumb formulas, specific to this backend, to compute the available bits and the dynamical range in dB are:
  n_bit = fix(ln(1800*t)/ln(2))-2
  n_dB = 20 * log10(1800*t)
where t is the sampling interval in milliseconds (minimum value is 1 ms).

8.2.2 Mark IV

Total Power observations can also be performed using the Mark IV terminal and the "Field System" software.

The terminal is made of two parts: IF distributor (receives the input from the Front End and splits them in sub-bands), Videoconverters (14 units that operate the base-band conversion and the integration). It is possible to choose between two outputs :

A) 28 narrow bands : minimum width 0.125 MHz, maximum 16 MHz*, central frequency user-defined (maximum total bandwidth 400 MHz).

Fig. 8.3 : Maximum bandwidths (MHz) processed by the Mark IV, example at 22 GHz

* 0.125 MHz and 1 MHz band are available only with external filter, please ask if available at the site.

B) Processing of the whole input:  2x400 MHz centered at 300 MHz and 1x400 MHz centered at 700 MHz

Fig. 8.4 : Maximum total bandwidths (MHz), example at 22 GHz

At 1.4, 1.6, 2.3 GHz available bandwidths may be considered less then what is reported because of interferences.

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