10MHz wide base band, converted from input 1420MHz by the RF receiver, is directly converted to digital by an high speed Analog/Digital converter card.
Our choice were the expensive AD1280DX, built by Ultraview. It is a dual channel 40Ms/s 12 bit A/D card with a PCI bus interface for common PC use, very powerful and easy to use under Windows or Unix. It has a wide input range (6Vpp) and a good resolution (approx. 2mV as LSB).
It let to make a 20Ms/s sampling of the output receiver signal in order to achieve a correct digital conversion of the entire 10MHz receiver bandwith, with a computing speed high enough to be near real time also with following operations. Infact, after this A/D conversion process, a FFT ( Fast Fourier Transform ) will be calculated to convert the receiver output signal from time domain to frequency domain. This part will be illustrated in next Data Acquisition System section.
Two main disadvantages of using this card are:
high cost of the card itself, expecially if compared with lower levels A/D cards such as Audio PC cards;
high sensitivity to internal PC generated RF interference, since that the card is placed inside the PC box, a very noisy environment.
The use of a common audio PC cards it is a strong compromise with this kind of project, due to their limited sampling rate, usually 44kHz .By this way it could be possible to realize digital receivers/analyzer with a maximum bandwith of 22kHz. In next chapter will be clear how the receiver simultaneously analized bandwith is one of more important parameter regarding the powerful of a SETI microwave system.
