Our SETI receiver project was designed with the collaboration of some people of Istituto di Radioastronomia di Bologna .
Considering our limited economic resources, we tried to realize the cheapest design that should permits to obtain reliable measurements, comparable with scientific results obtained in professional radioastronomical sites.
It is basically a direct conversion receiver that convert input signals around the hydrogen line frequency (1420MHz) to an intermediate frequency of 5MHz, with an output banwidth of 10MHz.
Some parts were bought from Mini Circuits Europe (mixer, 10MHz low pass filters, IF amplifier ) and some others were designed and built by ourself ( input band pass microstrip filter, four stage RF amplifier with MMIC).
Frequency stability was one of most critical issues, considering the kind of signals that we are looking for (narrow band microwave carriers). It is primarly determined by local oscillator stability.
Currently the local oscillator signal is generated by an HP8660C synthsizer, with high stability option, that provide a sufficent frequency stability for our kind of research ( ~ 5 * 10-9 Hz ).
A considerable improvment should be obtained using the Motorola Oncore UT+ GPS receiver to generate a more stable 10MHz external reference to lock the synthsizer; we are thinking to use for this purpose the interesting Brooks Shera's GPS based frequency standard, in order to achieve up to 1 * 10-11 Hz of stability.
The following picture shows a general schematic, with the main features of each receiver component: links on bottom of the page will display more details regarding most critical components.
The complete receiver is assembled into a metal box. Each component of the receiver is visible in the picture below, including the interconnections between elements realized by rigid SMA cables: