[Gb-ccb] RFI screening requirements

[John Ford]

Martin Shepherd writes:
 > It occurs to me that I really don't know what Green Bank's RFI
 > screening requirements are. Discussions at different times have
 > implied widely differing requirements.
 > 
 > For example, at the preliminary design review I asked whether a
 > honeycomb RFI ventilation filter would be ok for the cooling fan,
 > given that the attenuation of this filter dropped off rapidly above
 > 10GHz. I was told that this fan-screen would meet GB's RFI
 > requirements. Now however, there is concern at GB that the 2.5mm hole
 > in an all-metal optic-fibre feedthrough connector would not meet Green
 > Bank's RFI requirements. These two data-points appear to be
 > contradictory.

They are not contradictory.  The honeycomb filter is a waveguide
beyond cutoff structure.  A hole in a connector is not.  We've just
about convinced ourselves that the connector is fine.  Wait another
couple of days.

 > 
 > I am guessing that the difficulty in specifying this revolves around
 > the unknown of how much radiation a single-board computer and FPGA
 > will generate at microwave frequencies. I don't have the experience
 > necessary to make an educated guess at this, whereas I assume that
 > people at GB do.

This is the hard part, and this is why we are making suggestions to
you.  In general, any well-designed digital electronics in the
receiver room will need 60 to 100 db of shielding from DC to 10 GHz to
avoid interference with observations.

 > 
 > So, could somebody at GB please provide me with a specification for
 > the cutoff frequency needed of the RFI screening?

Our requirements  are very clear.   Jeff, would you please send Martin
the plot that Chuck generated of allowable emissions vs frequency?  Or
other documents that you think would be more helpful.

John