I have a test instrument that measure spectral energy density versus frequency. The measurement is extremely low noise. The RF input on the test instrument is an SMA-connector where the inner conductor is for the signal and the outer conductor is electrically connected to earth ground (the 3rd prong in the power outlet; as typical for high-performance scopes and other RF test equipment).
I connect an SMA cable between this input and an SMA bulkhead connector that sits in a penetration panel of a shielded enclosure. The cable's outer conductor is electrically connected to the shielded enclosure, which is a solid (welded) metal box that is only grounded via the shielded cable to the test instrument. On the inside of the shielded enclosure, the SMA bulkhead connects via another SMA cable to a device under test (DUT) that produces a signal for the test instrument to measure.
Using the test instrument, I see a lot of low and high frequency spurious noise on the signal.
If I DISCONNECT the DUT, I see the same spurious noise, which leads me to believe the shielded enclosure (which is roughly a 4 foot cube) is acting like an antenna and picking up electromagnetic fields, which I'm sensing as a small voltage via the outer conductor of the SMA cable connected to the test instrument.
Is there any way to remove or reduce this noise (whose frequency could be anywhere from 1 Hz to 18 GHz)?