The shield electrostatically isolates the RF amp
on the right from the 3496Hz LO signals on the left. The cover must be
installed with at least the 2 shield screws in order to completely eliminate
the undesired signals. The shielding is adequate when the deep audio null
does not shift when the RF amp is switched from Hi to Lo gain. For
this test, the RF gain pot is set to about 2/3 of max, and the loop antenna
is NOT connected to the BNC input jack. The lock alarm is on the
left to be as far away from the RF amp as possible.