Pulse loop
From Tardis Wiki, the free Doctor Who reference
A pulse loop was a simple device that could be used to trace faults on the master circuits of the Matrix using a photon pulse.
Chancellor Thalia suggested using one to distract Omega, as he would have to track it down to check if the High Council were attempting to bypass the Matrix control. In the confusion, the Fifth Doctor was able to leave Gallifrey in his TARDIS. (TV: Arc of Infinity)
