Abstract: A portable device 12 is capable of communicating with a host device 10 remotely, via a communications network 22 which may be prone to interception, and directly (e.g. via an I.R. Link 24, 40), when the devices are in close physical proximity and when the risk of interception is minimal. The host device 10 and the portable device 12 update a shared security key or the like when they communicate directly and this key is used to authenticate the portable device 12 and/or encrypt the communication when the portable device 12 attempts to communicate remotely.

Abstract: A method for tracking an incrementally sweeping generator by a frequency selective receiver is disclosed. A sequence of test signals is generated each of which has a discrete and different frequency which is periodically and incrementally modified according to a program, and the dwell time of each frequency is sufficient to obtain at least two receiver measurements. This sequence of test signals is supplied to a device under test, and the output signals from the device under test are fed to the frequency selected receiver which operates at one of the discrete frequencies supplied according to the program. The operating frequency of the receiver is modified according to the same sweep program and with at least the same sweep rate. The levels of at least two consecutive measurements at the receiver output are compared with at least one reference value which is selected such that it discriminates against other signals that are not part of the sweep program.