This invention relates to a method of receiving a first potentially unreliable clock signal having a first frequency; receiving a second reliable clock signal having a second frequency; wherein the first frequency and the second frequency have an expected relationship; determining whether the first potentially unreliable clock signal has changed with respect to the second reliable clock signal by: determining an actual relationship between the first potentially unreliable frequency and the second reliable frequency; and generating an alarm signal if the actual relationship is different to the expected relationship.