An alarm system having the capability to self-adjust or self-calibrate itself is disclosed. A differential interference contrast microscope includes a beam-shearing assembly that includes a beam-shearing component. The dome has an inner semi-hemispherical surface located close to the signal converter to improve the signal strength. When the selected frequency band does not include an unused channel or one in which no disturbing wave is present or there is no transmission rate associated with the selected frequency band at which the received field strength value exceeds the threshold value, the next highest frequency band is successively selected and the above is repeated.