Quote Originally Posted by Mike Valentine
"The failure to alert to a particular speed sign might have one of several causes:

1) The speed sign may employ an FM-CW modulation technique to sense the direction of vehicles in its field of view. This feature is used to isolate departing from oncoming traffic. The V1 does not recognize FM-CW modulation as a valid police radar signal and tries to reject it. FM-CW is allowed for license-free "Part 15" devices by the FCC. FM-CW is not allowed for police radar. There are numerous traffic-volume sensors using FM-CW spread all around metropolitan areas these days that detector owners would rather not produce an alert. Otherwise freeway driving (in Cincinnati, at least) would be very annoying. It's a good thing that the V1 rejects FM-CW modulated signals.

2) The speed sign may have drifted out of the police radar band from aging or temperature. "Part 15" devices are somewhat notorious for this behavior. Automatic door sensors and Doppler security sensors are both "Part 15" devices, too. Few automatic door sensors use FM-CW because of the added expense. The speed sign devices can afford the added cost of including the FM-CW modulating hardware.

3) The V1 in question may need for our service department to have a look at it to make sure it is operating correctly."