EAON circular # 8

Radio-controlled clocks

        Many observers are tempted to use a so-called "radio controlled clock" as a time-keeping source. Such a clock contains a receiver for radio signal (usually 77.5 kHz), a decoding circuitry, and a quartz clock. The clock is reset from the data decoded from the radio signal.

        To save batteries, most of such clocks connect the receiver only for 5 minutes, once a day (usually at 3h or 4h in the morning). If the radio reception is not good at that time, the bips are NOT decoded, and the clock IS NOT RESET.

        UNFORTUNATELY, the quartz clock is not often of good quality (this is not supposed to be useful, since the clock SHOULD be reset every day) : tests show that a drift of 0.05s per an hour are not unusual, that is to say 1.2s a day !

Such a clock CANNOT be used for timing !

        Please, FIRST test the drift of your clock against a reliable time source : to avoid a reset during the few days of the test, simply wrap the clock in some aluminum sheet, or use any other kind of radio shield.

        Check up the indication of the clock a first time with a reliable time source, and again one day (or more) later. So you will know the drift, and be able to take it into account.

For a better radio reception :

1/ Do not leave your clock at less than 2 meters from TV or PC screens.

2/ Avoid the nearness of metallic elements, such as metallic doors or windows.

3/ Inside a concrete building, the radio reception can be very weak. The radio receiver uses a ferrite antenna which must be oriented perpendicular to the direction of the transmitter (Frankfurt am Main for DCF). A metallic window shield, usually closed at night, can prevent any clock reset.

4/ When far from the transmitter, the reception can be weakened by ionospheric perturbations and eletrical storms.

5/ Avoid mechanical impacts and vibrations, sudden changes in temperature, sunshine, extreme cold and humidity.

6/ To be sure that the clock will reset (if the radio reception is good enough), you can remove the battery, short-circuit the contacts during half a minute, and then replace the battery : the clock should reset within a few minutes.

    Have a clear sky !

Raymond DUSSER, 2003.05.21