Meinberg UA32S - Manual (Page 8)

8
Antenna and LF Receiver
An external ferrit antenna is used to receive the signal from DCF77. Optionally, a
weather-proof antenna is available which can be mounted outdoor. A coaxial cable
which can be up to more than 100 meters in length is used to pass the antenna's output
signal to the on-board LF receiver where it is demodulated by a synchronous detector
with automatic gain control. The demodulated time marks are fed to the clock´s micro-
processor.
Mounting the Antenna
The antenna case should be installed at least 30 cm away from the board and from steel
girders or plates. The brighness of the LED “Mod” in the front panel depends on the
signal strength of the 77.5 kHz carrier. In order to get the maximum signal, the antenna
should be aligned in two steps. First it should be turned slowly until the “Feld” LED is
mostly dimmed. Finally the antenna must be turned by 90° from this position to obtain
maximum signal.
If the antenna is installed properly and the signal from DCF77 can be received without
strong distortions, the LED labeled "Mod" starts blinking exactly once per second,
corresponding to the time marks from DCF77. If this LED flashes intermediately, there is
some electrical noise around which prevents the microprocessor from decoding the time
message. In this case, a better location for the antenna must be found.
After reset, the LED labeled “Freil.” indicates that the clock is running on xtal and has
not synchronized with DCF77 yet. Due to the plausibility checks, it can take up to three
minutes after power-up until the clock is synchronized and this LED is turned off. The
state of this LED only changes when a new minute begins.
Microprocessor Circuit
Time marks from the receiver circuit are filtered and decoded by the microprocessor. If
no errors are detected in the current time message an additional plausibility check against
the previous time message is performed. If that plausibility check passes, too, the real
time clock on the board is adjusted corresponding to the decoded time and date. The real
time clock is read periodically and it's date and time are passed to the display driver and
the serial port drivers. Additionally, the microprocessor controls the switch outputs. An
on-board microprocessor supervisory circuit provides a watchdog timer which lets the
microprocessor recover from malfunction, along with a power-fail comparator which
resets the microprocessor if the supply voltage drops below a specified threshold.