Meinberg USB 5131 - Manual (Page 7)

7
Installation
Power Supply
The USB5131 is powered via the PC’s USB port. After connecting the USB port the
radio clock is ready to operate. For proper operation it is essential to pay attention to the
following points.
Aligning the antenna
Generally, the exact alignment of the USB5131 module with its internal antenna is
important. It has to be placed in longitudinal direction to the transmitter (Frankfurt/Main),
see arrow printed on the label of the module. A distance of several meters to TV- or
computer monitors, to computers or microprocessor boards should be kept. These
devices can produce interference fields that lead to a poor or no DCF77 receiption. The
USB5131 should be installed with a minimum distance of 30cm to all metal objects.
Otherwise the ferrite antenna will be detuned and the received power decreases. If it is
not possible to archieve a sufficient reception with the internal antenna, it is possible to
connect an external antenna to find the optimal antenna location. An indoor antenna
AI01 or a weatherproofed outdoor antenna AW02 can be connected to the SMB
connector using a coaxial antenna cable with length of up to some hundret meters. An
external antenna is recommended whenever difficult conditions regarding the reception
reign. Alignment of the antenna (internal as well as external) can be done by watching
the modulation LED. Like described in chapter „General information about DCF77“,
undisturbed pulses per second (except 59. second) without any interfering pulses are
required for corrcect decoding of the DCF77 time telegram. Therefore, a modulation
LED blinking exactly once per second is a criterion for good receiption of the DCF-
signal.
Status LEDs
The two LEDs „Mod“ and „FR“ reflect the state of synchronisation of the decoded time
marks of the USB5131. If the antenna is installed properly and the signal from DCF77
can be received without strong distortions, the green 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 micro-
processor from decoding the time message. In this case, a better location for the antenna
must be found. After start-up, the red LED labeled FR (free running) 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. Without or with a disturbed RF signal the clock runs on XTAL with
an accuracy of 10
-6 (after 24 hours of synchronous operation, otherwise: 10 -5 ). If the
clock have lost reception for more than 12 hours the FR LED starts blinking.