Meinberg LANTIME, Operating Instructions Manual

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General information DCF77 PZF
The German long wave transmitter DCF77 started continuous operation in 1970.
The introduction of time codes in 1973 build the basic for developing modern radio
remote clocks.
The carrier frequency of 77.5 kHz is amplitude modulated with time marks each
second. The BCD-coding of the time telegram is done by shifting the amplitude to
25% for a period of 0.1s for a logical '0' and for 0.2s for a logical '1'. The receiver
reconstructs the time frame by demodulating this DCF-signal. Because the AM-signal
is normally superimposed by interfering signals, filtering of the received signal is
required. The resulting bandwidth-limiting causes a skew of the demodulated time
marks which is in the range of 10 ms. Variations of the trigger level of the
demodulator make the accuracy of the time marks worse by additional +/-3 ms.
Because this precision is not sufficient for lots of applications, the PTB (Physical and
Technical Institute of Germany) began to spread time information by using the
correlation technique.
The DCF-transmitter is modulated with a pseudo-random phase noise in addition to
the AM. The pseudo-random sequence (PZF) contains 512 bits which are transmitted
by phase modulation between the AM-time marks. The bit sequence is build of the
same number of logical '0' and logical '1' to get a symmetrical PZF to keep the
average phase of the carrier constant. The length of one bit is 120 DCF-clocks,
corresponding to 1,55 ms. The carrier of 77.5 kHz is modulated with a phase
deviation of +/-10° per bit. The bit sequence is transmitted each second, it starts
200ms after the beginning of an AM second mark and ends shortly before the next
one.
Compared to an AM DCF77-receiver, the input filter of a correlation receiver can be
dimensioned wide-bandwidth. The incoming signal is correlated with a reconstructed
receiver-PZF. This correlation analysis allows the generation of time marks which
have a skew of only some microseconds. In addition, the interference immunity is
increased by this method because interference signals are suppressed by averaging the
incoming signal. By sending the original or the complemented bit sequence, the
BCD-coded time information is transmitted.
The absolute accuracy of the generated time frame depends on the quality of the
receiver and the distance to the transmitter, but also on the conditions of transmission.
Therefore the absolute precision of the time frame is better in summer and at day than
in winter and at night. The reason for this phenomenon is a difference in the portion
of the sky wave which superimposes the ground wave. To check the accuracy of the
time frame, the comparison of two systems with compensated propagation delay is
meaningful.
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