Meinberg PZF511 - Manual (Page 5)

General information
The German long wave transmitter DCF77 started continious operation in 1970. The
introduction of time codes in 1973 build the basic for developing modern radio remote
clocks.
The carrier frequency of 77.5kHz 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 10ms. Variations of the trigger level of the demodulator make the accuracy
of the time marks worse by additional +/-3ms. Because this precision is not sufficient for
lots of applications, the PTB (Physical and Technical Institute of Germany) began to
spread time informations 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 1,55ms.
The carrier of 77.5kHz is modulated with a phase deviation of +/-10° per bit. The bit
sequence is transmitted each second, it starts 200ms after the beginning of a 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 meaning-
ful.
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