Meinberg IMS-PZF Clock, Руководство пользователя

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4 PZF - DCF77 Long Wave Receiver
4 PZF - DCF77 Long Wave Receiver
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 DCF77 frequency and signal is derived
from the atomic clocks of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, the
national institute for science and technology and the highest technical authority of the Federal Republic of
Germany for the field of metrology and physical safety engineering.
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 addi/-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 built 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 wideband
width. 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.
M Start of Minute (0.1 s)
R RF Transmission via secondary antenna
A1 Announcement of a change in daylight saving
Z1, Z2 Time zone identification
Z1, Z2 = 0, 1: Daylight saving disabled
Z1, Z2 = 1, 0: Daylight saving enabled
A2 Announcement of a leap second
S Start of time code information
P1, P2, P3 Even parity bits
0
10
20
30
40
50
R
M
1
4
2
1
20
10
8
4
2
1
P
2
0
2
0
1
8
4
2
1
2
4
8
10
1
2
4
8
10
2
0
4
0
8
0
3
P
A1
Z1
Z2
A2
S
1
2
4
8
1
0 2
0
4
0
1
P Minute
(reserved)
Hour
Day of Month
Day of Week
Year of the Century
Month of Year
The PZF radio clock is a precision receiver system for the time signal transmitter DCF77. It is available as a
module for use in systems such as Meinberg IMS, LANTIME M300 models and as a computer plug-in card. The
microprocessor of the system performs the correlation of a reproduced pseudo-random bit sequence with the PZF
of the transmitter side and simultaneously decodes the AM time and date information of the DCF telegram. By
evaluating the pseudo-random phase noise, a time raster can be generated which is up to a factor of a thousand
IMS-PZF180 Setup Guide Date: 31st January 2020 7