Meinberg DCF77 PZF Receivers

Most simple DCF77 receivers just decode the amplitude modulation of the longwave signal received from the DCF77 transmitter, and the signal strength value displayed by most DCF77 receivers depends on the amplitude of the received 77.5 kHz signal.

However, unfortunately a high signal level can also originate from electric noise in the 77.5 kHz frequency range, so a high signal level alone is no guarantee for a proper reception of the original longwave signal. If the signal is weak, and there is only a low noise level, reception can be better than in a case where the original signal is strong, but the noise level is very high.

Meinberg PZF receivers also decode the phase modulation of the 77.5 kHz carrier used by DCF77, which significantly increases immunity against electric noise, and also results in a significantly increased accuracy of the derived time.

The phase modulation signal is a 512 bit digital pseudo-random noise (PRN) code, where a '0' bit causes a carrier phase shift in one direction, and a '1' bit causes a phase shift in the opposite direction.

Since a PRN code sequence has the same number of '0' and '1' bits, the mean carrier phase is not affected by the phase modulation.

The PRN code is modulated onto the carrier every second between the end of the last AM second mark and the beginning of the next AM second mark. See also:

• DCF77 phase modulation published by the PTB
  https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-phase-modulation.html

A PZF receiver also generates the well-known PRN code locally, compares it to the PRN code derived from the carrier phase shift of the incoming DCF77 signal, and measures the time in which both signals match, i.e. have the same logic level, both '0' or both '1'.

As long as the two PRN signals are shifted in time by more than 1 bit time, both signals have the same logic level only half of the maximum time, i.e. the match is 50%, which means the 2 signals are not correlated to each other. This is due to the noise characteristics of PRN codes in general.

If the signal generated by the receiver is shifted in time so that the effective time shift between the 2 PRN signals becomes less than 1 bit time, the signals are correlated to each other, i.e. they match more than 50% of the maximum time.

If the generated PRN signal is shifted such that it matches the received PRN signal as good as possible, the highest possible correlation is achieved. In theory this can be 100%, if both signals really match exactly.

However, due to the limited bandwidths of the antennae, electrical distortions, etc., the shape of the received PRN signal may more or less deviate from the ideal waveform, so the best possible correlation may be less than 100%.

Anyway, if the correlation value is e.g. 90% or 95%, this is an indicator of good reception, while a correlation value of 60% or 65% indicates that reception is poor. A correlation value less than 50% is not possible. Such a value would just indicate correlation where one of the 2 signals is inverted.

BTW, this technique is basically also used by GPS receivers, where each receiver channel is a correlator for an individual, well-known PRN code assigned to a specific satellite. The advantage of the PRN codes used by GPS is that they have a much longer PRN code sequence which is generated at a much higher clock rate than for DCF77.

For GPS and similar signals this is known as spread spectrum technology, which also greatly reduces the susceptibility of the receiver to electrical noise.

— Martin Burnicki martin.burnicki@meinberg.de 2019-09-25