Some folks were asking about the accuracy of the DCF77 10 MHz standard described earlier, DCF77 10 MHz – which has an Piezo brand OCXO, steered by a long-time-constant PLL locked to the DCF77 77.5 kHz carrier.
But, how to assess the short and long term stability of such a ‘standard’ in practical terms? Well, short term accuracy – it will simply be that of the Piezo OCXO, and some noise injected by the power supply. Mid- and long term, the drift will be determined by the DCF77 master clock (which is dead accurate), and the propagation conditions of the long wave signal (which is by far worse).
With my location at Ludwigshafen, Germany, I’m reasonably close to the DCF77 transmitter – maybe 70 miles? So there is hope that the transmission induced effects are not all that bad.
To measure the mid and long term stability, see below two plots of the DCF77-locked phase of the Piezo OCXO, vs. the instantaneous phase of GPS, stable to 40 ns or better, and obtained from a Motorola M12+ timing receiver. Measurements were done by measuring the time interval from the GPS 1 PPS signal, to the rising edge of a 10 kHz signal – derived from the 10 MHz OCXO by a good divider (using a ADF41020 REF input – R divider routed to MUX output) by HP 5335A counter.
In short – DCF77 is tracking GPS extremely well, and the OCXO phase is stable to within a few 10 to 100 ns. In practical terms, 1 second of observation time would be well enough to calibrate any frequency standard to 1 ppm or better, by comparison with the DCF77 locked OCXO. In other words, the DCF77 locked OCXO instability appears to be dominated by the propagation of the DCF77 signal more then anything else.
2 thoughts on “DCF77 vs. GPS time comparison: not a lot of uncertainty…”
hello, very interesting blog ! I found a lot of good information. My problem is to measure the difference in time between DCF77 and 1pps GPS to determine the height of the D layer in ionosphere… do you have a trick for me, how to proceed ? I am lost
Hi, at which time constant or frequency would you need to measure the difference, every second, or can you have some averaging – this will determine the bandwidth of the DCF77 receiver.
My general approach to this would be to lock a 10 MHz clock to the 1 pps GPS signal, and then to generate timestamps of the DCF77 signal zero crossing at a fixed interval (maybe set by the second pulse of the DCF to avoid any interference of the amplitude modulation with the zero crossing detection), and then to reconstruct the phase information/delay from the time stamps. This can be done easily with a small microprocessor, running a counter, and with a hardware interrupt reading the counter at the zero crossing of the DCF77 signal. Another way may be to lock a VCO to the 77.5 kHz with a suitable PLL bandwidth, like, some minutes, and to divide by 77.5k, and to determine the phase between the 1 pps signal and such DCF77 1 second pulse. Using the usual second pulse of the DCF77 from the amplitude modulation will not be accurate enough, so you need to reconstruct the phase from the carrier, or better a clean 77.5k VCO locked to the carrier.