Tag Archives: GPS timing receiver

Perfect time: upgrade to a Motorola M12+ receiver, and new GPS antenna

For years, a Motorola UT+ GPS timing receiver has served me well as a frequency reference and source of accurate time (and location). While I primarily use a DCF77 locked 10 MHz OCXO, the GPS time is useful for various purposes, be it, to confirm that DCF77 is actually delivering the proper time.

One drawback of the Motorola UT+ is the rather large “sawtooth” error, which is caused by the quantization of the 1 pps signal derived from a 9.54 MHz clock. This results in a +-52 phase inaccuracy – which can be corrected, but only with further effort.

The later model, which is dated by now and available at low cost, the Motorola M12+, is much better in this respect, featuring a +-13 ns sawtooth, which is not a lot, and good enough for most purposes without any further corrections.

Below, some tests on an OCXO vs. GPS 1 pps pulses, for a OCXO under test (10 MHz, divided down to 100 kHz, and phase displayed in microseconds).

ocxo-vs-ut

ocxo-vs-m12-100ns

This is the small board, not a thing of beauty, but working. The only parts needed are +3.0 V and +5 V (actually using +4.4 V) voltage regulators: 3.0 V for the M12+, 4.4 V for the GPS antenna.
The 3.0 V also powders a MAX3232 TTL to RS232 converter.

m12plus-board

Also procured a second-hand GPS timing antenna – this one has a nice radome, a quadrifilar helix element, and a 26 dB amplifier to compensate any cable losses. The cable, LMR-195, features N to SMA connectors, and a considerable of PVC tape was used to protect the N-connector from the elements. Still it would be better to use some special outdoor N connectors, but, sorry, don’t have.

m12plus-antenna

m12plus-tac

A handy program to control the GPS – TAC32. Usual procedure is to carry out a location survey, which will take about 2-3 hours, and then continue in position hold/timing mode.

One drawback of my location – there is no way to get full 360 deg view, so reception is limited to the more southern satellites. But usually 6-8 satellites are in sight.

Still contemplating if it is worthwhile to put this in a larger box, together with a 10 MHz OCXO, and possibly a DS1023-50 delay line to implement a hardware sawtooth correction. Maybe a good project for winter time.

DCF77 vs. GPS time comparison: not a lot of uncertainty…

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.

dcf dcf vs gps time day 57603

dcf dcf vs gps time day 57604

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.