Rapco 1804P9
GPS Disciplined Precision Frequency Source
Page 2

Unlike my ‘RUH-Clone’ which makes use of the GPS-Locked 10KHz output from the Jupiter ‘GPS-engine’, the Rapco GPSDOs lock their OCXOs to the 1pps Timing Pulse from the Trimble GPS RX. This pulse is Time-Locked when the GPS acquires a geographic fix.


From documentation (1804L17) …

After power-on, the GPS receiver will normally provide a position fix in less than 5 minutes. If the unit has been moved a long way from its previous position, or if a long time has elapsed since it was switched on, then the 'Time to First Fix' could be as long as 15 minutes. When the GPS receiver has locked on to sufficient satellites the green GPS status indicator will come on, and the time output data (1 Hz and serial RS232 data) will be corrected to satellite time. …


Behaviour during warm-up

After a short period (2 to 3 min. typical, 15min. max) the GPS STATUS light will switch on, indicating that the GPS receiver has locked onto the satellites. At this point, the Time/Date information at all interfaces will be corrected from its initial (arbitrary)

setting to the correct Time and Date derived from the satellite data, and the 1Hz Timing pulse output will be phase-stepped to align with UTC.


At this stage the frequency error of the reference oscillator may still be of the order of 5 in 10E7. Because of this frequency error, following the initial alignment sequence the 1Hz Timing marker will drift away from UTC at a rate of about 30 microseconds per minute. This drift rate will decrease as the oscillator continues to approach its correct frequency, and will continue to drop as the unit approaches its normal stand-alone accuracy.


A time delay of approximately 20 minutes from power-up is counted down by the software, after which the oscillator should have reached 'parts in 10E8' accuracy. At this point the 1Hz Timing output is again realigned with the GPS(UTC) pulse to correct the timing error that has accrued during the oscillator warm-up period.

The 1Hz marker pulse will continue to drift away from UTC, but now at a reduced rate of about 3 microseconds per minute.


After a further period, of approximately 10 minutes, the CONTROL STATUS light will switch on; this indicates that the frequency control system has started drift correction. At this point, the Date/Time outputs (at the RS232 ports) will again be corrected to satellite time and the 1Hz marker pulse will be finally realigned with UTC. Thus the complete warm-up cycle from power-on to full performance is completed in less than 30 minutes, although in many applications the unit may be regarded as serviceable after half this time.


Once the CONTROL STATUS indicator is on, there will be no more phase-jumps in the 1Hz Timing signal, all further adjustment to maintain UTC alignment being achieved by 'steering' the oscillator frequency.

The frequency drift-corrections involved in this process will take place at intervals of 512 seconds (8min 32sec).


The CONTROL STATUS indicator will remain on during normal operation unless the frequency reference should be disturbed by (say) a power break, GPS failure, severe temperature shock etc. Under such conditions, the oscillator control voltage is locked at the previous value (held in nonvolatile memory) until the drift-correction process has re-started.

August 2016: