24 MPC User Manual Rev 0D
Chapter 3 MPC Setup Considerations
3.3.1 I/O Connectors and Cables
Below the VFD screen on the front of the MPC are three BNC female connectors: 10 MHz Out, 1PPS
Out and Mark In.
3.3.1.1 10 MHz Output
The 10 MHz output provides a high-stability reference 10 MHz frequency that is by default phase
locked to GPS time. It permits the synchronization of other equipment requiring an extremely stable
reference signal of 10 MHz. See also Figure 4 on Page 20.
The steering of the internal OCXO is controlled by the CLOCKADJUST, EXTERNALCLOCK and
CLOCKCALIBRATE commands. The status of the clock steering process is provided by the
CLOCKSTEERING log and the time history plots on the Modulated Precision Clock website.
Refer to Volume 2 of the OEM4 Users’ Guide for more information on these commands and logs. For
details on time plots, please see Section 6.1.3.1, Time Plots on Page 67.
3.3.1.2 1PPS Output
Upon determination of position-time, the receiver will align the one pulse per second (1PPS) signal to
the GPS epoch. Its specifications and electrical characteristics are described in Front Panel 1PPS Out
on Page 78. From this time onwards the 1PPS triggers every second. By default, the OCXO frequency
is adjusted to maintain the 1PPS signal within 10 ns of GPS time. The bandwidth of the OCXO clock
steering loop is adjustable with a default value of 1/100 Hz or approximately a 15 minute time
constant.
The CLOCKADJUST command is used to enable or disable the clock steering. The
CLOCKCALIBRATE command is used to set the parameters of the clock steering loop. It also
provides an auto-calibration procedure to find or reset the centre point and gain of the control loop.
Please refer to Volume 2 of the OEM4 Users’ Guide for details on these commands.
Immediately on start-up of the receiver, the 1PPS is active (at 1Hz) but it is not aligned to the GPS
epoch. The timing of the 1PPS will be adjusted when position-time is known.
1 Hz GPS measurements are taken on the 1PPS. These measurements include pseudorange, carrier
phase and Doppler.
3.3.1.3 Mark Input
The Mark In connector provides a connection for an input mark with an input pulse greater than 55 ns.
Time tags output log data to the time of the falling edge of the mark input pulse. Logs with the
ONMARK trigger will output when a trigger event passes the Mark In connector.
If you supply the reference frequency and 1PPS signal, your 1PPS signal will be connected to the
Mark In connector. The receiver can be configured to adjust its internal 1PPS to match your 1PPS to
within 50 ns. Refer to the ADJUST1PPS command in Volume 2 of the OEM4 Users’ Guide.
