Meinberg FDM509 - Manual (Page 5)

Generally
The module FDM509 was designed to calculate and monitor the frequency in 50/60Hz
power line networks. A preconnected reference is necessary that provides a high
accuracy 10MHz frequency, a serial time string and a PPS (pulse per second).
Additional to the frequency the "PLT" (Power Line Time) is calculated that would be
displayed by a clock using the mains frequency as its timekeeping reference. The
accumulated differece between the drifting power line time and the high accuracy
reference time (REF) is called "time deviation". Because the time deviation has the
long-term accuracy of the reference it is suitable to supervise the frequency stability.
The time deviation can be read out either via the serial interface or via one of the two
integrated analog outputs for further evaluation or regulation.
Functional Principle
The power line frequency to be monitored is applied via the rear VG edge connector
(optional: via a mains socket in the frontpanel) of the FDM509, then filtered and
transformed. After that the sine-wave signal is converted into a TTL signal using a
schmitt trigger. The rising edge of the TTL signal is used to start/stop a counter that is
clocked by the high accuracy 10MHz pulse of the reference. Because of that the period
length of the mains frequency is measured with a resolution of 100ns and read out by
the micro controller of the FDM509. The values are averaged over a period of one
second or one minute (selectable) and then the frequency is calculated with a resoluti-
on of 1mHz.
Calculation of the PL time occurs by counting the periods of the power line
frequency. Depending on the nominal frequency the PL time seconds are incremented
after counting 50 or 60 periods. To initialize the PL time it is necessary to get the exact
time via the serial interface. The time deviation is calculated once per second or once
per minute and is limited to ±100 seconds.
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