Meinberg FDM511 - Manual (Page 5)

Features of the Frequency Deviation Monitor FDM511
The module FDM511 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 clock, a serial time string and a PPS (pulse per second). In addition to
the frequency, the time is calculated that would be displayed by a clock using the mains
frequency as its timekeeping reference (PLT = Power Line Time). The accumulated
differece between the drifting power line time PLT and the high accuracy reference time
REF is called the time deviation (TD). 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 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 FDM511, then filtered and
transformed. After that the sine-wave signal is converted into a TTL signal using a
schmitt trigger. The rising edge of this TTL signal is used to start/stop a counter that is
clocked by the high accuracy 10MHz pulse of the reference. Due to this, the period
length of the mains frequency is measured with a resolution of 100ns and read out by the
micro controller of the FDM511. The values are averaged over a period of one second
and then the frequency is calculated with a resolution of 1mHz.
Calculation of the power line time PLT occurs by counting the periods of the mains
frequency. Depending on the nominal frequency, the PLT seconds are incremented after
counting 50 or 60 periods. To initialize the PLT, it is necessary to get the exact time via
the serial interface (REF) and the pulse per second (PPS) from the preconnected
reference. The time deviation TD is calculated once per second and is limited to ±100
seconds.
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