| 摘要 |
A frequency monitoring system comprises a plurality of circuit cards. Each circuit card, such as a primary multiplexer card and a backup or redundant multiplexer card, has an oscillator that provides a reference clock signal. On each circuit card, a respective frequency compare element is configured to receive a clock signal to be measured and to provide a frequency error signal indicating a frequency error of the clock signal relative to an average frequency of a plurality of reference clock signals. Accordingly, the frequency measurements for the circuit cards are based on the same reference frequency (e.g., the average frequency of the reference clock signals from the oscillators).
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