TND301

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3

Clock Management Highways

Clock Management is included in electronic systems that

contain backplanes (see Figure 4). Backplanes are the

physical highways for clocks. They are multilayer printed

circuit boards that are on the back of a card cage and have

connectors that each circuit card plugs into. The design of

the backplane is very critical to the performance of the Clock

Management system. Many factors must be considered for

a good backplane design.

The Clock Generator is typically on a circuit card with

Clock Distribution circuits. The clocks are distributed

throughout the cards on the backplane and each card may

then redistribute, delay, divide, and translate these clock

signals.

Backplanes are noisy due to the high amount of electronic

signal traffic. Standard connectors are also a problem on a

backplane since they do not offer a good transition due to

impedance mismatch. Most connectors do not offer

differential signal capability and do not provide adequate

ground pins for elimination of crosstalk. Backplanes tend to

slow down signals because they have multiple layers which

add capacitance and delay.

Figure 4. Example of Backplanes

Clock Management systems distribute clocks over backplanes in super–and mini–computers,

communication equipment like PABX, SONET/SDH systems, ATM, and advance test equipment.