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Getting the jitter bug

What is a jitter transfer function and how does it help designers? By Edmund Suckow.

As jitter specification becomes the norm when comparing interface ics, several techniques have emerged to summarise the jitter performance of a particular device. One such method is the jitter transfer function (JTF) and recording a JTF for a device containing a phase lock loop (PLL) gives a summary of performance across jitter frequencies, indicating areas where the device either decreases or amplifies jitter.

Today's high clock rates mean PLLs are becoming more prevalent. In a serialiser, for example, it is simply not practical to supply a 1.25GHz clock and maintain highly accurate device to device skew. The PLL feedback loop maintains itself in constant check with the original system clock; in serialisers, this would be the ttl input data frequency.

All serdes PLLs have an input frequency, typically set by a ttl data rate applied via CLKIN, and an internal frequency that maintains serialisation timing and which needs to be synchronised with this frequency.

A phase detector is used up front in the loop to assign a phase delta value. This delta is typically fed into a filter that controls the voltage sent to a voltage controlled oscillator, which modifies its square wave ring frequency according to this input. A 'divide by' transfer function then generates the final CLKIN to PLL frequency ratio (see figure 1).

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Graham Pitcher

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