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U. Glaeser

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Circuit Summary<br />

In general, all DLL and PLL circuits must be designed from the outset with supply and substrate noise<br />

rejection in mind. Obtaining low noise sensitivity requires careful orchestration among all circuits and<br />

cannot be added as an after thought. Supply noise rejection requires isolation from one supply terminal,<br />

typically with current source isolation. Substrate noise rejection requires all fixed-biased devices to be<br />

well-type devices to minimize threshold modulation. However, the best circuits to use depend on both<br />

the loop architecture and the IC technology.<br />

Self-Biased Techniques<br />

Achieving low tracking jitter and a wide operating frequency range in PLL and DLL designs can be<br />

difficult due to a number of design trade-offs. To minimize the amount of tracking jitter produced by a<br />

PLL, the loop bandwidth should be set as high as possible. However, the loop bandwidth must be set at<br />

least a decade below the lowest desired operating frequency for stability with enough margin to account<br />

for bandwidth changes due to the worst-case process and environmental conditions. Achieving a wide<br />

operating frequency range in a DLL requires that the VCDL work over a wide range of delays. However,<br />

as the delay range is increased, the control becomes increasingly nonlinear, which can undermine the<br />

stability of the loop and lead to increased jitter. These different trade-offs can cause both PLLs and DLLs<br />

to have narrow operating frequency ranges and poor jitter performance.<br />

Self-biasing techniques can be applied to both PLLs and DLLs as a solution to these design trade-off<br />

problems [8]. Self-biasing can remove virtually all of the process technology and environmental variability<br />

that affect PLL and DLL designs, and provide a loop bandwidth that tracks the operating frequency. This<br />

tracking bandwidth sets no limit on the operating frequency range and makes wide operating frequency<br />

ranges spanning several decades possible. This tracking bandwidth also allows the bandwidth to be set<br />

aggressively close to the operating frequency to minimize tracking jitter. Other benefits of self-biasing<br />

include a fixed damping factor for PLLs and input phase offset cancellation. Both the damping factor<br />

and the bandwidth to operating frequency ratio are determined completely by a ratio of capacitances<br />

giving effective process technology independence. In general, self-biasing can produce very robust designs.<br />

The key idea behind self-biasing is that it allows circuits to choose the operating bias levels in which<br />

they function best. By referencing all bias voltages and currents to other generated bias voltages and<br />

currents, the operating bias levels are essentially established by the operating frequency. The need for<br />

external biasing, which can require special band-gap bias circuits, is completely avoided. Self-biasing<br />

typically involves using the bias currents in the VCO or VCDL as the charge pump current. Special<br />

accommodations are also added for the feed-forward resistor needed in a PLL design.<br />

Characterization Techniques<br />

A good DLL or PLL design is not complete without proper simulation and measurement characterization.<br />

Careful simulation can uncover stability, locking, and jitter problems that might occur at the operating,<br />

environment, and process corners. Alternatively, careful laboratory measurements under the various<br />

operating conditions can help prevent problems in manufacturing.<br />

Simulation<br />

The loop dynamics of the DLL or PLL should be verified through simulation using one of several possible<br />

modeling techniques. They can be modeled at the circuit level, at the behavioral level, or as a simplified<br />

linear system. Circuit-level modeling is the most complete, but can require a lot of simulation time because<br />

the loops contain both picosecond switching events and microsecond loop bandwidth time constants.<br />

Behavioral models can simulate much faster, but are usually restricted to transient simulations. A simplified<br />

linear system model can be constructed as a circuit from linear circuit elements and voltage-controlled<br />

current sources, where phase is modeled as voltage. This simple model can be analyzed not just with<br />

transient simulations, but also with AC simulations and other forms of analysis possible for linear circuits.<br />

Such models can include supply and substrate noise sensitivities and actual loop filter and bias circuitry.<br />

© 2002 by CRC Press LLC

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