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U. Glaeser

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Open-loop simulations at the circuit level should be performed on individual blocks within the DLL<br />

or PLL. The VCDL and VCO should be simulated using a transient analysis as a function of control<br />

voltage, supply voltage, and substrate voltage in order to determine the control, supply, and substrate<br />

sensitivities. The phase detector should be simulated with the charge pump, by measuring the output<br />

charge as a function of input phase different and possibly control voltage, to determine the static phase<br />

offset and if any nonlinearities exist at the locking point, such as a dead-band or high-gain region. The<br />

results of these simulations can be incorporated into the loop dynamics simulation models.<br />

Closed-loop simulations at the circuit level should also be performed on the complete design in order<br />

to characterize the locking characteristics, overall stability, and jitter performance. The simulations should<br />

be performed from all possible starting conditions to insure that the correct locking result can be reliably<br />

established. The input phase step response of the loop should be simulated to determine if there are<br />

stability problems manifested by ringing. Also, the supply and substrate voltage step response of the loop<br />

should be simulated to give a good indication of the overall jitter performance. All simulations should<br />

be performed over all operating conditions, including input frequencies and divider ratios, and environmental<br />

conditions including supply voltage and temperature as well as process corners.<br />

Measurement<br />

Once the DLL or PLL has been fabricated, a series of rigorous laboratory measurements should be<br />

performed to insure that a problem will not develop late in manufacturing. The loop should first be<br />

characterized under controlled conditions. Noise-free supplies should be used to insure that the loop<br />

generally locks and operates correctly. Supply noise steps at sub-harmonic of the output frequency can<br />

be used to allow careful measurement of the loop’s response to supply steps. If such a supply noise signal<br />

is added synchronously to the output signal, it can be used as a trigger to obtain a complete time averaged<br />

response to the noise steps. The step edge rates should be made as high as possible to yield the worstcase<br />

jitter response. Supply noise steps swept over frequency, especially at low frequencies, should be<br />

used to determine the overall jitter performance. Also, supply sine waves swept over frequency will help<br />

determine if there are stability problems with the loop manifested by a significant increase in jitter when<br />

the noise frequency approaches the loop bandwidth.<br />

The loop should then be characterized under uncontrolled conditions. These conditions would include<br />

worst-case I/O switching noise and worst-case on-chip core switching noise. These experiments will be the<br />

ultimate judge of the PLL’s jitter performance assuming that the worst-case data patterns can be constructed.<br />

The best jitter measurements to perform for characterizations will depend on the DLL or PLL application,<br />

but they should include both peak cycle-to-cycle jitter and peak input-to-output jitter.<br />

Conclusions<br />

DLLs and PLLs can be used to relax system-timing constraints. The best loop architecture strongly<br />

depends on the system application and the system environment. DLLs produce less jitter than PLLs due<br />

to their inherently reduced noise sensitivity. PLLs provide more flexibility by supporting frequency<br />

multiplication and an unlimited phase range. Independent of the chosen loop architecture, supply and<br />

substrate noise will likely be the most significant cause of output jitter. As such, all circuits must be<br />

designed from the outset with supply and substrate noise rejection in mind.<br />

References<br />

1. M. Johnson and E. Hudson, “A Variable Delay Line PLL for CPU-Coprocessor Synchronization,”<br />

IEEE J. Solid-State Circuits, vol. SC-23, no. 5, pp. 1218−1223, Oct. 1988.<br />

2. I. Young, et al., “A PLL Clock Generator with 5 to 110 MHz of Lock Range for Microprocessors,”<br />

IEEE J. Solid-State Circuits, vol. 27, no. 11, pp. 1599−1607, Nov. 1992.<br />

3. F. Gardner, “Charge-Pump Phase-Lock Loops,” IEEE Trans. Communications, vol. COM-28, no. 11,<br />

pp. 1849−1858, Nov. 1980.<br />

4. S. Sidiropoulos and M. Horowitz, “A Semidigital Dual Delay-Locked Loop,” IEEE J. Solid-State<br />

Circuits, vol. 32, no. 11, pp. 1683−1692, Nov. 1997.<br />

© 2002 by CRC Press LLC

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