News Article

Phase Coordination to Achieve More Ideal PLL/DLL
Date: Aug 05, 2013
Source: EDN Network ( click here to go to the source)

Featured firm in this article: Gain ICs of Colorado Springs, CO



In Figure 4, the erratic behavior of the TPC can be seen as jitter peaking where the coordinating phase is not tracking with the reference phase. TPC was observed to have erratic behavior with minimal loop filtering (light gray trace Figure 4a) as well as when there is no loop filtering (light gray trace Figure 4b). The UPC, in contrast, shows that it is coordinating very well with the reference phase, both with minimal loop filtering (blue trace Figure 4a) and without any explicit loop filter (neon green trace Figure 4b). Figure 4 shows how the UPC method virtually eliminates jitter peaking as compared to TPC which amplifies jitter to jitter peaking.

The control factors for both UPC and TPC in Figure 4 are as follows:

identical fast loop gains
modeled in a 180 nm process
a carrier frequency of 500 MHz
a modulation amplitude of 10 MHz (modulation index of 1)

Results from measurements show that UPC has 4 times the coordinating frequency range, with 1.2 times higher frequency on the high end. Jitter peaking and jitter accumulation at high loop gain with a non-modulated reference in Figure 5 show a 400 us jitter spread for TPC and 1 ns spread for UPC (tracking actual variation on the reference clock, as expected for an ideal PLL). In essence, TPC shows 400,000 times more jitter, as the math models indicate. Jitter Transfer Functions (JTF) show coordinating bandwidth to be merely in the hundreds of hertz for TPC with high loop gain. Due to limitations of existing test equipment (designed to meet only the expected coordinating bandwidth of TPC) UPC was only able to be directly tested to 200 kHz, even though simulations and math models show that it can go well into the 100 MHz range. JTF show UPC coordinating bandwidth does reach 200 kHz, with less than 0.02% jitter transfer across that range, even though it can go more than 1000 times higher.




Figure 6 illustrates the coordinating and reference signals achieve 1000 times and above increase in phase coordinating bandwidth due to the advantages of more ideal phase coordination. This high data transfer bandwidth enables Digital FM. In a 180 nm process (a slower process) with 1 GHz carrier frequency a 100 M b/sec transfer rate can be observed in the graph below. The result is a 25 times increase in data transfer rates over conventional communication schemes that currently struggle to get to 4 Mb/sec. UPC offers the additional benefit of the much greater signal integrity of FM when transmitting through the atmosphere. A further 8 times increase in speed would also be expected in more integrated processes such as 28 nm. Thus, UPC enables bit transfer rates more than 100 times faster than currently achievable theoretically approaching 1 G b/sec.

Measurements for TPC were not obtained due to the fact that TPC came nowhere near operating (let alone coordinating phases) at 1 GHz at the same fast loop gain of UPC. TPC exhibited excessive instability due to jitter peaking and, as a result, no reliable measurements were possible.





Conclusion
A new method of phase coordination demonstrates that jitter peaking can be eliminated and coordinating bandwidth can be increased by more than 1000 times over old methods of phase coordination. The new UPC method has significantly improved clocking accuracy over traditional phase coordinating methods in spread spectrum clocking and reduced jitter accumulation and peaking in typical clocking applications. As also demonstrated, this new phase coordinating can be employed in DLLs and PLLs to also increase data transmission rates by several orders of magnitude, with transfer rates 100s of times faster than current communication schemes. UPC significantly reduces the acquisition time of a PLL from often seconds or milliseconds to nanoseconds while greatly reducing silicon area by eliminating the large loop filtering required by TPC in order to prevent jitter peaking.