Re: "For PCIe 2.0, there are two jitter SPEC named high-freq jitter and low-freq jitter. Can anyone explain what are the definitions of the two jitters?" I think you are looking at Table 4-14, which is referred to in Note 1 of both Table 4-16 (Refclk Compliance parameters for Common Refclk architecture) and Table 4-18 (Refclock parameters for Data clocked Rx architecture). Table 4-14 and the text above it explains the purpose without actually supplying test limits. The purpose is this: The low frequency range (0.01 - 1.5MHz) is assumed to be inside the tracking bandwidth of the Rx PLL. There is a separate spec for the residual of SSC, so the ability of the Rx PLL to track SSC adequately is specifically checked. The high frequency jitter (>1.5MHz) is assumed to be outside of the tracking bandwidth of the receiver's PLL, so this spec on the clock is how much jitter it can have in the frequency range where the Rx PLL may not follow it, so it could really appear to the receiver as jitter. Below 10kHz is not checked because the assumption is that any Rx PLL can follow those frequencies "completely". What the spec actually says (in 4.3.7.2.1.) is: "Lower edge of filter removes 1/f jitter contributions that are completely trackable by CDR. " (Personally, I think such an assumption is a test spec is suspect, but this one seems to have worked out). Tables 4-16 and 4-18 give the actual limits for these jitter tests and notes explain the conditions under which the tests are made, for the stated architecture type. --- Joe S. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu