Hello Bala, these two specs have very different meanings: Frequency in Hz gives the nominal (expected) frequency of the oscillator. E.g. 100 MHz. The tolerance in ppm (parts per million) gives the possible deviation of any particular oscillator from the nominal frequency. Typically this is a combination of manufacturing tolerances and environmental influences (temperature, aging). E.g. 10ppm So if you by an oscillator rated at 100 MHz and 10 ppm that means the actual output frequency may lie anywhere between 100MHz *(1 +/- 10*1E-6), i.e. 99.999 MHz and 100.001 MHz. How much frequency deviation your system can deal with depends on a variety of factors, there is no general answer. Regards, Wolfgang -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of bala Sent: Wednesday, December 14, 2011 11:17 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] ppm(parts per million)-Crystal OSC Hi all, I have a little confusion on PPM vs frequency of Crystal Oscillators.How important is the term 'ppm' than frequency.Why all the datasheets have both units,ppm and Hz.Please clarify,Also how much important is the value of ppm on the Signal integrity side,if any.Thanks for your time. -- bala ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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