Thanks Azelio, I have also thought that the EFC range probably says something. The datasheet for the OFC4834 says EFC range +-0.4ppm and that that should be enough for 15 years. The OFC MC834, I have seems very similar, but without EFC. So my 68ppm drift is well within the +-400ppb Lars >From: Azelio Boriani<mailto:azelio.boriani@gmail.com> >Sent: den 19 november 2016 02:01 >One starting point to figure out the 10 years aging can be the datasheet of an OCXO: MTI240: per year, 3.0E-7, in 10 years must be less than 30.0E-7 MTI220: per year, 1.0E-6, in 10 years < 10.0E-6 Bliley NV26R: 17 years, 5.0E-6 OSA8663: per year, 3.0E-8, in 10 years < 30.0E-8