Ulrich is right about the use of MTI 260 use in GPSDOs. I have a Korean-made Z3815A clone (it's labelled Agilent) which has a little daughter board with a 5MHz MTI 260 DOCXO in place of the original E1938A used in these models. I understand that there were production and reliability issues with the E1938A. 73, Murray ZL1BPU