Rigol DG1022Z internal frequency reference

[Circlotron]

What exactly does the above sig gen use as a frequency reference? Had one for about 5 or 6 years now and wondering what it’s long term accuracy is like. If I zero beat it against the national AM radio station there is about 0.2 ppm difference, but of course I don’t know how accurate the radio station is. For my purposes 0.01% accuracy for the sig gen would be good enough. Just wondering how good these things are.