[time-freq] Frequency Standards - Which to believe?

[Arthur Shulman]

For the past 3 years, I have been depending (blindly) on the accuracy of
a surplus Effratom Rubidium Standard as an external reference in my Lab.
My faith was based on the theory of 'What can go wrong with the natural
resonant frequency of the Rubidium atom?'

Recently I was fortunate to acquire a HP Z3801A  GPS Standard. It seems
to work correctly, but I have concerns. Those more knowledgeable will
know the next line! "The man who has two clocks doesn't know the time."

I set up an X-Y display on a scope to compare the phase of the two
signals. I am able to get quite a nice Lissajou figure, with a period of
about seventy (70) seconds. But my reasoning is, 'Why should there be
any difference at all since I am expecting short-term frequency accuracy
better than 1E-11 for both devices?'

Also, assuming there is in fact a difference, this gives rise to several
other questions:
a)    what part of the physics do I not understand?
b)    how does one tell which device is really correct (or more
accurate)?, and,
c)    how can one get both instruments to agree with what they output?

Arthur Shulman ve3zv
Waterford, Ontario