[time-nuts] 5070B once more.... (actually 5370A fans)

[Tom Van Baak]

One comment, and one idea on airflow...

I've noticed that none of my high-end frequency standards use
fans at all. That includes every Rb, Cs, and H-maser. This does
not mean they are all cool to the touch, but my guess is it's better
to allow temperature gradients to exist than use fans in an attempt
to even then out.

I'm sure this requires no small effort in thermal engineering on
their part. But it also has the side effect of 100% quiet instruments
and no mechanical wear-out mechanism. Of course, you don't
want to exceed a high temp limit on any one component, but the
point is passive heat transfer with metal seems to be the better
choice than active transfer with air and fans.

Now, a suggestion for the 5370 question.

I'm sure we could discuss for some time what airflow is too much
or too little or where to place the thermal probes or how to take IR
scans or how then interpret them. But it seems to me the bottom
line is not what the temp is inside, but how the unit performs; how
stable the time interval counter is. voltco; tempco; airco?

It doesn't matter if you measure a +10C change inside but it has
little effect on the jitter or accuracy of the counter. Similarly, if a
tiny +1 C change at some special point causes many picoseconds
of drift then that's something more to worry about.

So my suggestion is for someone to take a 5370 and set it up to
take continuous phase measurements between two stable inputs
(or delay generator, or a tee from a common reference) and then,
vary the airflow from way too low to way too fast. You could do
this in steps every 10 minutes, or linearly over hours, etc.

The plot you get is then simply accuracy (or jitter or drift) versus
air flow. This plot, more than one or more raw temperature probe
measurements or IR pictures, will tell you what the ideal range of
air flow is. I have no idea what you'd find. But it would settle the
question about fan replacement for the 5370.

/tvb