[time-nuts] Surface Mount OCXO Questions

[John C. Westmoreland, P.E.]

Bob,

OK - that makes sense.  If you follow good analog/digital layout rules then
this may not be a problem.  But your point about the EFC sensitivity is
well taken.
But, that is always a problem.

Yes, do a lot of people violate good analog/digital layout rules,
especially on the ground planes - yep.  This is exactly why I am asking
these questions.

Even in manufacturer's recommended layout instructions you can find
mistakes.  Unless you have a dev board that you have used and have the
gerbers from
that board so you know exactly how that part behaves with that layout - you
cannot know for sure you have a sound layout for your design.  You also
have
to be careful with board material, dielectrics, and copper weight, not to
mention controlled impedances.

Remember the early days of DC/DC converters?

Thanks!
John / AJ6BC



On Wed, Oct 30, 2013 at 5:52 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> Consider that microvolts matter on the EFC. Unless you have a separate
> return for the oven current it’s going to be tough to keep everything
> separate. One might  ask “why no separate return”. Well when you design one
> in, and then go look at people’s layouts - you might as well not have
> designed it in …..
>
> Bob
>
> On Oct 30, 2013, at 8:37 PM, John C. Westmoreland, P.E. <
> john at westmorelandengineering.com> wrote:
>
> > Graham and Time Nuts,
> >
> > (thanks for the answers.)
> >
> > I have another question - I am looking at a part from MTI.  I wanted to
> use
> > one of their 3.3V parts.  They are telling me to use the 12V part because
> > the 3.3V part can have an issue with ground loops due to the higher
> current
> > requirements at that voltage for the oven.
> >
> > Have any of you experienced this?  Makes me wonder a little why they
> offer
> > the 3.3V part.  It would seem good layout can control any possibility of
> > ground loops becoming a problem.
> >
> > Thanks and Regards,
> > John W./AJ6BC
> >
> >
> >
> >
> > On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H <timenut at austin.rr.com
> >wrote:
> >
> >> John:
> >>
> >> All SMT OCXO's will either have a recommended PCB footprint in the spec
> >> sheet
> >> or will refer you to a recommended footprint in another document.
> >>
> >> Some don't care about a ground plane under the part, some require it
> >> with no crossing signals, some require an open thermal hole underneath
> >> the oven.  I have seen all three cases.  As usual, it is suggested that
> you
> >> read the [] manual.
> >>
> >> Best regards,
> >> --- Graham / KE9H
> >>
> >> ==
> >>
> >>
> >>
> >> On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote:
> >>
> >>> Hello,
> >>>
> >>> I was wondering if I could get some recommendations on surface mount
> >>> OCXO's
> >>> vs. the traditional through hole.
> >>>
> >>> I was also wondering on the board layout - if you found it necessary to
> >>> leave a thermal moat so to speak - and what worked best.  Maybe the
> OCXO
> >>> has an internal air barrier that maybe would make this unnecessary -
> not
> >>> sure.
> >>>
> >>> Your input and experience appreciated.
> >>>
> >>> Thanks,
> >>> John Westmoreland
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