[time-nuts] Surface Mount OCXO Questions

[Bob Camp]

Hi

You may have a good layout on your board, but they have to get the current to that pin / pad somehow. In all likelihood the amps of current through the pad it’s self are going to cause issues. Also remember that the ground is likely an RF return as well. Multiple rules and layout issues all collide at that pin…..

Bob

On Oct 30, 2013, at 9:02 PM, John C. Westmoreland, P.E. <john at westmorelandengineering.com> wrote:

> 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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