[time-nuts] Surface Mount OCXO Questions

[Bob Camp]

Hi

The control voltage on the 12 V OCXO is likely 0-10V or 0-5V. The tune on the 3.3V part isn’t going to be above 3.3V and it may be 0-2.5V. The 3.3V part is going to be at least 8X more sensitive to grounding issues. 

To put this in perspective, you can see a change on a normal 12V part grounding it on the top side of a PC board vs grounding it on the bottom side of the board. The 0.032” of solid ground lead has enough drop to be noticeable. 

Bob

On Oct 30, 2013, at 9:47 PM, Graham / KE9H <timenut at austin.rr.com> wrote:

> John:
> 
> Look at the ppm (or however they express it) as to the sensitivity of the
> frequency stability of the OCXO relative to Voltage input.
> 
> Say the oven power drops from 3 watts to 1 Watt as the oven comes up
> to temperature.  At 3 Volts, relative to 12 Volts, for a given resistance,
> it is four time the Voltage change due to the higher currents, and an additional
> four times the percentage of the operating Voltage as a ratio.  So additional
> design consideration for Voltage control/stabilization is needed.
> 
> If you have a solid (wide, thick, multi-layer) ground, then that can
> work.  To reduce the voltage drop feeding the OCXO, you might consider
> putting a dedicated LDO regulator, right at the OCXO, that shares the ground
> reference with the OCXO, so any voltage drop in the feed side is removed,
> as well as any Voltage variability with current in the ground system.
> 
> As to why they are selling the 3.3V part, they probably started selling it
> before they had some customers get into performance issues per the above.
> But once offered, they have to continue to support their customers.
> 
> I think they are just telling you that it is somewhere between 4 and 16 times
> easier to get the full performance out of the part with a 12 Volt power feed
> than a 3 Volt power feed, not that you can't get full performance with a 3.3V feed.
> 
> I am sure their parts meet specs, you just need to understand them.
> 
> P.S. - I would stick with linear regulators feeding the OCXO, not a switcher.
> 
> --- Graham
> 
> ==
> 
> On 10/30/2013 7:37 PM, John C. Westmoreland, P.E. 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
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.