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 <time...@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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