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 <time...@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 <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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.