Graham, Good points - yes, I have this part currently in the design: TPS75833KTTT (LDO from TI) - putting another one down (just) for the OCXO isn't a problem.
And a nice 12V rail isn't a problem either since this is for a radio with a nice 12V source. Could I boost the 3.3V rail to 12V or maybe 5V to 12V - sure - but your point about the switcher is well taken and I agree. Having a nice, fat, analog ground plane isn't a problem either. And, this is just a 'dev' board so we can do what we need to make the OCXO work as good as possible. >From their spec sheet: '... the supply voltage sensitivity and load sensitivity is 5E-11 for a 5 % change in voltage or load impedance.' Thanks, John On Wed, Oct 30, 2013 at 6:47 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<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.