[time-nuts] Bye-Bye Crystals
kb8tq at n1k.org
Wed Mar 15 18:03:12 EDT 2017
> On Mar 15, 2017, at 3:55 PM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
> Hi Bob,
> If one prototypes any crystal oscillator, and runs it on a bench. Then builds an 'oven' of sorts and runs it on the same bench. Would you expect to see any improvement?
Sure, the XO likely moves 50 to 100 ppm over -30 to +70. You will cut that down to a couple of ppm. It’s much easier these
days to just buy (or salvage) a cheap TCXO to get the same level of stability.
One very basic issue: If I just grab a random crystal, it likely is a cut that does *not* have a useful turn temperature at all. For
a proper OCXO you need a crystal with a turn temperature in the practical range for your oven. There are many other
The key point (just as in the previous message) is that you must have good frequency vs temperature data to know if
you are improving things or not. That involves having a real temperature test chamber than can be slewed in a controlled
fashion and repeatably set to a sequence of temperatures. A typical run starts at room, steps down to cold (or up to hot).
It then steps to the other end and finally steps back to room. Data is taken every 10C or so and analyzed to be sure that
things are not all messed up. One obvious problem / issue would be drift during the run. A typical run takes several hours to
most of a day.
> In other words for a given oscillator (crystal and electronics, etc), will there be any improvements in an oven compared to not in an oven? Or, are there other things that outweigh the gains by temp controlling the whole thing.
> Yeah, this is a pretty open question, but I don't really have a feel for how good an oscillator needs to be before an oven starts to improve things...
> On 3/15/2017 12:00 PM, time-nuts-request at febo.com wrote:
>> Where do you plan on getting an OCXO grade crystal at an odd frequency like
>> that? Much of the performance of a good OCXO is in the crystal. Doing a proper
>> design on one is a lot of work. You *might* think that having a design for 5.000000
>> MHz would give you a good design for 5.000050 MHz. I have empirical evidence that
>> this isn’t the case. Many years later, I’m still utterly amazed that this is the way things
>> work in the crystal business ….( = it’s not just a design issue, it’s also a business decision)
>> More or less the crystal needs to be:
>> 1) Cut specifically to have a turn at a temperature that makes sense for your application.
>> 2) A “large blank” design (for it’s frequency)
>> 3) In a cold weld package (most of the normal crystals are resistance weld)
>> 4) Run through a high vacuum / high temperature process
>> 5) Be plated with gold rather than something like silver or aluminum (unless it’s at VHF).
>> 6) Have a motional capacitance that makes sense for your EFC range ( normally = minimize)
>> 7) Preferably be an SC or modified SC cut.
>> This is for a high stability part. The list does keep going on for a while, but that should
>> give you a pretty good idea.
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