[time-nuts] EFC divider resistors

Bob Camp lists at rtty.us
Sat Feb 1 13:51:10 UTC 2014


Hi

OCXO’s with separate ground for the oven have indeed been made. They do show up in catalogs and on data sheets. Customers don’t buy them….. The same is true of the other approach, which is to bring out a separate (isolated) EFC return.

Bob

On Jan 31, 2014, at 11:14 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 
> lists at rtty.us said:
>> A 7805 has a *rated* temperature coefficient of 2 mv / C. Most parts do way
>> better than this. The problem is knowing if you have a good part or not. On
>> a 5V part, your reference could be running 400 ppm/C. Since the divider has
>> no impact on this number, it over-rides what ever you do with the divider
>> resistors. A good voltage reference will set you back about $50 or so.  
> 
> Plan B would be to keep the 7805 at a constant temperature.
> 
>> You commonly see a couple of mv of variation in the ground pin voltage with
>> oven current depending on exactly where the pin is terminated. Any change in
>> the ground setup can be an issue on an OCXO with the EFC in common with the
>> oven (which is most parts). 
> 
> Again, plan B would be to keep the OCXO at a constant temperature so the 
> heater current is (close) to constant and the offset on the ground pin due to 
> heater current would be constant.
> 
> ---------
> 
> I've heard comments about offset due to a single ground pin for several/many 
> years, so it's nothing new.  Why are manufacturers not providing separate 
> pins?
> 
> Are there OCXOs (that I haven't noticed) with 2 ground pins?
> 
> Is the problem backward compatibility?  In that case, I'd expect there would 
> be a pinout with a new second-ground pin that offered an interesting 
> "beat-this" challenge to other vendors.
> 
> 
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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