[time-nuts] EFC divider resistors

Hal Murray hmurray at megapathdsl.net
Fri Jan 31 23:14:58 EST 2014


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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