[time-nuts] EFC divider resistors

Volker Esper ailer2 at t-online.de
Sat Feb 1 07:21:11 EST 2014


As far as I know, they make the internally used Vref externally
available for a divider, so the EFC input has a natural rejection
against fluctuations of it's own Vref. I would recommend using it rather
than building your own reference supply. However, a 78xx won't be a good
idea at all. An LTZ1000 is a good choise but to expensive and doesn't
give you the above mentioned natural rejection. If you absolutely want
to use an external reference, the LM399 can be an appropriate choice, at
a price of 10$. Today there are a lot other good references at prices
around 5$.

Volker



Am 01.02.2014 02:03, schrieb Bob Stewart:
> Any idea what the usual temperature coefficient is for the VRef output of  the OCXO?  I looked under the Trimble label of my spare and found: DOC2127 and 44/1416.  The one I have installed is probably the same.
>
>
>
>
>> ________________________________
>> From: Bob Camp <lists at rtty.us>
>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Friday, January 31, 2014 6:17 PM
>> Subject: Re: [time-nuts] EFC divider resistors
>>
>>
>> Hi
>>
>> 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. 
>>
>>
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