[time-nuts] V refs

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Nov 30 22:53:32 UTC 2008


Richard Moore wrote:
> On Nov 30, 2008, at 4:00 AM, Bruce wrote:
>
>   
>> Message: 1
>> Date: Sun, 30 Nov 2008 17:26:28 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] V refs
>>
>>     
>
>   
>> <clip>
>> Does the Datron use a series stack of zeners or does it use the once
>> popular ring of 2 reference circuit?
>>
>> Bruce
>>     
>
> Bruce, the 1082 uses two sets of two zeners in series. Each series  
> set of zeners has its own current setting resistor set. The out put  
> from each zener string is fed thru a 9.1k resistor into the + input  
> of an opamp mixer which drives an emitter follower, and that drives  
> the current setting resistors for the zeners, and, divided down, the  
> ref. output for the meter. The system actually works very well, but  
> given the ages of these meters, I suspect that the zero TC current  
> point of the zeners might need tweaking. The opamp is a Fairchild  
> uA714CH. I'm not familiar with its noise performance or, in this  
> system, its input temp characteristics -- I'll bet this also would be  
> a good spot for a chopper amp, if I switch to a LTZ1000....
>
> Dick Moore
>
>   
Dick

The uA714 is claimed to be an OP07 equivalent.
A chopper stabilised amplifier would have lower drift and flicker noise,
however the zener drift and flicker noise probably dominates.

Stacking LTZ1000's in the same way is a little tricky.
The only satisfactory method I have devised so far uses a PJFET as a
voltage controlled resistor to sink the zener current whilst allowing it
to be reused to bias lower LTZ1000s in the series stack.

Bruce



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