[time-nuts] TCVCXO Adjustment

Bob kb8tq kb8tq at n1k.org
Sat Apr 14 17:28:08 EDT 2018


It’s reasonable to expect the slope of the EFC to vary 2:1 over it’s range. Could be more,
could be less. How much depends a lot on what the original OEM wanted on surplus parts. 
In the case of “bought new” you would have to check the data sheet. If there’s no spec, it’s 
reasonable to assume 4:1 slope ratio over the range. 

Note that none of this tells you what the curve looks like. It only gets into the change in slope. 
Why? Well, if you are looking for a tuning increment, that is what you are after. It is still not 
a perfect measure, but it’s about as close as you are going to get. 

The shape of the curve depends on a lot of things. The crystal overtone and its parameters
certainly get in there. The exact varicap (or set of varicaps) used get in there. Finally the offset
from series (or anti-resonance) gets into it all. 

So lots of variables, lots of curves …..


> On Apr 14, 2018, at 4:03 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> kb8tq at n1k.org said:
>> The gotcha is that you do not have a calibrated adjustment. Put another way,
>> there isn’t a perfect correlation between DAC bits and ppm. Each adjustment
>> you make is subject to a bit of error. When you are trying to get within a
>> ppm,  your measurements are quicker, so the larger error ( percentage of
>> step) may not be as big a deal. When you get close, it is likely to become a
>> big deal.  
> Interesting.  Thanks.
> How much does it vary from unit to unit?  How non-linear is it likely to be?  
> How much does it change with temperature?
> Anybody have data?
> Are fancy OCXOs more linear?
> -- 
> These are my opinions.  I hate spam.
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