[time-nuts] Know thy components

[James Flynn]

Hal Murray <hmurray at ...> writes:

> > Also the carbon composition resistor  drifted with time at elevated
> > temperature.
> 
> I assume the drift is slow enough so that any any control loop will 
easily 
> track it.
Actually, "ageing" is a better word than "drift". After the major change 
in resistance due to temperature, the change in resistance was a slow 
exponential, taking several weeks to level off.  The change in the first 
day or so was pretty dramatic, causing an error of about 2E-9 per day in 
the oscillator. Since my application was looking at the source signal 
over the course of a day, the error was appreciable. Even with a GPSDO, 
the window has to be in the range of 10000s for the level of precision I 
am shooting for. 

After the oven stabilized, the changes due to temperature were quicker 
and on the order of 2e-10 in a few minutes. This was unacceptable for my 
application due to the averaging times. Even the average diurnal change 
in ambient temperature was visible in the error plot.
 
> 
> What is the drift like at non-elevated temperatures?
I have not studied this, but I would think that, like other ageing 
processes, it would be less.

> 
> What is the drift like for good resistors?
Also not studied yet.

> If I'm using 2 resistors as a divider, how well do they track?  Does 
it 
> matter if they have same or different values?
IF you assume they have the same change in percent of their value per 
degree C, it would not make any difference, regardless of their relative 
sizes.  However, I cannot write that "IF" big enough. Even resistors out 
of the same batch may not change in the same way. I would not rely on 
them tracking. Also if other resistors are included in a configuration 
that is not a divider, all bets are off.

> 
> The problem I see with resistors (or voltage reference or ???) 
drifting is 
> that it gets tangled up with the crystal drifting.  ...

Exactly. And exactly why I have changed the resistors to ones I have 
tested to be less than -1ppm/deg C.