[time-nuts] Replacement A9 boards for the HP 5065A

Charles Steinmetz csteinmetz at yandex.com
Sun Mar 4 14:17:24 EST 2018


Poul-Henning wrote:

> The input signal to the integrator is continuous and jump-free and
> the relevant time constant is sub-second.  Dielectric absorption
> doesn't matter when there are no voltage jumps.

That is a very common misconception, probably fostered by the usual test 
methodology for DA.  But it is not correct.

DA is a charge-migration phenomenon, so it occurs *whenever the charge 
on a capacitor changes.*  A step function in the charge very obviously 
represents such a change (and for this reason is used for testing the 
parameter in most circumstances), but so do gradual changes in the 
driving charge.  These can be less obvious (than step changes), 
depending on the time scale of the change relative to the time scale of 
the DA process in the particular capacitor.

There are several dimensions to DA -- not just how much charge isn't 
returned immediately, but what the "tail" looks like (i.e., the time 
profile of charge return, generally measured as current vs. time). 
Different dielectrics (and, to a certain extent, different construction 
techniques) exhibit different charge-return profiles.

Long-TC integrators do tend to mask the effects of DA, because the time 
scale of changes in the driving charge is closer to the time scale of 
the delayed charge return caused by DA.  But that does not mean that DA 
isn't important, because slow integrators are frequently used where 
extremely high precision is required (such as integrators for 
high-resolution DACs and EFC servos for precision oscillators), so the 
tolerable amount of delayed charge return is extremely low.

As with any circuit, the A9 integrator is "good enough" when the errors 
it causes are swamped by other accumulated errors.  But I'm no fan of 
"just good enough" design, particularly in a case like the 6065A where 
time nuts are diligently working to improve the performance of the 
instrument.  Some times, "just because it's better" is a sufficient 
reason to overdesign, particularly where the incremental cost is low and 
especially where the projected number of units is low, both of which are 
true WRT the improved A9 board.

Best regards,

Charles




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