[R-390] 390A IF cans/ caps

[Bob kb8tq]

Hi

“Back in the day” ceramic caps were not what they are today. Ceramic cap technology
has come a *long* way since then. A NPO ceramic cap at 0+/- 30 from -55 to +125 is
a very stable capacitor. It also is stable in humidity which some of the early ceramics
were not. The other good news is that they are very low loss at RF and even microwave
frequencies. 

Since we’re talking about an IF can here, it’s a pretty good bet it’s at 455 KHz (to lazy
to look up the designator ….). In your shack the radio likely changes by < 20C. 30 ppm 
on the cap will be 7.5 Hz / C. Total “drift” of that coil will be 150 Hz. 

Tempco of the coil it’s self will depend on what slug material they used. None of the ones
you use at 455 are in the 30 ppm / C range. Simply put - the coil is worse than either the
mica cap or the NPO. It likely is a positive tempco so it will add to the mica tempco in this
case. 

Simple answer is - this kind of stuff is *why* you have mechanical filters …..

Bob

> On Jan 18, 2019, at 2:39 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Gary wrote:
> 
>> Digi-Key has 200 PF mica caps 1% tolerance and an F tempo with is essentially no drift.
> 
> Not exactly.  "F" tempco is 0 to +70 ppm/K = "+35 ppm/K +/- 35ppm" or "35ppm/K +/- 100%."  This is almost exactly the same magnitude as the tempco of the ceramic cap (if Larry's data is correct), but of course they have opposite signs.  (This is not "essentially no drift.")
> 
> Also, note that the combined tempco of the pair of capacitors is NOT zero (if it were, we could just use a 239pF C0G/NP0 instead).  Rather, it is the tempco that will compensate the tempco of the coil so the overall tempco of the LC tuned circuit is near zero.
> 
> Best regards,
> 
> Charles
> 
> 
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