[R-390] Ceramic Temp coefficients

[Bob Camp]

Hi

There's a lot more to it than the simple EIA information. Ceramic caps (as previously mentioned) change capacitance with applied voltage. The X7R does this less than a Z5U. An NPO will beat any of the rest. They also have leakage (usually dimensioned in megohms per microfarad). Again, NPO wins the race compared to other ceramics. Next on the list is either microphonics (as in it's a ceramic mic) or dielectric absorption (as in short out the cap, come back in a while and there's voltage on it). NPO wins both of those as well, X7R comes in second.

So far that's all DC performance. In a radio IF, you likely have RF on the caps. Caps have both a Q and an effective series resistance. For most caps, the Q gets worse as frequency goes up past some magic point. Again NPO wins, X7R works pretty well at any R-390 type frequency. 

No, that's not the whole story. But it's already getting awfully repetitive. Simple answer is always - if it's ceramic use NPO if you can. If you can't get NPO, use X7R. If you can't use X7R, dig very deeply into what you are doing.

--------

That's just talking about ceramics. For DC performance, you can beat ceramics with *good* film caps in *good* packages. For purely RF specs, film caps generally are not going to perform as well as an NPO ceramic. 

Lots of choices. 

Bob

On Feb 10, 2013, at 7:55 PM, quartz55 <quartz55 at hughes.net> wrote:

> I've always been a bit confused about the TC on caps.  Here's a good one on the ceramics.  http://www.niccomp.com/Products/TC_Ceramics.pdf
> 
> N3DT
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