NPO cap tempco spec confusing

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From: David Shalita (af389@lafn.org)
Date: Thu Dec 07 1995 - 11:17:10 EST


I must be misunderstanding the tempco specs for NPO class 1
ceramic disk capacitors.
 
Samsung NPO's, class 1 500 volt, shows temperature coeffcients
as plus/minus 0.25 percent per degree C max on a graph.
Centered and zero at 20 deg C, capacity increases for increasing
temperature above 20 deg C, decreases when going below 20 deg C.
 
Murata shows class 1 500 volt ceramic disk caps with a tempco of
30ppm per deg C max...it may be plus minus 30ppm max, not sure.
 
I believe 0.25 percent per deg C implies 2500 ppm per deg C change in
capacity which is a huge change compared to 30 ppm per deg C, for a
40 deg temperature shift....assume 100 pf NPO cap.
 
Are NPO class 1 ceramic disk caps available which have
        a 30 ppm per deg C capacity change
                or
        a 3000 ppm per deg C capacity change
 
depending upon which vendor's NPO's I purchase? I hope not.
 
I think I see many examples of this, even in DIGIKEY catalog
         where Panasonic caps are advertised as 0.5ppm per deg C
        and Philips caps are offered at 30 ppm per deg C max.
        Both are class 1 parts.
 
I am convinced I am misunderstanding the specifications for
tempco and really need some advise.
 
My application for NPO caps is for a VFO(S) and LO(S) for
my homebrew projects.
 
Thanks,

--
af389@lafn.org
Dave Shalita,


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