Hi Two different use cases, bypass and resonant circuits:
The X7R gets used when replacing a normal *bypass* capacitor. In a bypass situation, there likely is some minimum C that is required. Going over that amount typically causes no big problems. There also is some maximum ESR that can be tolerated. Again, going under that normally isn’t a big deal. Could you replace all your bypass caps with NPO’s? I suppose you could. In some applications, NPO’s *do* get used for bypass. This is pretty rare. Their cost and size make them a bit tough to deal with for bypassing. Bob > On May 1, 2024, at 10:56 AM, Jacques Fortin <[email protected]> wrote: > > Hi Bob, > > I'm sorry but I do not agree with your last statement below. > A X7R is a class 2 capacitor and it will most probably perform badly in a > resonant circuit because it's Q factor will be too low. > A C0G (NPO) is a class 1 capacitor and some can have ESR / Q values at par > with the mica ones. > More info on the ceramic caps at: > https://en.wikipedia.org/wiki/Ceramic_capacitor > But the final test is the application circuit: if a ceramic cap used in a > resonant circuit fail to provide sufficient stage gain, or output signal > amplitude, try a mica one instead. > BTW, some small polypropylene or polystyrene foil capacitors can replace > silvered mica caps (sometimes) but their temperature coefficients do not > match the mica ones. > > 73, Jacques, VE2JFE in Montreal > > -----Message d'origine----- > De : Bob Camp <[email protected]> > Envoyé : 1 mai 2024 10:01 > À : Jacques Fortin <[email protected]> > Cc : David Wise <[email protected]>; R-390 Mailing List > <[email protected]> > Objet : Re: [R-390] R-390 Q of Caps and tuned circuits > > Hi > > Another approach: > > Never *assume* a ceramic is an NPO unless it is labeled as such. Just *why* > folks would inventory (and then sell off as surplus) non-NPO 100 pf caps … > who knows? > > My bet: Somebody who didn’t know better found that the X7R (or whatever) > parts cost 1/3 as much as the NPO’s. They then switched all the purchase > orders over to X7R. A month or two later, you have chaos in the factory. A > few weeks after that a whole bunch of X7R’s go on the surplus market *really* > cheap. Yes, I have first hand experience with that happening. > > Indeed the same thing applies to parts in bypass applications. Various > dielectrics have *big* differences in a number of parameters. A Z5U or Y5V > put into a location that had an X7R just might be a bit “close to the edge” > when it gets nice and hot in the radio. The gotcha there is that the part you > just pulled out probably isn’t very clearly marked in terms of “what was it”. > Simple answer: go with an X7R (if it will fit). > > Bob > >> On Apr 30, 2024, at 11:38 PM, Jacques Fortin <[email protected]> wrote: >> >> Hello Dave, >> >> Thank you for all those measurement results. >> For the ceramic caps, I already mentioned that some ceramic caps >> (especially the NPO ones) can be as good as the SM ones, but some made >> of different ceramic grades are really worse, to the point that a >> given one prevented the 17MHz xtal oscillator stage to drive the >> R-390A first mixer with sufficient amplitude. >> And this was the starting point for this topic. >> I believe that Larry recommendation is: use a SM replacement cap where >> mica ones are used in the R-390A, unless you can measure that a >> ceramic replacement is good enough for the task. >> >> 73, Jacques, VE2JFE in Montreal >> >> ______________________________________________________________ >> R-390 mailing list >> Home: http://mailman.qth.net/mailman/listinfo/r-390 >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net Please help support this email >> list: http://www.qsl.net/donate.html > ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
