Hi The X7R isn’t going to be as good as the NPO for Q. It will beat the Y5V and Z5U by a wide margin. It also will win out over them for delta C over temperature or delta C over voltage.
To see a major change on a 3KV cap, you likely will need to get the voltage up into the 500 to 1,000 V range. Further complicating things, package size matters if you are looking at a modern part. “Back in the day” there was one chart the described any Z5U (or X7R). If you get into really small modern packages … not so much. Bob > On May 1, 2024, at 11:03 AM, David Wise <[email protected]> wrote: > > My 3kV ceramic cap might be X7R. It is physically small. I will test it > today and report back. I will look for capacitance change as it receives DC > bias. > > I was disappointed that I didn't have a 100pF polystyrene cap. I do have > some 10nF ones, and maybe I will compare polystyrene, polyester, > polypropylene, C0G, and mica. Probably not today, I had the 4342A on the > kitchen table yesterday so I should stay out of my wife's way today 🙂 > > Dave Wise > From: Jacques Fortin <[email protected]> > Sent: Wednesday, May 1, 2024 7:56 AM > To: 'Bob Camp' <[email protected]> > Cc: 'David Wise' <[email protected]>; 'R-390 Mailing List' > <[email protected]> > Subject: RE: [R-390] R-390 Q of Caps and tuned circuits > > 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 <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
