[R-390] R-390 Q of Caps and tuned circuits

[Bob Camp]

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 <jacques.f at videotron.ca> 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 <kb8tq at n1k.org> 
> Envoyé : 1 mai 2024 10:01
> À : Jacques Fortin <jacques.f at videotron.ca>
> Cc : David Wise <d44617665 at hotmail.com>; R-390 Mailing List <r-390 at mailman.qth.net>
> 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 <jacques.f at videotron.ca> 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:R-390 at mailman.qth.net
>> 
>> This list hosted by: http://www.qsl.net Please help support this email 
>> list: http://www.qsl.net/donate.html
>