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

[David Wise]

That 3kV cap is definitely Type II since its capacitance changes as DC bias is applied.  I didn't see before, it has the circled-2 shorthand mark for Sprague.

Dave Wise
________________________________
From: Bob Camp <kb8tq at n1k.org>
Sent: Wednesday, May 1, 2024 8:13 AM
To: David Wise <d44617665 at hotmail.com>
Cc: Jacques Fortin <jacques.f at videotron.ca>; R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] R-390 Q of Caps and tuned circuits

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 <d44617665 at hotmail.com> 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 <jacques.f at videotron.ca>
Sent: Wednesday, May 1, 2024 7:56 AM
To: 'Bob Camp' <kb8tq at n1k.org>
Cc: 'David Wise' <d44617665 at hotmail.com>; 'R-390 Mailing List' <r-390 at mailman.qth.net>
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 <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
>
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