Good RF caps - Page 1
Forum section: electronics / rf-microwave

author: Lee697
author_rank: Regular Contributor
timestamp: April 15, 2021, 12:46:15 am
content: This is probably a pretty basic question, but I've become a little bewildered on Mouser to say the least.....I'm building a lightning detector circuit from techlib.com ('Sferics detector'). One of the caps the author has suggested 'use a good RF cap' - 2.2nF, part of an LC tank on the inputs of a TL592 amp chip. I'll probably need a 200pF one to match the inductor I have... but what one??My research suggests maybe a ceramic in C0G? Kemet seem to have a bunch on Mouser, like this one 80-C322C222J5G ?Is this what I should be getting a few of?Thanks for any assistance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2021, 05:11:10 am
content: Is this, say, <30MHz?Yes, C0G will do fine, as well as PP film if you need somewhat bigger values anywhere. X7R, Y5P or even Z5U ceramics are fine for bypass or coupling capacitors, or electrolytics for much larger bypass.SMT chips are desirable at higher frequencies, but if this thing isn't doing anything crazy or powerful, leaded should be fine.Note that leaded MLCCs don't handle much voltage beyond their rating. Duh, I guess, but ceramic disc types can often survive many times their rating, a handy tip in a pinch, or when transients are expected.(Which, heh, not like picking up distant lightning, more like nearby ESD or lightning-induced surge.)Tim

author: A.Z.
author_rank: Super Contributor
timestamp: April 15, 2021, 10:32:29 am
content: Quote from: Lee697 on April 15, 2021, 12:46:15 amDid you try contacting the man behind the Techlib website ? As far as I can tell, he replies to emails, so you may just ask him your question!

author: radiolistener
author_rank: Super Contributor
timestamp: April 15, 2021, 01:12:13 pm
content: Quote from: Lee697 on April 15, 2021, 12:46:15 amyou're needs low noise high Q capacitor for such purposes, such asATC100B series.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 15, 2021, 02:02:18 pm
content: Generally "RF" caps are meant for UHF and higher. Good general purpose MLCC C0G SMD will be fine up to a few hundred MHz, which is more than enough for your application.High RF current applications might be different, you would need high-grade RF caps even at HF as thermal dissipation becomes an issue so you want the absolute minimal losses.

author: A.Z.
author_rank: Super Contributor
timestamp: April 15, 2021, 04:59:25 pm
content: just in case, the circuit should be this onehttp://techlib.com/electronics/spherics.htm

author: jmw
author_rank: Frequent Contributor
timestamp: April 15, 2021, 05:08:54 pm
content: Silver mica is what comes to mind for RF + capacitor, and they're still reasonably priced at 200 pF. At 2.2 nF, they're starting to get a little chonky and expensive, but still usable.

author: Lee697
author_rank: Regular Contributor
timestamp: April 15, 2021, 11:47:37 pm
content: Quote from: A.Z. on April 15, 2021, 10:32:29 amYes, and he is excellent at replying, very helpful, and actually generous too, at sending components (internationally, for free too).... but I've asked him so many questions lately, thought I'd spread the load a bit....

author: Lee697
author_rank: Regular Contributor
timestamp: April 16, 2021, 12:10:55 am
content: Quote from: M0HZH on April 15, 2021, 02:02:18 pmCheers, this circuit only runs in the 300kHz range, I'll give the C0G's a try.

author: cdev
author_rank: Super Contributor
timestamp: April 20, 2021, 01:31:58 pm
content: I am building a loop antenna that I hope to transmit into in the future, so I am wondering about good capacitors for RF at medium power. Beeft caps for power?Preferably high Q. Mica? Vaccum? Ceramic? What are the best/cheapest caps for filters, loops and other high Q applications if you're on s budget?

author: A.Z.
author_rank: Super Contributor
timestamp: April 20, 2021, 01:44:36 pm
content: Quote from: cdev on April 20, 2021, 01:31:58 pmYou're OT here, if you want, please open a new thread.

author: cdev
author_rank: Super Contributor
timestamp: April 20, 2021, 02:16:57 pm
content: Yes, and that is, according to the experts, a huge problem with this kind of application. For example trimmer caps get hot or melt. Even at low power levels, if they are not really high q.Quote from: M0HZH on April 15, 2021, 02:02:18 pm