Hi,

I'm designing my own hardware, using the AT89Rf23x.

I have a question for those who already design their own hardware.


I was wondering what kind of capacitors do you use for RF purpose, like coupling capacitor between the IC and the balun.

Can I use standard capacitors or do I have to use capacitor especially designed for this purpose (typically, capacitor which present self-resonant frequency above 3 GHz).

E.g. I saw this one http://www.avx.com/docs/catalogs/accuf-p.pdf: Thin-Film RF/Microwave Capacitors

The drawback of these capacitors is the price which is 10 times higher than "classical" ones.


Thanks a lot for your precious advices.


P.S. Sorry for the possible mistakes that I've done, english isn't my mother tongue.

Yes, you'll need to use RF class caps. Anything in the RF path will require this. Usually, you'll be looking for low-loss, high Q types of devices. They'll usually also say RF in the description. Digikey has quite a few. If you're worried about the additional price, then you shouldn't be doing wireless

Thanks for the quick reply !

In fact, I was expecting of this.

But the datasheet of AT86RF230 makes me hesitate.
In the BOM at page 9, the RF coupling capacitor that they give seems to be generic type capacitor because of their low SRF:
AVX - 06035A220JAT2A (http://www.avx.com/docs/Catalogs/ccog.pdf).

or EPCOS B37920 (http://www.epcos.com/inf/20/10/db/cc_03/00140033.pdf).

Did they make a mistake in the datasheet or am I wrong ?

If they included it in the datasheet, then it should be okay. It requires testing though. Parts designated for RF are usually made with the knowledge that the value is critical. They also have low loss, high self-resonance, and characteristics beneficial to high frequency applications. The reason they're more expensive is because of the material used to make them and the testing involved.

Using standard parts requires that you do a lot of the testing yourself. Many standard parts are not very stable over a large temperature range and can vary significantly. This means that a good match at 25 deg C might not be a good match at 0 deg C or 100 deg C. Since wireless apps are used in a lot of outdoor situations, then you need to be aware of these factors. Thats why RF parts are generally better to use. The manufacturers know that they'll be used in matching circuits or that the value is critical and the tempcos (temperature coefficients) are usually more stable.

The choice to use standard or RF class parts is an engineering decision, but in my case, I don't mind spending extra on the RF front end since that's the main feature of a lot of my designs. Also, it usually only amounts to about $0.50 or so in extra BOM cost.