Homebrew PCBs

----- Original Message ----- 
From: "kilocycles" <kilocycles@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, February 20, 2006 11:50 PM
Subject: [Homebrew_PCBs] Re: Film Caps at RF frequencies


> John,
> Which type of film capacitors would be used at RF frequencies (say,
> 1.8 to 30 MHz)? I recently had the opposite problem...finding a low
> value 150 pF cap to use in an audio bandpass filter circuit in an Op
> Amp (NE5532 used as the audio final amp).  I had to use a silver mica,
> figuring it would be better than a ceramic disc.
>
> I've used foil caps, polystyrene ones, in VFO circuits for their
> thermal stability, but I'm interested to get your comments on other
> types of "plastic" film at these frequencies.  Generally I use
> ceramic, monolythic or multilayer ceramic for bypassing, ceramic
> NP0/C0G in temperature-frequency sensitive circuits, in addition to
> the polystyrene, and dipped silver mica in transmitter output filters
> at higher power (1 watt and above).

Silver mica are difficult to find and very expensive. Film capacitors aren't 
very suitable for RF, I use low-K ceramic plate capacitors, like these:

< 
http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=80010&CTL_CAT_CODE=30151&STK_PROD_CODE=M32340&XPAGENO=1 >

Leon