Homebrew PCBs

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).

I appreciate your input,
Ted

--- In Homebrew_PCBs@yahoogroups.com, "John Popelish" <jpopelish@...>
wrote:

> You are welcome.
>  
> > Your explanation provides a good example of why we don't use film
> > capacitors at RF, only at audio frequencies.
> (snip)
> 
> Actually, film capcitors are often used in RF design.  But details of
> construction methods and dielectric properties count for a lot.  Every
> commercial capacitor is a compromise between lots of competing
> factors.  None is perfect and free and minimal size.  Best design
> practice involves getting to know the relative costs and imperfections
> of each kind of dielectric and construction method so you can select
> the best overall compromise for any given application.