Re: gEDA-user: OFF: capacitors for RF power amplifier

[Gabriel Paubert <paubert@xxxxxxx>]

On Wed, Apr 13, 2011 at 09:31:24AM +0100, Andy Fierman wrote:
> If you are going to model the PA - particularly to look at resonance
> effects - then you should include reasonably accurate models for the
> inductors and capacitors which include their major parasitic
> components.
> 
> The Murata Chip S-Parameter & Impedance Library is a handy tool for
> looking at their ceramic capacitor and inductor behaviour.
> 
> http://www.murata.com/products/design_support/mcsil/index.html
> 
> or there's an online version:
> 
> http://ds.murata.com/software/simsurfing/en-us/index.html
> 
>          Andy.
> 
> signality.co.uk
> 
> 
> On 12 April 2011 23:57, Wojciech Kazubski <wk0@xxxxx> wrote:
> >> I'm currently designing a power amplifier for the HF (3-30MHz) radio
> >> band.
> >>
> >> I am selecting capacitors for the low pass harmonic filter bank at the output.
> >> My question is what kind of capacitors should I use? I apply not more then
> >> 100V of say 30MHz maximum.
> >>
> >> My best bet is to use X7R capacitors with as much DC voltage rating as I can
> >> get. I don't know if there's any connection between the DC and AC losses.
> >>
> >>
> >> Thanks,
> >> Levente
> >>
> > The biggest problem can be the current handling capacity. Very few capacitor makers specify this. Usually if a capacitor is rated for 100V this means 100V DC wthout any current. Only special purpose capacitors for RF power applications have HF current ratings, see Anerican Technical Ceramics for example:
> > http://www.atceramics.com
> >

Another manufacturer is Dielectric Labs: 

http://www.dilabs.com

but they are not cheap, especially the porcelain ones.
However, they have very good temperature characteristics
and high to very high voltage ratings. Dilabs basically 
only manufactures capacitors for high frequency applications.

> > For RF do not use X5R/X7R (good only for supply bypassing), use NP0/C0G or similar low loss ones.

Agreed, well X5R/X7R can be used for loop filters in PLL and servo
applications, when the bandwidth is not too critical, as long as you
meet the stability criteria (but I use 25 or even 50V rated capacitors
when the voltage on the capacitor never exceeds 3.3V). The problem
is that you can't use anything else when you have size constraints
and need capacitors in the 0.1 to 10 microfarad range: very low loop 
bandwidths for either crystal (VCXO) oscillators and even much lower
for thermal stabilization loops.

On the other hand, class Y and Z dielectrics should not even
be allowed; a look at temperature and voltage dependence graphs
should suffice to convince any circuit designer.

	Gabriel


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