2 wrote:
>
>When DC blocking caps are paralleled, a parallel-resonance is created
>between the capacitors in the RF output path. The additional resonance
>may be problematic.
>
This is only relevant when the two capacitors are very *different*. You
then get a parallel resonance, between the lower capacitance and the
self-inductance of the higher-value one.
For example, if you connect a typical 0.1uF polyester cap in parallel
with the 1000pF screen bypass cap built into a tube socket, you're
likely to get a parallel resonance at HF. This is from 1000pF with
several hundred nH self-inductance of the polyester cap. The resonance
is very sharp, but it means that the screen grid is completely
un-bypassed at that frequency!
Fortunately these sharp resonances are easily damped out. Often the
losses in the components (generally the higher-valued capacitor) or
elsewhere in the circuit will do it automatically, so you never even
notice. A more positive cure is a low-inductance resistor of 10-100 ohms
inserted between the two capacitors, or in series with the higher-valued
one. (The resistor can be shunted with an RF choke if DC continuity is
required.)
But none of this applies when *identical* capacitors are paralleled.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
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