>
>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*.
With two doorkob caps in parallel of equal value, I got a sharp dip at c.
80MHz. I could repear rhe experiment if you like, Ian.
>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.
>
// Why not? Is the sharp dip I see on the dipmeter a pseudo-dip?
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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