>
>
>2 wrote:
>>
>>>
>>>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.
>[...]
>
>>>
>>>But none of this applies when *identical* capacitors are paralleled.
>>>
>>// Why not? Is the sharp dip I see on the dipmeter a pseudo-dip?
>>
>Certainly it's real. When you connect the two capacitors together at
>each end, you make a series-resonant loop. Guessing at values, two
>1000pF caps, in series with a total of 8nH lead inductance + self
>inductance, comes out at 80MHz.
>
>But dipping the two capacitors on their own tells you *nothing* about
>their behavior in-circuit.
>
// It tells you that an additional resonance is in series with the anode
circuit.
>However, you can make some real measurements using an MFJ or Autek SWR
>analyzer:
>
>* To look for a series resonance, connect the components in series with
>a 50 ohm resistor and sweep the frequency - SWR dips to 1 wherever the
>total reactance is very low compared with 50 ohms.
>
// To simulate the condition caused by paralling two blocking caps, they
need to be measured in paralleled.
>* To look for a parallel resonance, connect the components in parallel
>with a 50 ohm resistor and sweep the frequency - SWR dips to 1 wherever
>the total reactance is very high compared with 50 ohms.
>
// A garden-variety dipmeter can be used to find the resonance. The
solution is to use one DC blocker of sufficient current rating.
>A network analyzer it isn't, but it does show what kind of resonances
>you have,
// At VHF, only with a zero-L, 50? resistor.
> and where they are.
>
// So will a dipmeter. According to what Mr. Rauch said, I would trust
a dipmeter to find a VHF resonance.
>--
cheers, Ian
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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