>
>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.
>
// The issue at hand is using two, paralleled capacitors as a DC
blocker. My position is one DC blocking cap with more current capability
is a better choice
>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.
// So Mr. Rauch was in error about the MFJ analyzer's dipper
disfunction?
>
>* 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.
>
// The hat-trick is finding an elusive zero-L 50-ohm resistor.
>A network analyzer it isn't, but it does show what kind of resonances
>you have, and where they are.
>
// Dipmeters don't show where resonances are?
?
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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