[AMPS] Re: 6m amp blocking caps

[2]

>
>
>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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