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.
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 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 network analyzer it isn't, but it does show what kind of resonances
you have, and where they are.
--
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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