[AMPS] Re: 6m amp blocking caps

[Ian White, G3SEK]

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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