[Elecraft] K3 6m pre-amp

[Jack Smith]

Indeed such is the case, and it is quite pronounced with high Q inductors.

You may wish, however, to model and/or measure two series chokes when 
both are wound on lossy ferrite material. The response looks quite 
different. The resistance in parallel with the inductors radically 
modifies the response when the material causes the inductor Q to be < 1.

I'll E-mail you some early measurements and a quick simulation plots.

Jack K8ZOA

Alan Bloom wrote:
> When you wire two chokes of different value in series, you almost always
> get a series-resonant "hole" in the attenuation somewhere between the
> parallel-resonant frequencies of the two chokes.  It's not hard to see
> why that's true if you model each choke as an ideal inductor in parallel
> with a capacitor.  So you need to design the combination so that the
> series-resonant frequency falls somewhere unimportant.
>
> Al N1AL
>
>
> On Thu, 2008-07-31 at 09:14, Jack Smith wrote:
>   
>> Jim:
>>
>> Thank you for the reference.
>>
>> Indeed, the self-resonant frequency of  the  typical small (FT50 size) 
>> chokes I've wound are in the 5-10 MHz range, but at 100 MHz some 
>> (depending upon the  core material) still show enough Z to be useful. 
>> For truly wideband 10 KHz - 100 MHz choke action, it's necessary to 
>> series two wound with different core material and turns, e.g., 35 turns 
>> on Steward 40 material for the 2.5 mH, followed by, e.g., 10 turns on 
>> FairRite 43 material for > 50 MHz.
>>
>> You have to pay particular attention to the u' and u'' values of the 
>> ferrite material and how they change with frequency as this is a case 
>> where core loss at high frequencies can be good.
>>
>> Jack K8ZOA
>>
>>
>> Jim Brown wrote:
>>     
>>> On Wed, 30 Jul 2008 20:33:53 -0400, Jack Smith wrote:
>>>
>>>   
>>>       
>>>> Yes, I have several ferrite core 2.5 mH chokes here, including the 
>>>> Hammond one you mention. There's a significant difference in high 
>>>> frequency performance of the pi wound on ceramic form versus the smaller 
>>>> pi-wound over ferrite and that's one of the things I'm hoping to 
>>>> illustrate in the article. 
>>>>     
>>>>         
>>> The RFI and Ferrite tutorial on my website includes clear explanations of 
>>> the nature of ferrite materials from a circuit point of view. You may find 
>>> it helpful in explaining why your very correct in your analysis. You are 
>>> welcome to cite it as a reference. 
>>>
>>> http://audiosystemsgroup.com/RFI-Ham.pdf
>>>
>>> However -- ANY 2.5 mH choke has a good chance of looking capacitive at 50 
>>> MHz. 
>>>
>>> 73,
>>>
>>> Jim Brown K9YC
>>>
>>>
>>>
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>
>