Thoughts of K2 xtal prefilter

[Benny Aumala]

Installing prefilter IN STEAD of KNB2 gives the possibility to
place 5dB 140 ohm attenuator on the same board in front of the prefilter.
Then the 5dB pad should be temporarily taken out of function on main rf-board.

Installing prefilter on top of KNB2 :
-Input of KNB2 seems to have the necessary attenuation.
-Output can be taken from top of C6 and L2 and noise gate also
 connected here through C8. This saves the step-up coil in xtal filter.
-Original 5dB pad off in main rf-board
-For removal, this KNB2 + prefilter unit must be replaced with a
 simple board with that 5dB pad.

Benny      OH9NB
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[Stewart Baker]

Hi Benny,

Thanks for your comments on the Pre-filter.
I have put my comments in between yours.

73
Stewart G3RXQ

 On Sat, 19 Mar 2005 11:53:33 +0200, Benny Aumala wrote:

>
>
> Installing prefilter IN STEAD of KNB2 gives the possibility to
> place 5dB 140 ohm attenuator on the same board in front of the prefilter.
>
>> The Pre-filter has some loss, but if absolute MDS is not a criteria, then
>> your idea may prevent some of the IMD performance being lost.
>>
> Then the 5dB pad should be temporarily taken out of function on main rf-board.
>
>> That needs to happen anyway when fitting the noise blanker.

> Installing prefilter on top of KNB2 :

>> Might cause a height problem for those fitting KPA100 in the K2 case.

> -Input of KNB2 seems to have the necessary attenuation.
> -Output can be taken from top of C6 and L2 and noise gate also
> connected here through C8. This saves the step-up coil in xtal filter.
>
>> Yes, that might work. The terminations for the mixer are rather different
>> between 5dB pad and noise blanker input. I don't know if Elecraft have
>> published IMD figures with/without the noise blanker.

> -Original 5dB pad off in main rf-board
> -For removal, this KNB2 + prefilter unit must be replaced with a
> simple board with that 5dB pad.
>
>> KE1L has a website with 'Un-modules' which allow various K2 modules
>> to be removed while retaining signal paths etc. There is one for the KNB2
>> which would be ideal for those not having purchased a noise blanker.
>> http://w5big.home.comcast.net/K2_Un-modules.htm
>
> Benny      OH9NB
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[Stewart Baker]

Hi Benny,

Thank you for your comments.
Yes, I think that a diplexer in front of the pre-filter and and a 5dB amplifier
after it would work.
I do not have the necessary test equipment here to show the before and after
results, so I will probably wait until someone like yourself (or Elecraft) makes
those modifications.
To date I have found no IMD problems caused by the Pre-filter, but there again I
am not resident in a country where we are 'legally' allowed to run kW's.

73
Stewart G3RXQ


 On Mon, 21 Mar 2005 20:27:44 +0200, Benny Aumala wrote:

>
>
> Dear OM Steward,
> how about using a diplexer in front of prefilter.
> If impedance there is 150 ohms, try 4,86uH and 213pF combination.
> If not, I would like to simulate other impedances.
> Between prefilter and 2N5109 amplifier this could save the 5dB attenuator
> loss for
> prefilter.
> Looking components, the hi-Q coils are not necessary, but care must be taken
> to
> choose a ferrite without IMD. I am looking for 15mm tubes of Siferrit U17.
> That is a component very hard to find now. But 30 years ago I made the best
> minimum-loss
> bandfilters with them.

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