K2 Pre Filter

[Brian Broggie]

We are getting a lot of requests for the parts to make G3RXQ's K2 Pre
Filter.  To facilitate this, we have made a build list with all the
parts that we can supply for a nominal charge.  The list is based on the
description and schematic found on G3RXQ's website.  Included are the
crystals, caps, toroids, magnet wire, a pc board and an 8 pin
connector.  The total cost for the parts is $29.00, plus shipping (and
CA tax to orders inside California).  Please order by part "K2PREFILTER".

While we encourage hams like you to explore and try new circuits and
ideas with our products, please be aware that any modifications to our
products come at a tradeoff to the original design. Here are some
guidelines from the owners of Elecraft and designers of the K2, Wayne
Burdick and Eric Swartz...

1. Overall receiver gain and noise figure may degrade because the added
crystal filter will have some loss of around 5 dB.

2. K2s with older crystals in their RF board and KSB2 filters may not
have the exact same center frequency as the ones we're using (and
shipping) now. The effect would be that the new filter would overlap the
old, reducing the effective bandwidth on receive. The only way to ensure
that this doesn't happen on older K2s would be to replace all 14
crystals (RF board and KSB2).  A new set of 14 matched crystals can be
ordered from Elecraft as part "K2KSB2XTLS".

3. The crystal filter does not have the pad in front of it. The Pad
normally is either part of the KNB2 or the PCB if no KNB2 is installed.
(The pad gives the 2N5109 amp stage before it a good return loss match.)
It will degrade K2 IMD Dynamic range by over 10 dB (IMDDR3 and IP3)
without this pad. Reinstalling the PCB pad will not help as it would end
up between the two SSB xtal filters.

4. Finally, you should be aware that the KDSP2 does the same thing for
ultimate rejection, since the regular filter more than filters enough to
keep off freq signals from pumping the K2s AGC, allowing the DSP to
clean up any residual blow-by.

Thanks, Wayne and Eric!

So, evaluate the data on G3RXQ's website and what we are presenting in
this email and order accordingly.

73,
Brian Broggie - W6FVI
Elecraft Parts
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[Stewart Baker]

Hi Brian,

Thanks for your comprehensive post on the implementation of my Pre-filter.
I have inserted my comments between yours.

73
Stewart G3RXQ

 On Thu, 17 Mar 2005 09:50:24 -0800, Brian Broggie wrote:

> We are getting a lot of requests for the parts to make G3RXQ's K2 Pre
>
> Filter.  To facilitate this, we have made a build list with all the
> parts that we can supply for a nominal charge.  The list is based on the
> description and schematic found on G3RXQ's website.  Included are the
> crystals, caps, toroids, magnet wire, a pc board and an 8 pin
> connector.  The total cost for the parts is $29.00, plus shipping (and
> CA tax to orders inside California).  Please order by part "K2PREFILTER".
>
> While we encourage hams like you to explore and try new circuits and
> ideas with our products, please be aware that any modifications to our
> products come at a tradeoff to the original design. Here are some
> guidelines from the owners of Elecraft and designers of the K2, Wayne
> Burdick and Eric Swartz...
>
> 1. Overall receiver gain and noise figure may degrade because the added
> crystal filter will have some loss of around 5 dB.
>>
>> If the Pre-filter is used in place of the noise blanker I have observed
>> negligible additional signal path loss.
>> As I said in my article one of the benefits of this mod is that it can
>> be easily installed or removed. You then have the choice to listen to weak
>> signals on a quiet band (with the noise blanker removing pulse interference)
>> or with the Pre-filter in to work weak signals next to S9+40dB contest
>> stations.
>
> 2. K2s with older crystals in their RF board and KSB2 filters may not
> have the exact same center frequency as the ones we're using (and
> shipping) now. The effect would be that the new filter would overlap the
> old, reducing the effective bandwidth on receive. The only way to ensure
> that this doesn't happen on older K2s would be to replace all 14
> crystals (RF board and KSB2).  A new set of 14 matched crystals can be
> ordered from Elecraft as part "K2KSB2XTLS".
>>
>> Yes, it is very important that new crystals should not be put in front of a
>> KSB2 filter that contains old ones. Anyway, it is a good time to upgrade
>> your crystals to get the best performance from your KSB2.
>>
>
> 3. The crystal filter does not have the pad in front of it. The Pad
> normally is either part of the KNB2 or the PCB if no KNB2 is installed.
> (The pad gives the 2N5109 amp stage before it a good return loss match.)
> It will degrade K2 IMD Dynamic range by over 10 dB (IMDDR3 and IP3)
> without this pad. Reinstalling the PCB pad will not help as it would end
> up between the two SSB xtal filters.
>
>> This may be true. I have no test equipment that is up to the task of
>> measuring the K2 IMD (which is already very good).
>> In a recent contest I noticed that the benefits of the Pre-filter
>> outweighed the slight loss of IMD dynamic range.

> 4. Finally, you should be aware that the KDSP2 does the same thing for
> ultimate rejection, since the regular filter more than filters enough to
> keep off freq signals from pumping the K2s AGC, allowing the DSP to
> clean up any residual blow-by.
>
>> I'm not so sure about about this statement. The KDSP2 processes audio after
>> the AGC loop, so although the received signals may sound cleaner it has no
>> effect that I can see on AGC pumping caused by either a lack of filter shape
>>factor or blow-by.
>>

> Thanks, Wayne and Eric!

>> Seconded - nice to get feedback...
>
> So, evaluate the data on G3RXQ's website and what we are presenting in
> this email and order accordingly.
>
> 73,
> Brian Broggie - W6FVI
> Elecraft Parts
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

_______________________________________________
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