[Elecraft] K3 Buffer Mod

[Larry Phipps]

I have addressed this on my reflector, but for the benefit of those on 
the Elecraft reflector...

95% of LP-PAN users will be happy with the NF and sensitivity without 
any modifications to the K3. On most bands, the resulting NF caused by 
the loss in the K3 buffer will be masked by atmospheric noise. On the 
higher bands, keeping the K3 preamp on will help a lot. This is usually 
OK at most locations because the K3 preamp is quite strong.

The reason I came up with the mod is because a couple users were trying 
to use CW Skimmer for weak signal detection with LP-PAN feeding it. The 
mod improves the NF of the IF output by up to 10dB (K3 preamp off, 
somewhat less with the K3 preamp ON). Even though the NF of LP-PAN 
itself is quite good, it can't do anything to improve the system NF 
since it's downstream of the loss.

My advice is to try it without modifying the K3 first. You will most 
likely be quite happy with the result. Be sure to read all the 
instructions before attempting the mod. It is comparable in complexity 
to the HAGC mod that Elecraft offers... but still requires reworking SMT 
parts.

73,
Larry N8LP



Jack Smith wrote:
> Larry's LP-PAN uses my Z10000 amplifier design in its front end, with 
> a couple of modifications to provide variable gain.
>
> The AD8007 amplifier chip in both my Z10000 and the LP-PAN is a decent 
> amplifier with a respectable noise figure.
>
> If the K3's IF sample circuit had been built with closer to 0 dB 
> transfer gain, then the composite noise figure would be closer to the 
> K3's noise figure.
>
> To answer the two questions directly (I don't have an LP-PAN, so Larry 
> might wish to modify my answers):
>
> 1. Is the mod necessary with the LP-PAN, or is it only needed with other
> panadapters? That is, is the noise figure of the LP-PAN's internal 
> amplifier
> sufficient, or is the mod to the K3 required to lift weak signals 
> above the
> noise floor of the LP-PAN?
>
> I believe you will see a small improvement in composite K3-LP-PAN 
> noise figure if the changes are made to the K3's IF sample circuitry. 
> However, I doubt that under most band conditions you will observe a 
> difference as there is sufficient external noise to mask the improvement.
>
> 2. Would the external buffer amplifier solution be a viable 
> alternative to
> the mod?
>
> Yes, I believe it is if you are using something other than the LP-PAN 
> as the panadapter. Panadapters are not normally designed to be as 
> sensitive as a receiver. There are many reasons for that that I won't 
> bother to go over now. The consequence is that designers of receivers 
> that provide a pan scope RF sample have the levels set so that there 
> is at least a net 0 gain, and positive gains of 10 dB or more are not 
> uncommon. I'm speaking mostly of commercial and military grade 
> equipment, but my Kenwood TS-940 has reasonable transfer gain as well.
>
>
> Jack K8ZOA
> www.cliftonlaboratories.com
>
>
> Richard Ferch wrote:
>> I am scratching my head wondering whether any of this applies to the 
>> LP-PAN,
>> or whether it is only relevant to other panadaptors such as the 
>> Softrock and
>> the Z90.
>>
>> K8ZOA's web page (the same one everyone is quoting) says:
>>
>> "I looked at two potential panadapters; a Softrock and my Z90.
>> (Telepostinc's LP-PAN has a built-in amplifier to overcome the 
>> transfer gain
>> problem.)"
>>
>> and in the recommendations near the bottom of the page, refering to 
>> the mod,
>>
>>
>> "These steps are not necessary with an LP-PAN panadapter as it has a
>> built-in isolation amplifier."
>>
>> That same web page also suggests that an alternative to the mod would 
>> be to
>> use an external buffer amplifier.
>>
>> So, I have two questions:
>>
>> 1. Is the mod necessary with the LP-PAN, or is it only needed with other
>> panadapters? That is, is the noise figure of the LP-PAN's internal 
>> amplifier
>> sufficient, or is the mod to the K3 required to lift weak signals 
>> above the
>> noise floor of the LP-PAN?
>>
>> 2. Would the external buffer amplifier solution be a viable 
>> alternative to
>> the mod?
>>
>> 73,
>> Rich VE3KI
>>
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>