[Elecraft] OT: Softrock for K3 IF

[Paul Christensen]

A final note:  The author of that circuit created a pseudo low-Z output 
through the use of a 20 dB resistive pad.  The 1K output Z of the final FET 
is feeding the low Z pad.  Other than creating a lower final source Z at the 
expense of further circuit loss, I cannot imagine why he was satisfied with 
a total loss that may approach 32 dB into the Softrock -- except perhaps to 
build in more reverse isolation through the use of the 20 dB pad.

Considering the parts count of his circuit, there's an even stronger case 
for building a low-loss, low noise buffer amp using a high-grade isolation 
amp with a low source Z.

Paul, W9AC



----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: <elecraft at mailman.qth.net>
Sent: Friday, January 08, 2010 7:25 AM
Subject: Re: [Elecraft] OT: Softrock for K3 IF


>> I found an article here
>> http://www.kingsqueak.org/stuff/softrock_IF_tap.pdf
>> that shows a buffer amp that can be built from parts most people probably
>> already have...
>
> His buffer consists of a pair of cascaded FET source-followers.  It would 
> be
> a good idea to measure reverse isolation of the circuit for use with
> Softrock and if necessary, use a shielded enclosure if circuit layout 
> cannot
> be optimized.   Also, typical A (gain) value for a source-follower is less
> than 1.0 and is often in the area of 0.5, depending on the 
> transconductance
> (gm) of the FET and the source resistance.  The overall gain is set by the
> product of the two source-follower A values:
>
> A = A1 * A2
>
> So, it's possible to easily end up with an A value circuit loss of ~ 0.25
> which represents a combined voltage loss of 12 dB which may, or may not be 
> a
> problem with the desired display resolution through Softrock.
>
> The source resistor in a source-follower is primarily responsible for
> setting the output Z and is generally chosen to be close to the reciprocal
> of the gm value.  In his circuit, that's about 1K.  I would prefer to see 
> a
> much lower output Z (i.e., 50 ohms), especially if high-capacitance cables
> are used for RF sampling.  It would be better to find another way to 
> achieve
> a high degree of isolation and low driving Z without that much loss (e.g.,
> the Z10000B amplifier or similar) and build it into the K3.
>
> Paul, W9AC
>
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