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" <w...@arrl.net> To: <elecraft@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 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
