> 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
