Re: K3 keying an amplifier

Posted by P.B. Christensen on Feb 16, 2011; 9:48pm
URL: http://elecraft.85.s1.nabble.com/K3-keying-an-amplifier-tp6033183p6033677.html

>I don't think the diode isolation will work in this case.
>
> Scott   K9MA

If the Icom used an amp key line from an NPN transistor with an open
collector and a high value (e.g., 10K) pull-up resistor to a positive supply
rail, then all one needs is a steering diode added to the Icom key line
configured with the cathode to the junction of the Icom's collector and
pull-up resistor.  The diode's anode points to the Alpha 9500.  The K3 with
its MOSFET output simply parallels the 9500 key line.  It doesn't care.

If the Icom is unpowered or powered, a Schottky steering diode will isolate
the pull-up resistor while still allowing the driving transistor's junction
to pull low enough to ground.  An unbiased or unpowered NPN open-collector
switching transistor should not pull to ground potential if the rig is
powered down.

The only remaining obstacle is that when the Icom is powered down, the Alpha
9500 key line would see 10K resistance through the pull-up resistor to the
power supply positive rail.  Powered down, that rail may be close to ground
potential.  So, whether a steering diode alone will work is really a matter
of: (1) the value of the pull-up resistor; and (2) whether the Alpha 9500's
key line circuitry is sensitive to high-impedance switching.  No problem if
the Alpha uses a photo-transistor on the input key line, for example.  But
if say a pair of both pull-up and pull-down resistors are used ahead of a
CMOS gate, then the value of the resistors need to be known.

If a steering diode still doesn't work, then a separate switching transistor
can be used, using an open-collector without a pull-up resistor -- or a
MOSFET without a pull up on the drain.

Paul, W9AC

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