Hi all,
Thanks to Jim M0BMU, Alan G3NYK and Mike G8YMB for comments. I agree that
incidental use of a diode for other than its intended purpose needs care
with verification of the desired incidental use. My evaluation was on a
small sample of the red cased BYX10 diodes. I reported my findings on BYX10
tests as an incidental PIN diode as the cost is well under a Euro and fairly
available, but there is a risk that a particular batch may not be so
co-operative with PIN controlled resistance or have a "cutoff" higher than
the LF band of interest.
I'm not sure of the cost or availability of a small signal PIN diode
designed for LF, and I don't need a transmit rated device for small signal
control. If there are other readers who have price and availability
information on PIN diodes designed for small signal LF use I would
appreciate a reply, even if it is private email.
The ALC circuit produced by Mike uses a bipolar transistor shunt, which is
high impedance going low when driven. There are other variants of the
circuit using a JFET shunt, but that is initial low impedance going high
with bias applied. In essence each is a current controlled resistor type of
application. My opinion is that a PIN diode (when used in the appropriate
frequency range) is the better device for smooth gain control and linearity.
I'm not sure of the finer points of intermodulation with any ALC circuit,
but in any case intermod in the PA final is likely to be the most demanding
part of a linear transmitter to control quality of the emission.
73, Bob ZL2CA
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