Two views of the original Auto-Band-Switching-Unit. I have to admit that it is somewhat over-engineered, in that rather than asking ICOM for the band switching voltage tolerances, I worked them out and designed a ‘ladder’ comparator network to perform the selection. I have since discovered a simpler albeit similar design. I have also come across an even more ingenious design using an LED driver array to perform the same task. Subsequently, I have also made a universal version based on a PIC design by Peter Rhodes, G3XJP.
Inclusion of the 8dB attenuator allows more control over the level of drive to the amplifier. It might also be said that running the exciter’s output at a higher level, improves the linearity. Although intended for a max input of 100W, giving about 15W into the amplifier, the attenuator has been designed deliberately to handle up to 150W.
The Completed 300W Linear Amplifier Installed, July 2001
A point probably worth noting here is the fact that an increasing number of transceivers will actually deliver full output (typically 100W) for several milliseconds before ‘throttling back’ to the power level set by the RF Power Level control. I discovered this to my annoyance recently when a good friend loaned me his FT920 and Ameritron 811A amplifier. Even with the power turned right back on the 920, the 811A still managed to deliver a few milliseconds of power which was sufficient to take out the forward detecting diodes in my Peak Power meter! Although, my trusty IC725 doesn’t exhibit this characteristic, I still keep the attenuator in line. My recently acquired IC706 MK2 does produce a full power spike on transmit . . . . Hence, the 725 is connected to the amplifier, not the 706.
Click on the play button below to hear what the signal sounds like in North America