Transmitter Band Select Feature

[NZ0I]

While reviewing the schematic design for the dual-band transmitter one more time, it occurred to me that one more small feature might be very beneficial. We could have the processor detect which antenna(s) is(are) connected, and automatically switch to the appropriate band. 

We could still have the software allow the user to override the band selection if desired, but I can't think of any likely situations where that would ever be desirable. In automatic mode, when an antenna is connected to the 80M antenna jack, the transmitter will automatically output an 80M signal to the 80m jack; and likewise for 2M. If two different connector types are used for the two different bands, it will be impossible for someone to connect an antenna for the wrong band. 

We are already having the antenna jack/plug control power to the transmitter final amplifier power, so adding a detection circuit to determine which antenna is connected is trivial. It does require a couple additional processor binio pins though. So I think I will change the port expander (U903) to a 16-port device. I'll also make it a standard totem pole push-pull output device, instead of the quasi-bidirectional outputs used in the current design. The standard port-expander pins will be able to drive and sink 25mA of current, which should eliminate all doubt about them being able to drive the load switches and other near-the-threshold control lines.