[Lowfer] The forgotten Lowfer

[Lyle Koehler]

Speaking of exciters, the ZL1BPU DDS exciter is hard to beat for price and
versatility. You can sometimes find three of them at the watering hole: UWL,
NC and LEK.

Early last summer Dex told me about this deal on an Atmel AT90S2313
development board for under 20 bucks:
http://www.futurlec.com/ATDevBoard.shtml
That's the same microprocessor that is used in the DDS exciter. I finally
ordered one of the boards and received it yesterday. It comes with a
parallel-port programming cable and is complete and ready to go -- except
that the documentation sucks, and it took me a long time to figure out how
to make the programming cable work. The board will function as a constant
carrier generator without any modification, although I replaced the 8 MHz
crystal on the board with a 12 MHz crystal from my junkbox. A 12.8 MHz TCXO
would be even better.

The signal can be taken from the most significant bit of the
microprocessor's output if you don't mind the jitter in the square wave,
although I recommend adding the R/2R ladder network as shown in Murray's
article at
http://www.qsl.net/zl1bpu/micro/EXCITER/Index.htm
An alternative to the R/2R network would be a D/A converter, and I plan to
try that soon. Some kind of output driver like the TDA7052A audio amplifier
in Murray's design is probably necessary to drive the final. In order to
take advantage of the multimode keyer function, you also need to supply a
bit clock, which can be a simple R-C oscillator. Futurlec has an inexpensive
RS-232 serial cable that probably works with the development board, but I
made my own cable (didn't find it until after I had placed the order). But I
usually do all the programming via the parallel port anyway.

More to follow...

Lyle, K0LR