[RTTY] AFSK to FSK converter

[Kok Chen]

On Sep 16, 2012, at 11:43 AM, Bill, W6WRT wrote:

> Can anyone recommend an AFSK to FSK converter, either kit or assembled?

1) write to K4DSP directly to ask for a programmed Atmel controller if you don't have a way to program an AVR chip.  I'll bet that if you beg hard enough, Doug has a PC board squirreled away somewhere, or he might be willing to part with the CAD drawings so you can fab your own board.

2) build a simple AFSK demodulator using an Exar XR-2211 chip (DigiKey.com has it for $2.64) to convert AFSK from the computer into FSK keying signal.

http://www.exar.com/common/content/productdetails.aspx?id=XR2211

Feed AFSK from a sound card output from the computer into the 2211's audio input and drive an open collector transistor with the Exar's FSK demodulator output.  Use the carrier detect output from the chip to key the PTT with a second open collector transistor.  Use optoisolators if you need galvanic isolation.

3) use the MicroHAM USB Interface III's p-FSK mode.  To do this you need to turn off one of the transmit tones (either keep Mark or Space off) since the Interface III does not have any selectivity (like the K4DSP circuit or the XR-2211 chip).  

Both fldigi and cocoaModem support this single tone on-off transmit keying mode (also supported by the MicroHAM digiKeyer II), and other software modem probably can too, since it is such an obvious thing to implement, I just don't know enough about Windows programs to know for sure.  The USB Interface III supports serial port PTT.

4) wait long enough and perhaps Tigertronics will wise up and include the Exar chip into the SignaLink USB .  

There are probably a couple of hundred FSK ops who will buy that at the drop of a pin; of course, there are ten times more AFSK ops, so that is probably what drives Tigertronics' business :-). 

That is not to say that you cannot modify an existing SignaLink USB yourself to create a self contained (including the sound card) USB-to-FSK solution. There is a SOIC version of the XR-2211, and that can probably be squeezed into the SignaLink USB enclosure.

(I have no idea what the rise and fall times of the tone detector in the USB Interface III is like and if it works well enough at 75 or 110 baud.  W4TV probably has measured data.  The K4DSP and Exar circuits should be OK since they make use of both Mark and Space tones to threshold the output.)

73
Chen, W7AY