Friday, December 30, 2011

Look Mum no Hands, or - FSK without using an RS-232 port

In a recent QST (August 2011) I spotted an article showing a way to implement FSK keying using only the audio output from your PC.
The article was written by Doug Hall K4DSP who can take full credit for this brilliant design!

You can find more information on this project at -

That may sound easy but up to now the only solutions that provides true FSK without using an RS232 port have required conversion from the USB port which is quite complex in order to achieve a stable 45.45 baud rate.

The FSKit from Doug converts transmit AFSK tones from your PC into FSK keying and also PTT. In addition it provides isolation between the PC and transceiver.

I wanted to build a portable station for field use using my IC-706 and a lap top PC (No RS232 ports) and I didn't want to use AFSK as -

1 - You have to keep an eye on the drive levels to ensure a clean signal
2 - The displayed frequency is incorrect 
3 - You need a work around to use the narrow RTTY filter.

The article from Doug was easy to follow; at the heart of the project was an AMTEL AVR micro-controller (ATTiny 45). The binary is available online but in reality it was easier to email Doug and purchase the micro-controller and at the same time a PCB...

The circuit was fairly easy to assemble although obtaining the op-amp and transistors required a little searching around.

I decided to house the FSKit in a Maplin enclosure and add a few bells and whistles..

1 - Powered from the IC-706 8v line
2 - Incorporate CI-V control for logging and spotting
3 - Some indicator lamps (Power - PTT - FSK - CI-V)
4 - Audio out gain control for transmit of PSK and other modes
5 - Audio in gain control  and isolation
6 - Audio transmit side tone  

I tried using an L78L05 to run the board but it was a little fussy a 3.3 volt regulator worked perfectly. The board uses very low current so the low power version is best.
I have a simple CI-V and tried an isolation circuit for this, unfortunately I couldn't get this to fly so just routed the CI-V through the terminal unit with some in-line ferrites to provide a little protection. 

All the indicators were LEDs - 
  • Power just needed a 1K resistor in series
  • PTT and FSK lines run to LEDs with a 2k and 1k resistor in series to the 3.3v line
  • CIV was a high brightness LED across the CI-V line with a 2k resistor in series (Flashes to showing active data present).
Doug provides an isolated audio output on his board. I fed this to a potentiometer and then on to the rig.

Audio receive was from the rig to another potentiometer and then an isolating transformer to the PC audio input - I use N1MM with MMTTY RTTY which uses audio in so just needed some isolation.
Last was a simple moving coil microphone insert that was across the audio input line. This is useful to check the tones sound okay on transmit and gives the operator confidence that something is happening (I find complete silence on transmit a little disconcerting.
As you can see the whole project went into an aluminum enclosure. I could have squeezed it into something smaller but the box I used made for an easy life.

The cable from the rig to the terminal terminates in a 9way D-type so I can make up other cables for other rigs in future. I used an old VGA cable as it has ferrites pre-installed and several screened conductors which are useful to keep the audio lines quiet.

I haven't labeled the terminal unit yet but that's the only thing left to do.

Its not a microHam interface but its inexpensive & does the job for /P operation.

I think this probably the best and least expensive way of implementing true FSK. I have made a few contacts from home but I'm looking forward to summer when I can try it portable.


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