[GreenKeys] FSK Operation

Tue, 30 Mar 2004 16:41:44 -0500

Tom,

To send FSK you either need a transmitter that is designed with an FSK input, or you must modify your transmitter so that you can "pull" the frequency by 170hz, (or whatever your preferred shift). Most rigs with FSK I'm familar with use a "dry contact" switch to shift them from mark to space. A polar relay or (more modern) opto-isolator inserted in your high voltage loop translates it into a dry contact you use to interface with your transmitter. You don't usually want to just connect the keyboard contacts straight to the transmitter. These contacts were designed to be in the 60ma HV loop and need the "punch" of the HV to make/break accurately. At very low voltages (depending on condition) they are not very good switches.

For transmitters that don't have FSK the method used is to drive it in SSB mode with a pure tone through the microphone or perhaps a phone patch input. The effect of transmitting a pure 2,125hz tone through a SSB transmitter in LSB is a CW carrier 2,125hz below the set frequency. A shift in the audio tone results in a likewise shift in the output frequency. So by shifting the input audio tones the transmitter's output frequency is also "shifted". Most demodulators have an AFSK option that can output two mark/space tones for this use. These tones follow the HV loop so when you type the AFSK output also "types". AFSK has some drawbacks, mainly due to the possibility of overdriving the audio input of your transmitter, (which obviously you cannot do with FSK). Be sure the demodulator AFSK output level is compatible with the input level of your transmitter, especially if you use the microphone input, which is necessarily very sensitive.

All AFSK RTTY on the HF bands use LSB. If you use USB you'll be "upside down". Some hams will give you a hard time about it, but that's what the reverse switch on your demodulator is for!

While much of what's on these websites is focused towards computer/soundcard use, you can learn a lot by visiting:
www.rttyinfo.net
www.aa5au.com/rtty.html
www.rttycontesting.com

I'm no expert, but hope that much helps.
Hope to see you on the air! (I need Alaska).
Dave, WW8S

----- Original Message -----
From: "Tom Elmore" <[email protected]>
Date: Tue, 30 Mar 2004 11:24:16 -0900
To: "greenkeys" <[email protected]>
Subject: [GreenKeys] FSK Operation

> Being fairly new to RTTY can someone please explain how you send using FSK. I understand the receiving concept and currently have a Frederick Demodulator set up for this. Now if I want to send FSK what is the procedure for this. I have a Hallicrafters transmitter that has FSK capability do I just need to build a switching relay to interface between the RTTY machine and the radio ? If I want to use my Collins KWM-2 does the T.U.
> interface with the microphone / audio input ? What frequency is actually being shifted is it the audio frequency or the carrier frequency. When using RTTY is there a standard as to what mode to use LSB / USB ?
> 
> Thank You 
> Tom Elmore KA1NVZ
> Anchorage  Alaska
> 

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