[GreenKeys] Dovetron Advice

[Ralph Irish]

Jack, Steve and group

The later DOVETRON AFSK arrangement uses 'zero crossing' shift.  That is, a transition from Mark to Space,
or Space to Mark always takes place at a 'zero crossing' of the sine wave.  This prevents any 'transition
glitch' from getting into the audio chain during transmit.  If a sine wave were cut off at or near the
positive or negative peak of the sine wave, it could create a spike that would get into the audio chain
and end up possibly causing problems a few KHz away from the transmit freq.

The 'zero crossing' might rob a pulse of a few hundred micro-seconds of 'air time', but it is not noted
during receive at the other location.  DOVETRON didn't invent this but they used it on almost all, if not
all of their TUs at some point.  I think they used some kind of XR (brand) chip to generate the tones.

The most important setting is either the audio gain of the mike/audio path of the transmitter or the
audio out from the TU, if available, or seek some combo that gives good indications monitored from a mile
away, if possible.  Someone with a panadaptor or some sort if band monitor would be good.

- - - -

I used AFSK decades ago on my DRAKE T4X, wide shift.  I got lots of reports that the other station 
could not hear my SPACE tone, but heard the MARK going on and off.  That finally convinced me to use
the low tone pair, 1225 and 2125 for RTTY.  That was until narrow shift got a good toe-hold on the
bands.  Then I used 2125 and 2295, like the rest of the world.  The SPACE tone was down maybe a db
but the signals got good reports, and I never heard any contrary reports from anyone else on the band.

- - - -

There should be time enough to set things up and ask some nearby ham to listen critically to your
signal and also to tune up and down the band from it 5 to 10 KHz and listen for clicks or anything
that might be from your signal.  Coordinating on the phone would allow you to turn the test signal
on and off a few times, should the monitoring station hear 'something'.

"73 and good luck in the contest, old man!"

Ralph - W8ROI

- - - - - - - -

On Dec 4, 2017, at 10:43 AM, Jack wrote:

> Steve,
>  
> If the Dovetron has the AFSK option (sometimes it’s not there) make sure the
> tones are set for the 2125/2295 170 HZ shift. You can then plug the tones
> into your SSB rig’s mic input and set the carrier level (TX power output) to around
> 50% of rated CW output.  The tones generate the FSK signal.
> A good way to control transmit/receive function is to switch the tones on and
> off into the rig (use VOX in the transmitter).
>  
> I use the direct FSK feature in my Kenwood TS870 and key it with an opto-coupler
> in the TTY loop. That’s the best way for good FSK and it doesn’t need the
> Dovetron to do anything. Using AFSK tones into an SSB rig could cause
> splatter and possibly make the rig run hot. With this setup, I just use
> the Dovetron’s external “standby/receive” feature to put the TU in
> standby when transmitting. A suitable switch (DPDT) can put the TU in standby and
> turn on the transmitter.
>  
> If you don’t put the TU in standby, it could erratically trigger the transmitted FSK.
>  
> Jack K2TTY
>  
>  
>  
>  
> From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Steve Garrison
> Sent: Monday, December 4, 2017 9:56 AM
> To: greenkeys at mailman.qth.net
> Subject: [GreenKeys] Dovetron Advice
>  
> And How-To on using a Dovetron MPC-1000R with my M28 to key my radio for the upcoming ARRL TTY Round-up.  All advice appreciated.  While I have used the Dovetron on multiple occasions to copy off of the air, I have never transmitted RTTY, so need lots of advice.  Hope I haven’t waited to long to jump into the deep end of the pool with the contest just a month away.
>  
> Steve G./N4TTY
>  
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