[GreenKeys] On Freq or Off Freq???

[Ralph Mowery]

----- Original Message ----- 
From: "John Becker" <w0jab at big-river.net>
To: <greenkeys at mailman.qth.net>
Sent: Wednesday, March 08, 2006 10:27 AM
Subject: Re: [GreenKeys] On Freq or Off Freq???


> If you are using AFSK then you want a dial readout
> of 3,682,125....
>
> That will put your mark tone right on 3,680.
>
> The frequency you call out should be the one your
> mark tone falls onto. Otherwise say dial freq.
>
> Hope this helps.
>
> John, W0JAB
>
>
> At 09:03 AM 3/8/06, you wrote:
> >OK, got the RTTY going last nite.  Had a sked with Jack on 3.680.  Now,
> >what's the "correct"  procedure for this:  Do you put Mark on 3.680 or
> >your carrier?  I used LSB and injected the 2125 for Mark.
> >
> >Jack was looking for me on a  different freq.  How do most folks do
> >this?  I need to get my procedure correct.
> >
> >Many thanks to all,
> >
> >Larry
> >K2JIA
> >www.antiquetelephone.com

John, this is not really directed at you, but to clear up a few things.

First, AFSK is not permitted on most of the low band frequencies.  When you
feed a single audio tone into a SSB transceiver you get out a single
frequency.  If it is then shifted such as from the mark to the space tone,
you get another single tone either lower or higher in frequency depending on
if you are using lower sideband (as usually done on the ham bands) or upper
sideband.   This is just plain FSK and not AFSK that you get if you feed
audio into an AM or FM transmitter.  Often a difficult thing for many to
understand.

It has to be assumed that you are using the more or less standard 2125 hz
tone for a mark.  If you use another frequency, then you have to recalculate
the frequency.  Not everyone used the 2125 tone plus the needed tone for the
desired shift.  Other mark tones can be used.  I forgot what they are off
the top of my head, but there used to be what was called the low tones.
This is because some transmitters when fed audio would not pass the higher
frequency as well as it did the lower frequency and the amplitude of the
tones were not equal.  This was especially bad when the 850 hz shift was
being used.  Almost any two tones that are 170 hz apart (assuming 170 hz
shift) can be fed into a ssb transmitter.  The main limitations are they
fall in the passband of the filters.  That means from about 300 to 2500 hz.
The lower frequencies are not usually used because the second harmonic is
still in the pass band.  They should be around 1500 hz or more to prevent
this.  Anything much higher than 2500 hz will be greatly reduced.

There are atleast 3 ways the rtty frequencies have been called.  One is the
actual RF frequency of the transmitter.  Another is the dial frequency of
the receiver.  This is probalby used most often if you are using the
"standard 2125" audio tone for the mark.  Another is to split the differance
in the mark and space frequency, probably the least used.