[Elecraft] K2 RTTY

[Bill Coleman]

On Sep 5, 2008, at 1:30 AM, Logan Zintsmaster wrote:

> I hope the group will pardon me while I engage in a bit of RTTY  
> heresy.....
>
> When you use a PC sound card for decoding RTTY, the center frequency  
> is no
> longer important, just the tone spacings.  With MTTY, I can pick any  
> center
> frequency I want to use (typically 1000 Hz so I stay away from the  
> roll-off)
> and tune until I see it in the MTTY spectrum window.  I make any fine
> adjustments with the cursor to pick the signal I want if there are  
> multiple
> signals shown.  If I want a bandpass filter, I use the bandpass  
> function in
> MTTY.  MTTY uses the same audio frequency for encode as it used for  
> decode
> so the signal is inherently locked to the received signal.  It even  
> has an
> AFC function to track any drift.

All this is true for receiving. You can pick just about any center  
frequency you want.

One problem with using the waterfall display and selecting from a wide  
bandpass is you don't get the level of unwanted signal rejection that  
is possible when using narrow crystal or DSP filtering. If there is a  
strong signal adjacent to your desired signal, you may not be able to  
decode properly.

The K2 is equipped with a variable-bandpass filter that works very  
well on RTTY as it does on CW. In order to use this effectively,  
you've got to select a particular center frequency and adjust the  
filters around that frequency.

This does require one to "tune in" a RTTY signal, but that is a pretty  
easy process using the crossed ellipse display.

> Using just the SSB filter for receive and transmit avoids the  
> problem of
> center frequency tracking between the SSB filter that is always used  
> on
> transmit and whatever receive filter has been selected.

Except that you often don't want to use the SSB filter for receive. I  
generally use the variable-bandwidth filter set for 1.0 kHz. I also  
have settings for 0.5 and 0.3 khz. I only use the SSB filter to  
receive RTTY on a very quiet band.

One other consideration -- when you are transmitting, you want only  
the AFSK signal to fit in the passband. This is the reason traditional  
RTTY was so high in the audio bandwidth. If you send tones of about  
1000 Hz or so, then their second and possibly third harmonics may be  
transmitted as well. To avoid this, pick frequencies higher in the  
passband. With the limited BFO ranges on some K2s, it may be hard to  
center the variable-bandpass filter. I chose 1500 Hz as the center  
frequency for my RTTY tones for these reasons -- it places the second  
harmonic outside the passband of the SSB filter.

I also chose a whole number as the center frequency so I can bring the  
DSP passband filters into play.

> It also has the advantage of allowing you to hear what is happening  
> across
> the audio bandwidth so you know whether to go higher or lower for  
> the next
> signal.  Very useful in a contest.

In a contest, I don't want the radio's AGC pumping against nearby  
signals -- I want those signals rejected -- another reason to use a  
narrow filter.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901