[RTTY] Distortion

[Kok Chen]

On Aug 24, 2004, at 9:35 AM, W0YR at aol.com wrote:

> So, an S9 FSK signal has no unwanted garbage (aside from key clicks).

Unless your rig happens to be a "keyed FSK" rig, such as the FT-990, 
FT-1000D, FT-1000MP and many more.  The keyed FSK signals go through 
the same balanced modulator path as AFSK does.  So the same linearity 
rules apply to the stages following the balanced modulator as they do 
to AFSK.

> An S9 FSK signal STILL can be heard 2.125 kHz away at S2 to S3 !!  
> That's
> louder than a lot of QRP signals.

Here is where AFSK can do better than FSK...

FSK sidebands are pretty much controlled by the the rise and falls 
times of the modulating ("keying") signal.  With AFSK, you have at 
least two controls.

You can use continuous phase modulation to practically reduce the first 
order discontinuity to very small amounts.

Secondly, you can apply a steep bandpass filter around the AFSK signal 
(I have no idea how many software modems actually do that, but I know 
of at least one that does) to reduce the modulation sidebands -- you 
can get modulating sidebands that are 300 Hz from the center frequency 
(600 Hz total bandwidth) to fall off to nothing.

This AFSK "roofing"  does not practically degrade the printability of 
the signal, as observed by people being able to print RTTY perfectly 
fine using 500 Hz and even 250 Hz receive filters.  But it helps reduce 
your modulation sidebands (and allow you to apply that just little bit 
more of the transmit power to where it counts -- but don't tell any of 
the "QRO forever" guys :-).

The bottom line of course is that the linearity of stages in the rig is 
very important when you are using AFSK whether or not you apply any of 
the AFSK cleanup countermeasures.

BTW, the FCC has recently tightened the rules on spurii on commercial 
Amateur equipment. 40 dB now.

73
Chen, W7AY

P.S.  why don't we get on the air some weekend to do some signal 
comparison stuff?  Perhaps a roundtable where people can play with ALC 
levels and see what others report.  Everyone can benefit from hearing 
what well tuned and overdriven signals sound like.  Especially when 
they discover that overdriven signals are actually harder to copy!