[RTTY] Question

[Kok Chen]

For 170 Hz shift RTTY, a good AFSK signal (e.g., using fldigi and 2Tone in AFSK today) is not much more than around 280 Hz wide all the way down to -80 dBc.  

If RTTY signals are made much narrower than 280 Hz, reception will suffer.  At the same time, making a transmitted RTTY signal wider than 280 Hz does not improve reception perceptively when received by a modern software modem that uses a Raised Cosine Filter or variants of it (both fldigi and 2Tone use a variant of the Raised Cosine filter).

You can see the analysis here:

http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html

In practice, due to transmitter PA's 3rd order IMD, an RTTY signal will start widening at around -40 dBc to -50 dBc level, instead of staying narrow all the way down to -80 dBc.  

On the other hand, even the best FSK signal will be much wider than what I described above, as can be seen in the second figure here:

http://www.w7ay.net/site/Technical/RTTY%20Sidebands/sidebands.html

As can be seen, a perfect FSK signal is already about 800 Hz wide at -35 dB, with keyclicks extending much wider.  

The exception is the Elecraft K3 which applies filtering even when transmitting FSK.  The width of a K3's FSK signal is more like 350 Hz (so somewhere in between a good AFSK signal and a good FSK signal).  

(The K3 starts with a filtered analytic (I+Q) baseband signal, much like AFSK's baseband signal.  That is why it could be filtered.  However, the shifting from baseband to RF is done numerically in DSP instead of using analog product modulators in more traditional superhet transceivers with AFSK.)

73
Chen, W7AY