[aprssig] APRS Bandwidth

Stephen H. Smith wa8lmf2 at aol.com
Tue Jun 23 23:04:00 CDT 2009

Steve Noskowicz wrote:
> I never considered that this can be analyzed either as FM or AM.
> What Steve S. has is completely correct except for the fact that the modulation won't be square waves limited by the transceiver.  At least it shouldn't be or rather needn't be.  Only the first set if sidebands should be needed (and are in the calculation).  
> The SSB transmitter would allow more (>500 Hz.) if a real square wave is used.
> Thinking about it, I don't think FSK teletype does any band limiting (at least it didn't when I was doing it), but just switches between the two frequencies, therefore taking more BW than ncessary.
> I wonder what modern transceivers do that have FSK.

For AFSK,  the less-than-infinite bandwidth the TX modulator &b IF 
filters limit the bandwidth, especially if you use a 500-800 Hz CW 
filter in the TX path.  Further, the better hardware and software AFSK 
generators have limited transition rates between mark and space freqs, 
i.e. not instantaneous.jumps.   Usually the transition from one tone 
freq to the other is made precisely as the zero-crossing point of the 
wave form.

The old time direct FSK RTTY setups from the 50s-60s actually 
diode-switched a small capacitor across the analog mechanical VFO L/C 
circuit to shift the VFO freq slightly. Later, this was done with a 
varactor diode across the VFO.   Since the era of synthesized rigs, the 
shift has been achieved by commanding the synthesizer to two freqs 200 
Hz apart.

For direct FSK, most rigs have some sort of slew-rate limiting in the 
"data input" channel, sometimes controlled by a menu to select the 
appropriate time constant for either 45, 100 or 300 baud to "round off" 
the square wave.   This is essentially the same kind of   rise-time 
limiting/waveform "softening"   used for CW "key click" filtering.

In either case (AFSK with a good tone generator, or rise-time-limited 
FSK), the tone doesn't jump instantanously and discontinuously from F1 
to F2, but rather very rapidly sweeps from one to the other.

For PLL-synthesized rigs, the slew rate limitation of the synthesizer 
can inherently do this since the PLL osc can't jump instantaneously 
between two freqs, but rather very rapidly "slides" from one freq to the 



Stephen H. Smith    wa8lmf (at) aol.com
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