[RTTY] 300hz or 500hz IF filter?

[Kok Chen]

On Aug 23, 2013, at 3:34 PM, Bill Turner wrote:

> Not so. The "theoretical" bandwidth of ANY FM signal is infinite. The
> sidebands extend out forever. Check any book on FM theory, such as the ARRL
> Handbook. 

Yes so.  Sorry, but Kai is again correct.  

(Those who are not familiar with Dr. Siwiak can easily look up his work on the web.  He is new to the RTTY reflector, but not new to digital modulation techniques and methods.)

The spectrum tails of an FM signal may reach infinity, but there is also no "Lebesgue measure" in the tails.  Just think: if it really has infinite bandwidth, you would need infinite power, don't you?

You can always come up with an equivalent bandwidth for any signal that has finite power (what mathematicians call "square integrable").  ITU actually uses an equivalent "bandwidth that contains power within n-dB" concept.

The reasoning is the same as Zeno's paradox ("Achilles and the Hare").  Achilles finally catches the Hare because the series is convergent.  The same is true with the spectrum of any signal that has finite power.  All practical signals have finite equivalent bandwidths.

Moreover, modern FSK techniques no longer even transmit a signal that bears any resemblance to an FM signal -- MMTTY, fldigi, 2Tone and cocoaModem all have non-constant output power.  Heck, even the K3 has non-constant output power in (gasp) FSK mode.

73
Chen, W7AY