On Mon,9/22/2014 12:17 PM, Duane Calvin wrote:
If Jim wants to describe a
frequency-shifted carrier as modulation and treat it as an analog signal,
fine, then run a Class A amp with it if you like.
It's not how I want to describe it, it's how Mother Nature treats it. :)
If you look at any RTTY signal on a spectrum analyzer with sufficient
resolution, you will see sidebands that are the result of modulation,
and of distortion in the RF amplifier circuitry. If you look at FM in
any decent text, you'll see that sidebands are produced. FSK is, in
essence, FM where the signal is a square wave. Any square wave has
harmonics, which produce sidebands, and which combine with each other
and with the carrier to produce intermod in the RF amplifier.
Elecraft's initial implementation of FSK produced more sidebands than a
good AFSK signal driving the K3. I know that because I measured it on a
neighbor's K3. Others were also noticing this, so they later improved
the shaping of the FSK waveform, and reduced the TX bandwidth. I haven't
taken the time to measure it since.
If you look at the graphs of ARRL Lab data in k9yc.com/TXNoise.pdf you
will see huge variations in the keying bandwidth between radios. This is
largely the result of the shaping of the square wave that keys the
transmitter, but is also affected by the IMD of the output stage and TX
phase noise.
73, Jim K9YC
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|