> jgorman wrote: > > This is misinformation that can be harmful to newbies or folks that > don't have the knowledge of about how the two different types of > signals (FSK versus AFSK) are generated. > > At the most basic level, if you look at the math involved in > generating the two types of signals, i.e., FSK versus AFSK, you will > see that AFSK has signal components close to the operating frequency > that FSK doesn't have. These components are commonly called 'opposite > sideband' and 'suppressed carrier'. Depending on the quality of the > transmitter and the method of SSB generation, AFSK will have these > components more or less suppressed but they are still there.
Sorry, but I didn't see any "misinformation" going on there... Practically speaking, FSK generated by frequency shifting an audio tone into an SSB transmitter is more FSK at the RF spectrum than it is AFSK. A properly adjusted SSB transmitter is simply a means of heterodyne with the "Local Oscillator" suppressed (phantom carrier) and the "Image" suppressed (unwanted sideband). Typically we see -50dB or so of suppression with modern SSB rigs. Let's put this in practical perspective for typical ham rigs: Transmitter power 10 Watts. Suppressed carrier (L.0.) = 100 microwatts. Rejected sideband (image) = 100 microwatts. Transmitter power 100 Watts. Suppressed carrier (L.0.) = 1 milliwatt. Rejected sideband (image) = 1 milliwatt. Transmitter power 1000 Watts. Suppressed carrier (L.0.) = 10 milliwatts. Rejected sideband (image) = 10 milliwatts. It seems like my neighbor's plasma TV set radiates more RFI than my undesired sideband does :) There are often harmonics and spurious from most common transmitters that are 15 or 20dB stronger than that... even when you are running Class C with CW or a diode-shifted oscillator FSK modulator :) There may be hum and noise when a computer sound card is used to drive the audio of an SSB transmitter... but if an attenuator is used or a properly set up audio interface, the noise floor is usually better than -60dB below the desired signal. Of course, there are some ops who use other software programs that generate strange sounds and music while they are transmitting with their soundcard, but IMHO, these should be characterised more as operator error than unsuppressed noise. :) Bonnie VR2/KQ6XA Hong Kong, etc.
