[GreenKeys] True FSK vs AFSK

[Bob kb8tq]

Hi

The FCC sees this *very* differently as does any textbook I’ve ever
seen on the topic. It does not matter *how* you get to the transmitted
signal. What matters is what the transmitted signal actually looks like
on a spectrum analyzer. 

When you put an audio tone into your transmitter and set it to AM modulation, 
you get AM AFSK. If you set your transmitter to FM modulation, you 
get FM AFSK. Set it to SSB and you get FSK. Three very different types
of modulation all from the same input signal. 

What goes in is not the determining factor here. 

Bob

> On Feb 5, 2022, at 3:25 PM, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
> 
>    However, AFSK is really a shifting signal at audio frequencies that is beat up to some RF frequencies via the transmitter. If the transmitter is not adding distortion the output will be exactly the same in either case.  The problem is the extra circuits of the AFSK can add distortion products.
>   Someone pointed out that a FSK transmitter can run in Class-C with somewhat higher efficiency. The power gain is probably not very great. The advantage of AFSK for an SSB transmitter is that it is simpler to implement for the transmitter itself.
> 
> On 2/4/2022 11:13 AM, Bob kb8tq wrote:
>> Hi
>> AFSK = Audio modulation of a carrier with all the sidebands present.
>> FSK = One of only two tones with no carrier and no opposite sidebands.
>> You can generate this or that a number of ways. What counts in terms of
>> calling it this or that sort of modulation is the signal that hits the antenna.
>> Unless it has a carrier with tones spaced off of it (either as FM or AM) it
>> technically isn’t AFSK.
>> This is nice because you can’t use AFSK on some pretty important chunks
>> of the spectrum ….
>> If you want to go a bit nuts, you can look at FSK as a pair of 100% modulated
>> ASK (amplitude shift key) signals, each chugging along. No ASK is not
>> the same as AFSK.
>> Bob
>>> On Feb 4, 2022, at 1:54 PM, Paul Kasley <w9ts at comcast.net> wrote:
>>> 
>>> Don't know. I feed audio from my compy into my K3. DDS chips are easily shifted. And way back, I switched a cap in and out of the VFO of my AM-only DX100.
>>> 
>>> Agreed, more sophisticated digital modes require audio generation and processing.
>>> 
>>> Simply responding to the original question as posed - yes, there can be a benefit to FSK v AFSK.
>>> 
>>> -w9ts
>>> 
>>> On 2/4/2022 12:42, Jim Haynes wrote:
>>>> On Fri, 4 Feb 2022, Paul Kasley wrote:
>>>>> 
>>>>> Directly shifting a carrier oscillator eliminates the audio chain and is
>>>>> essentially CW.
>>>> 
>>>> However, is that the way modern radios actually generate FSK?  and CW?
>>>> Or do they actually generate AFSK internally and put it into the audio
>>>> chain?
>>>> 
>>>> And, given that we have dozens of digital modes these days that all begin
>>>> by generating audio frequency signals, is AFSK for FSK any worse than
>>>> any other digital mode?
>>>> 
>>>> Granted that it's possible, easy in fact, to generate crud on the air by
>>>> feeding audio into the SSB transmitter.
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> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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