[600MRG] FT Question Answered?

[Bill Cromwell]

Hi John,

Consider that Collins and others used a single audio tone to generate a 
CW signal in some of their SSB transmitters. In theory it is wonderful. 
It is less expensive. It actually seems to work as *theory* says it 
should. They dropped it because it did not *reliably* give a clean 
signal..even with 'linear' amplifier chains.

I understand that the shift is small for the newfangled digital modes 
but shift they do. It is not a single tone and then later another single 
tone. Use caution if you are going to add nonlinear amplifiers into the mix.

73,

Bill  KU8H

On 01/13/2018 03:45 PM, John Andrews wrote:
> Bill,
>
> What you say is correct for speech or any audio that has multiple tones
> <<at the same time>>. But FT8, WSPR, and the vast majority of sound-card
> generated modes only output one tone at a time. In that case, you can
> have non-linear amplifier stages after the SSB signal is generated.
>
> Consider a properly run SSB generator in USB mode at 474.200 kHz. The
> carrier frequency and the LSB are suppressed by maybe 50 dB. If you
> apply a single 1500 Hz tone to the input, you will get a carrier at
> 474.200 + 1.500 = 475.700 kHz and nothing else of significance. A
> non-linear amplifier will do no damage to a single frequency signal. No
> magic involved.
>
> So, as long as you avoid the use of simultaneous multi-tone or PSK
> modes, linearity is not required after the SSB generator. FT8 and the
> other "K1JT" modes are all single-tone.
>
> John, W1TAG
>
> On 1/13/2018 1:47 PM, Bill Cromwell wrote:
>> Hi Frank,
>>
>> That is what I was going to say in reply to your latest query. If you
>> are using audio modulation in upper or lower sideband SSB non linear
>> amps will ruinate and splaterate it.
>>
>> 73,
>>
>> Bill  KU8H
>>
>> On 01/13/2018 10:47 AM, Frank Lotito wrote:
>>> I may have answered my question - We use AFSK not direct FSK to
>>> generate modulation modes such as FT8.  AFSK as I recall requires
>>> "linearity" in the audio processing stages.  But once in the RF
>>> domain we can use non-linear devices, such as heterodyne converters,
>>> Class E amps, etc.  Did I get that correct?  73 Frank K3DZ  / WH2XHA
>>>
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>
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