[600MRG] FT Question Answered?

[Bill Cromwell]

Hi Ben,

I hope that my cautionary comments are taken as just that. We have to 
keep an eye on those things. To do that we need to know what we are 
looking for. Distortion in the system is detrimental whether we are 
using AFSK or FSK in in the carrier oscillator. Historically the RF FSK 
has been more reliable. I doubt whether 'modern' gear makes very much 
difference. Take a close look at some of the 'digital' signals on the 
air. As always, some are pretty good. Some should be cause for shame.

The OP asked about using nonlinear amplifiers. I think my reply is 
appropriate. I use linear amplifiers even for conventional CW just to 
promote a cleaner signal.

73,

Bill  KU8H


On 01/14/2018 11:47 AM, Ben Gelb wrote:
> I think what you are describing is distortion in the audio chain. That
> seems like a problem even with linear amplification (though maybe
> nonlinear would make it worse) ... I don't know that your comments
> really apply to modern equipment and AFSK techniques, though... audio
> should be pretty clean (and needs to be in order to control occupied
> bandwidth).
>
> 73 de N1VF
>
> On Jan 14, 2018 7:07 AM, "Bill Cromwell" <wrcromwell at gmail.com
> <mailto:wrcromwell at gmail.com>> wrote:
>
>     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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