On 7/23/2023 11:48 AM, Gmail wrote:
Unlike many other digital modes, FT8 is a constant carrier, constant
envelope mode.
This is true for any "N of M tone" FSK mode where N is 1. However,
the DSP filtering in both the K3 and K4 has a finite delay which
causes some "spreading" of energy from one bit to the next which
means there is *some* level of both tones present during the transition.
This two tone condition *can* result in IMD. Since both the K3 and K4
are "100W, 12V" final amplifiers, the finals will begin to go into gain
compression (the early stages of saturation) at around 80W (look at the
data sheets on the final devices) and will produce IMD if run with the
PWR control above the onset of compression - *EVEN WITH A "CONSTANT*
*CARRIER, CONSTANT ENVELOPE" MODE*
The proper solution to this "excess" IMD is not reducing the ALC (audio
drive to the ADC and DSP modulator) but reduce the POWER OUTPUT until
the amplifier is operating in its linear (no gain compression) range -
or about 80W for most "100W, 12V" rigs.
73,
... Joe, W4TV
On 7/23/2023 11:48 AM, Gmail wrote:
Unlike many other digital modes, FT8 is a constant carrier, constant envelope
mode. The ALC level will quickly settle at a constant value and should not
hunt. ALC does not affect FT8 transmissions. My K4 goes to 5 and never changes
and peak power on an external analog meter does not change.
Perhaps I will try 7, monitoring signal on my Rigol SA and see what happens.
I wonder if the 5 recommendation for the K4 is more for non constant
envelope modes, to keep the level below the ALC threshold. Some insight from
Elecraft would be helpful.
Oh, as has been pointed out to me previously unlike my K3, 5 on my K4 is not 5
bars.
Ray
W8LYJ
Sent from my iPad
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