[Elecraft] JT65A output power variations.

[Richard Ferch]

Julian, G4ILO wrote:

> Surely using a digimode set up for SSB with ALC operating and 20dB of
> compression is going to cause distortion and splatter? Admittedly, JT65A
> isn't PSK31 but even so I would have thought IMD products would occur,

Not all digital modes are equal. PSK31 depends on amplitude shaping to 
preserve its narrow bandwidth. It must be operated with no compression 
and with the requested power kept low enough to avoid clipping of 
instantaneous peaks. Some other digital modes such as MT63 are similar.

Compression in these modes produces IMD which cannot easily be filtered, 
since it results in unwanted output at frequencies very close to the 
intended frequency. The recommended way to operate these modes on the K3 
is with no compression, and with 5 bars showing on the K3's ALC meter, 
which puts the K3's ALC in an area where it acts to keep the 
time-averaged power at the desired level without clipping instantaneous 
peaks and causing IMD.

However, RTTY and many FSK or MFSK-type modes (including JT65A, I 
believe) feature a single constant-amplitude carrier that is stepped 
between tones. When there is only a single tone being transmitted at any 
time, compression does not cause intermodulation; it causes harmonic 
distortion (odd-order harmonics), which can be eliminated by filtering 
(the transmitter's crystal filter). The net effect after both 
compression and filtering should be a constant amplitude signal. I 
believe this is why JT65A users can safely use compression.

73,
Rich VE3KI