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Old 23rd Jan 2019, 7:13 pm   #31
g4wim_tim
Pentode
 
Join Date: Nov 2018
Location: Freckleton, Lancashire, UK.
Posts: 232
Default Re: 160 /80 / 60 mtr AM TX project

Yes negative cycle loading is required which is why there is a resistor in parallel with the diode to the PA such that is the transformer output goes below zero the diode is reverse biased and the resistor takes over.

However due to the ALC circuit I'm yet to detect the output going below zero and the RF spectrum always looks nice and clean.

Measuring the modulation distortion on my HP8901B is remains below 2.5% up to 75% mod above which point the ALC kicks in and the distortion rises to 8% at 95% mod - so not a bad trade off.

Regards Tim
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