K9ACT wrote:Removing the load, (50 ohm resistor and headphone) squared it up nicely.
Now I can see what the controls do but I need some help understand why and what they mean.
The carrier level changes the duty cycle but I have no idea how this relates to the RF world downstream or what determines where to set it.
The audio level increases the duty cycle starting where the carrier ends. It would seem like 100% modulation would be when the total is twice the carrier width but as this can be accomplished over a wide range of carrier levels, I don't get it.
The NPL is still more mysterious. I can move the point where the modulation begins relative the the edge of the carrier but I don't understand what this accomplishes or how.
I am trying to relate this to my CBS Volumax and I am lost.
And yes, it's a 20 mhz scope.
js
This is why it's called a Pulse Width Modulator....
The width of the pulse determines the DC voltage the modulator produces at the output of the filter.
The wider the pulse, the more DC voltage.
What determines where to set it (the carrier level)will depend on the RF deck design, it's impedance... and the modulator power supply voltage.
The modulation should add AND subtract from the width of the pulse... so if it only ADDS to the carrier level, then the NPL is set to low (or high?) that's why I made the comment...
The pulse should be almost non-existant at the peak of negative modulation.. (near 0 volts output)
In the picture of the scope trace, this would mean the "fur" would extend from the leading edge (left side) of the pulse to a point about the same distance to the right of the point where the carrier width of the pulse was. Assuming a audio generator is used to provide modulation...
Voice characteristics (assymetry) will be a different story...
A good place to start will be to adjust the pulse-width to approximately 1/3 of it's total\, or less.
Then later, with the RF deck up and running, you can adjust the carrier level to it's designed level.