Odd order harmonics are typical of a square wave. f, 3f, 5f, .....
This tells me somewhere in the amplifier chain there is a stage that is
being overdriven significantly and the output of the amp is
flat-topping, i.e. creating a psuedo-square wave by lopping of the top
and/or bottom of the signal it is amplifying.
The second harmonic is usually created by an amplifier that is driven
outside it's linear operating range but not driven into flat-topping.
In essence the operating line looks a little like a parabola. The
equation of a parabola has a squared term which, when the trig is done
reduces to a cos(2wt) term in the output.
It may be that the 2nd harmonic is generated in the final stage and
adjusting the bias is keeping stage more linear. But if it isn't
changing the 3rd harmonic then I would guess it is being generated
somewhere further back in the amp chain.
It would be interesting to see if reducing the drive potentiometer
reduces the 3rd order harmonic.
tim ab0wr
On Fri, 01 Mar 2019 12:27:26 -0800
"Jim Sheldon" <w0eb@...> wrote:
Ok Ashhar,
I did your bias change while watching the 2nd and 3rd harmonic on 80
meters.? Sorry, but changing the bias reduces the heck out of the 2nd
harmonic but has very little effect on the third.? The attached
picture shows the final results.
I now have the tone generator kit, and after I finish writing this
I'll start building it so I can do the single and two tone tests on
the rig.
Jim, W0EB
