Question

Modulation index is defined as : h= (Umax-umin)/(Umax+umin) Or h=M/A Please how can I determinate the value of M and A from the oscillogram???????

Answer 1

A Time versus Voltage amplitude plot of Amplitude Modulated wave on an oscilloscope looks like figure above. Observe that waveform below the `V=0` line is mirror of waveform above the line. Various points have been identified as crest `V_max`, trough `V_min`, crest to opposite crest `V_(max(p-p))`, trough to opposite trough `V_(min(p-p))`.

We also observe that the peak value of the modulating signal
is one-half the difference of the peak and trough values.

`V_m=(V_max-V_min)/2` ........(1)

Also that the peak value `V_C` of the carrier signal is the average of `V_max` and `V_min`

`V_C=(V_max+V_min)/2` .......(2)

The modulation index denoted as `m` is defined as

`m-=V_m/V_C=((V_max-V_min))/((V_max+V_min))` ......(3)
`(V_m/V_C=M/A=h, "as given in the problem")`

The values of `V_max` and `V_min` can be easily read off the oscillogram. Values of `Mand A` can be found using expressions (1) and (2).