To measure the depth of modulation of an amplitude modulated (AM) wave using an oscilloscope with the internal timebase switched off, follow these steps:
1. Connect the output of the AM transmitter to the vertical input of the oscilloscope.
2. Connect a DC voltmeter across the output of the AM transmitter.
3. Set the oscilloscope to DC coupling and adjust the vertical sensitivity so that the AM waveform is clearly visible on the screen.
4. Adjust the horizontal position of the waveform so that the unmodulated carrier is centered on the screen.
5. Adjust the DC voltmeter to read the unmodulated carrier voltage.
6. Adjust the AM modulation depth so that the AM waveform just reaches the maximum and minimum levels of the carrier waveform.
7. Read the DC voltmeter again to determine the new carrier voltage.
8. Calculate the modulation depth using the formula:
Modulation Depth = (Vmax - Vmin) / (Vmax + Vmin)
Where Vmax is the carrier voltage with modulation and Vmin is the carrier voltage without modulation.
The displayed waveform will show the unmodulated carrier waveform centered on the screen, with the modulated waveform superimposed on it. The depth of modulation can be determined by measuring the maximum and minimum levels of the modulated waveform and calculating the modulation depth using the formula above.