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4.10 Analog Modulation Analog modulation is a signal changing in level and frequency on a continual basis. This occurs when a microphone or telephone is used with sound waves being converted into electrical signals. Figure 4.36 Analog modulation The analog transmitter is a LED that emits an unchanging light intensity. A LED normally emits an unchanging light level or a steady signal. The signal has to be modulated by using the audio output from a radio to change the light levels slightly. Figure 4.37 Audio signal added to the DC voltage driving the LED. The result is an analog signal with a DC offset. The detector is a phototransistor that converts the light into electrical signals that can be seen on the oscilloscope and heard through the amplifier. Acton Instruments - ANU Page 1 5/12/2017 Activity Construct a fibre audio link based on analog modulation by reference to figure 4.38. An oscilloscope is useful to visualise the waveforms. Fibre Analog in Analog out Photodiode 1 Photodiode 2 BNC & LED BNC & LED trig volt/div volt/div ch1 ch2 secs/div Speaker Radio Figure 4.38 Analog modulation Tune in the radio station and display the signal on the oscilloscope. Try VOLTS/DIV on 0.5volts/div and SECS/DIV on 5msecs/div. To display the Phototransistor signal move the DC-GND-AC slider to AC. This blocks the DC voltage and allows the VOLTS/DIV setting to be increased to display the signal. Acton Instruments - ANU Page 2 5/12/2017