Wireless Sound Transmission Using Light

Design (E) – 344 Task 2

L.Briedenhann – 18972438

Part 1 : Carrier frequency and bandwidth.

With only the message signal frequency given it is difficult to choose an appropriate carrier frequency and frequency deviation. According to the datasheets, the IR LED can be used in a very large frequency range which makes it very useful. But according to the datasheet of the phototransistor, it has a cut – off frequency of 110 kHz which makes selecting a carrier frequency easier.

By looking at the crystals in the package, the best choice for the carrier frequency is around 75 kHz.  By choosing the frequency deviation we first must decide between using NBFM and WBFM. WBFM allows us to transfer high quality signals. This makes it a good choice for the design process. A frequency deviation of about 15 kHz should thus be fine. The bandwidth according to Carson’s rule is as follows: BFM  2f  30 kHz.[pic 1][pic 2][pic 3]

Note that the carrier frequency and frequency deviation may change when the actual circuit is designed and tested. These values are thus only starting blocks for the design.

Part 2 : Circuit design.

This is my circuit that I will be using to drive the LED.

Figure below shows current across the LED.

Design (E) – 344

Task 4

L.Briedenhann – 18972438

Spice analysis.

Op – amp circuits[pic 4][pic 5]

Virtual - ground[pic 6]

The circuit above is the spice model for my high pass filter, virtual ground and op – amp circuit. The second op – amp circuit is just to invert the output signal.

Figure 1

Figure 1 shows the output of the first op – amp when using a sinusoidal input signal. It can clearly be seen that the op – amp saturates and thus the signal clips at the top.

Figure 2

Figure 2 shows the output of the second op – amp. As seen above the output signal just gets inverted so that first peak is at 5 V and not 0 V.

Figure 3

Figure 3 shows the frequency spectrum of the circuit output. The message signal (100 kHz) can clearly be seen on the spectrum at 100 kHz which is the desired result. The signal also has a much larger gain then when it did at the output of the high pass filter (figure 5).