Transmitter

The purpose of the transmitter is to transform the information we want to send into a signal that can be propagated by the channel. In the case of our wired copper channel, this means we want the information to be transformed into a modulated voltage level, something like the pulse train in figure 1. For a wireless channel, however, the transmitter needs to encode the information onto an EM wave that can be easily propagated. In this lab, this means that the transmitter needs to take the voltage pulses generated by the $\mu$Stamp11 and it needs to put them onto an infra-red EM wave. We usually refer to this encoding as modulation.

Let $p(t)$ be a time-varying signal representing the bits of information we want to transmit over the channel. A possible waveform for $p(t)$ is shown in figure 1. The transmitter then uses this signal to modulate a sinusoidal (or square) wave with a frequency $\omega_c$. We refer to this sinusoidal wave as the signal's carrier wave. For the wireless channel we're building, standard receiver structures usually assume a carrier wave with a frequency between $38-44$ kHz. The carrier wave is added to assist in subsequent signal processing at the receiver end. So the information that the transmitter generates has the form

$$x_1(t) = p(t) \sin(\omega_c t + \phi)$$ (2)

This type of encoding is referred to as amplitude modulation or AM because we're modulating the amplitude of the carrier wave. When $p(t)$ is digital data of the form shown in figure 1, we refer to this modulation scheme as amplitude shift keying or ASK.

The signal $x_1(t)$ in equation 2 is still an electrical signal that has been generated by the transmitter. The transmitter must now use this signal to amplitude modulate the IR beam we intend to transmit through the channel. So the strength of the IR beam that is actually transmitted by the system has the functional form

$$x(t) = p(t) \sin(\omega_c t + \phi) \sin(\omega_0 t)$$

where $\omega_0$ is the frequency of the IR beam, somewhere on the order of 1-500 TeraHz ($10^{12}$).

Figure 4: Block Diagram of Transmitter

Figure 4 illustrates the steps outlined above. The information signal $p(t)$ is mixed with the carrier wave in the block label mod. At the bottom of the figure, you'll see representative waveforms for the voltages in the transmitter that enter and exit the mod system block. The output of the modulator is then used by the trans (transducer) block in the system. The transducer block uses this voltage waveform to modulate the IR wave. This IR beam is then sent out into the channel (air) to be caught by the receiver.

In this lab, you'll need to build a circuit that performs the basic modulation and transducer functions shown in figure 4. A schematic diagram for the entire transmitter circuit is shown in figure 5. The modulator function is carried out by the 555 timer IC shown in the top part of the schematic. The transducer function is performed by the photo-diode circuit shown on the bottom part of the figure.

Figure 5: Schematic Diagram of Transmitter

The 555 timer IC (TLC555) is a standard IC chip that is used to generate very precise clock pulses. It is also possible to use the $\mu$Stamp11 to generate the clock pulses, but this puts a heavy burden on the micro-controller. Using the TLC555 IC relieves the micro-controller from this burden, thereby freeing it up to do more useful things. The clock signals generated by the TLC555 are shown below in figure 6.

Figure 6: 555 Timer IC

The frequency of this periodic waveform can be controlled by two external resistors and one capacitor. In particular, the positive pulse width generated by the TLC555 can be computed from the equation

$$T_+ = 0.69(R_1+R_2)C$$

and the negative pulse width is

$$T_- = 0.69 R_2 C$$

Figure 6 illustrates the pin-out for the TLC555. You usually implement $R_1$ using a current limiting resistor and a trim pot (see figure 5). You can use the trim-pot shown in figure 5 to adjust the carrier frequency $\omega_c$.