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Next: Receiver Up: Input Capture Interrupts Previous: The Channel


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 44. 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 44. 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

$\displaystyle x_1(t) = p(t) \sin(\omega_c t + \phi)$     (10)

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 44, we refer to this modulation scheme as amplitude shift keying or ASK.

The signal $x_1(t)$ in equation 10 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

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

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

Figure 47: Block Diagram of Transmitter
\begin{figure}\centerline{\psfig{file=figs/transmitter.eps,width=3in}} \end{figure}

Figure 47 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 learning module, you'll need to build a circuit that performs the basic modulation and transducer functions shown in figure 47. A schematic diagram for the entire transmitter circuit is shown in figure 48. 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 48: Schematic Diagram of Transmitter
\begin{figure}\centerline{\psfig{file=figs/transmitter.eps,width=3in}} \end{figure}

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 555 IC relieves the micro-controller from this burden, thereby freeing it up to do more useful things. The clock signals generated by the 555 are shown below in figure 49. 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 555 can be computed from the equation

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

and the negative pulse width is
$\displaystyle T_- = 0.69 R_2 C$      

Figure 49 illustrates the pin-out for the TLC555.

Figure 49: 555 Timer IC
\begin{figure}\centerline{\psfig{file=figs/tlc555_1.eps,width=4in}} \end{figure}

next up previous
Next: Receiver Up: Input Capture Interrupts Previous: The Channel
Bill Goodwine 2002-09-29