ELEC 241 Lab

Design

In the previous section, we identified the factors that need to be addressed to improve the performance of our optical communication system. Now we need to come up with a specific circuit design and a set of component and parameter values.

Part 1: The Transmitter

As shown in the system diagram, the transmitter must amplify the signal from the microphone and convert it to a current to drive the LED. For linear modulation, we want to drive the LED with a current of the form
$i_D(t)=A[1+s(t)]$
Since the optical output power is approximately proportional to the diode current, and since the received power is directly proportional to the transmitter power, we would like $A$ to be as large as possible. However the components we are using place a limit on the maximum current we can achieve.

The LED has a maximum current rating of 50 mA, and the op amp has a maximum short circuit rating of $\pm 25 \rm mA$ . We can meet these limitations with a value of $A = 10 \rm mA$ , i.e. $i_D$ varies between 0 and 20 mA with an average value of 10 mA.

To achieve this, we need to amplify the microphone signal voltage, add a constant offset, and convert the sum to a current which drives the LED. As shown in the transmitter circuit diagram we have done this using two op amp stages. The first stage is basically the mixer amplifier we have been using all semester. It amplifies the microphone (or function generator) output to a 1 V peak-to-peak level.

The second stage (LED driver) converts the signal voltage to a current, adds a constant offset to it, and applies the result to the LED.

\includegraphics[scale=0.550000]{ckt9.2.ps}
LED Driver
It is a modification of the inverting op-amp circuit where the output voltage ($v_0$ ) is ignored. Using the ideal op amp rules we have
$i_D = -\frac{v_s}{100} -\frac{-15}{1500} = 10[1-v_s]\rm mA$
The test point $v_1$ provides a means for monitoring the diode current.

Part 2: The Receiver

From the communication equation, the parameters we have available to increase the received power are: the transmitter power, the transmitter antenna gain, and the effective aperture of the receiver antenna. Recall that the transmitter antenna gain is a measure of how narrow the transmitter beam has been focused. Since the lens moulded into the LED package focuses the beam into a cone of about $40^\circ$ , the LED package is actually the transmitter antenna.

The third parameter we have available is the receiver aperture. If you look closely at the photodiode, you will see a small blue-black square in the center of the package. That is the photodiode itself and it has an active area of about $1 {\rm mm}^2$ . We could get a larger photodiode, but larger photodiodes are considerably more expensive. A more economical way to increase the effective area of the receiver is with a lens. If we place a lens of diameter $d$ in the path of the transmitter beam, it will focus the beam to (approximately) a point. If we place the photodiode at this focal point, then the power intercepted by the lens (having an area of $\pi d^2/4$ ) will be delivered to the photodiode. I.e. the effective area of the receiver is now the area of the lens.

We have constructed some receiver antennas consisting of a 50 mm diameter lens with a mounting for the photodiode at the focal point. They also have a connector for the photodiode and a cable so that you can manipulate the antenna independently of the breadboard (which has the transmitter antenna on it and should remain fixed).

The photodiode amplifier is basically the same circuit we used in Lab 4, but with more gain. One consequence of this additional gain is that the circuit is more prone to oscillation. We can reduce this propensity (and reduce the wideband noise as well) by limiting the bandwidth of the amplifier. That is the function of the 100 pF capacitor in parallel with the feedback resistor.

\includegraphics[scale=0.550000]{ckt9.3.ps}
Photodiode Amplifier

Rather than try to get all the gain in a single stage, we have split the receiver into two stages. This keeps the gain of each stage reasonable, and allows us to insert a volume control between the two stages to compensate for the large variation in received signal level between minimum and maximum range. The earphone driver is a non-inverting configuration like we used in the last two labs, except that it now has a gain of 100 (actually 101) instead of one.

\includegraphics[scale=0.550000]{ckt9.5.ps}
Earphone Driver