ELEC 242 Lab

Experiment 9.1

Assembling the System



Note
In Lab 10 we will require all three of the phone jacks on the interface board (J1-4, J1-5, and J1-6). If one of your phone jacks is damaged, get a new interface board before you begin wiring.

We will also be using the motor and summing amplifiers from previous labs, so don't dismantle them.

Part 1: Motor

The first order of business is to get the disk spinning. Next week we will actively control the disk (using the motor amplifier) to synchronize the receiver to the camera. Today, it it will suffice to simply connect the motor to a constant voltage source (the 0-6V power supply).


Step 1:

 Set the output of the 0-6 V power supply to 4 V. Turn off the supply.

Step 2:

 Plug your BNC-banana adapter into the 6V supply terminals. Be sure that the prong with the ground bump is plugged into the negative (black) terminal of the power supply.

Step 3:

 Plug one end of a BNC patch cord into the adapter. Plug the other end into J1-3 on the interface board.

Step 4:

 The motor is connected to pins 20 and 21 of the interface board socket strip. Connect pin 21 to ground and pin 20 to the positive terminal of the 0-6 V power supply (pin 3 on the socket strip).

Step 5:

 Plug the camera cable into J2-1 on the interface board.

Step 6:

 Turn on the power supply. Verify that the disk is rotating in a clockwise direction as seen from the front of the camera. If it is rotating counterclockwise, reverse the two connections to the motor.

Step 7:

 Turn off the power supply.

Part 2: Sync Detector

Next week we will use the pulses from the synchronizing hole in the disk to synchronize the camera with the receiver. For now, we will use them to measure the speed of the disk.


Step 1:

 Wire the following circuit (the LED and phototransistor are on the camera).
\includegraphics[scale=0.500000]{ckt10.1.ps}


Step 2:

 Turn on the power supply. You should be able to see the LED (on the back side of the camera) glowing from behind.

Step 3:

 Observe the signal $v_{sync}$ with Channel 1 of the scope. You should see a stream of narrow pulses having a low level of 0V and a high level of 5V.

Step 4:

 Adjust the 0 TO 6V control on the power supply until the frequency of the pulses is exactly 30 Hz.

Step 5:

 Leave $v_{sync}$ connected to Channel 1 of the scope. We will use it later to synchronize the video signal.

Part 3: Video Amplifier

Because of the small amount of light reaching the photodiode, the photocurrent generated is very small. If we connected the photodiode directly to the camera cable and wired the amplifier on the breadboard, we would have a very noisy signal. So to improve the signal to noise ratio, we have built the video amplifier directly on the back of the photodiode, and run the amplified, high level signal to the breadboard. This means that you don't have to wire the video amplifier. However, since the op amp requires power, you do have to connect +15V and -15V to it through the camera cable. Here's the circuit for the video amplifier:
\includegraphics[scale=0.650000]{ckt10.2.ps}
Because of the very high gain of the video amplifier circuit, it is susceptable to high frequency oscillations. The 2 pF capacitor rolls off the high frequency response sufficiently to improve stability and reduce noise, but not enough to effect picture quality. The 10k resistor in series with the output helps to isolate the opamp from the capacitive loading caused by the cable, further reducing the susceptability fo oscillation. The 39 ohm resistors and the capacitors form a filter for the op amp power.


Question 1:

 What is the cutoff frequency of this circuit? For a 30 line picture, how does this limit the number of pixels per line we can resolve?

Step 1:

 Connect pin 17 of the interface board socket strip to +15 V. Connect pin 16 to -15 V.

Step 2:

 Turn on the power. Verify that $v_{sync}$ is still active on Channel 1 of the scope.

Step 3:

 Leaving the trigger source set to CH1, observe the signal $v_{video}$ with Channel 2. You should see a series of closely spaced pulses of varing amplitude.

Step 4:

 The pattern you see represents the variation in intensity of the scanned image. You should be able to see patterns in the scene reflected in this signal: Covering the lens with your hand should make it go to zero. Pointing the camera at a light or open window should produce very large peaks. Tipping the camera up and down should make the pattern move from side to side. Turning the camera from side to side should cause the pattern of each of the broad pulses to shift. Try holding your hand in front of the camera and see if you can identify your fingers in the waveform on the scope.

Part 4: LED Driver

We can now transmit images with the camera, but for a complete system we need some way to display them when they are received. The easiest way to do this is to use the camera in reverse, with a variable light source behind the disk synchronized to the signal from the camera.


Step 1:

 Plug the cable from the LED on the front of the camera into J2-2 on the interface board.

Step 2:

 Wire the following circuit. Remember to connect power to the opamp.
\includegraphics[scale=0.650000]{ckt10.5.ps}


Question 2:

 Explain how the above circuit works.

Step 3:

 Connect $v_{drive}$ to the MAIN output of the function generator.

Step 4:

 Set the function generator to produce a 30 Hz 1 V p-p sine wave.

Step 5:

 Turn on the power and look into the receiver eyepiece. You should see alternating red and black bands moving across the image.

Step 6:

 Vary the AMPLITUDE and DC OFFSET controls. What effect do they have on the image.

Step 7:

 Reset the AMPLITUDE and DC OFFSET controls to produce a bright band and a dark band of equal width. Increase the frequency while watching the pattern. What is the maximum frequency that produces a visible pattern?

Question 3:

 Explain the various patterns you have seen in this part.