Optical Modulation and Frequency of Operation

Transcription

1 Optical Modulation and Frequency of Operation Developers AB Overby Objectives Preparation Background The objectives of this experiment are to describe and illustrate the differences between frequency of operation and frequency of an electromagnetic wave through experimentation using optical modulation. Read sections concerning passive filters and op-amps in textbook and also read description of LEDs in section 3.13 of this text. A common technique when trying to transmit information is to modulate a signal. The practice of modulation involves taking a signal, or information, that isn t easily managed or transmitted and overlaying it onto a carrier signal that can be worked with much easier. A radio can use amplitude modulation (AM) or frequency modulation (FM) to transmit voice and musical instrument signals across long distances. Most music is contained in the frequency range of 20Hz-20kHz. If all music was transmitted at its own frequency this would obviously cause interference and the original sound could not be recovered. We work around this by taking this 20kHz range and overlaying it onto a carrier signal of a much higher frequency, around 100 MHz for FM, and then transmitting. At the receiving end this signal is demodulated and the 20kHz range can be recovered. Using this method we can now give each signal its own carrier signal before being transmitted and no interference will occur! The receiving end will see all of these signals arriving but can use simple electronic filtering to pick only one of these carrier signals and then demodulate that chosen signal. This is how you can have several radio stations transmitting all at the same time with no interference. Also, a common modulation technique when using fiber optics is to have several channels communicating with the same wavelength of light, but being modulated at different frequencies of operation. In this experiment you will be using a multicolored LED to create two separate signals carrying information. In this case the information we are interested in is the intensity of each detected wavelength of light. However, the photodetector we are using is equally sensitive to the two wavelengths of light we want to detect and it has no way of differentiating between the two. To overcome this we will modulate the two signals at different frequencies and then use an oscilloscope to view the frequency spectrum. Two peaks should be seen at the frequencies that correspond to the modulated signals. A common way to optically modulate signals is by varying the current to the light source. By using a sinusoidal signal to drive the source we create a carrier signal to transmit information on. To do this with LEDs we will provide a constant 5V to the anode and a varying voltage on the cathode. By controlling the sinusoidal voltage on the cathode, we can generate a signal that will cause the current in the LED to range from the maximum allowable to near the turn-off point. To make this properly work, a DC offset is required so that we can prevent too much current from passing through the LED and so that the LED does not turn off. An LED turning on and off resembles a square wave and as such has a lot of harmonics that can interfere at the detector. 33

2 These extra harmonics are also why a diffuser is needed. If the detecting circuit saturates it also creates a pseudo-square wave with extra harmonics. To begin defining the parameters for your modulating signal first determine the maximum current that the LED can safely handle from the data sheet. Once you ve found this current determine the current limiting resistor by using the 5V at the anode minus the lowest voltage in the sinusoid at cathode minus the forward voltage of the LED. You will need to set an offset with the function generator so the lowest sinusoid voltage should be around 0V. When in doubt it s best to choose a larger resistor and then adjust it as necessary. The highest point of the sinusoid should be such that 5V minus this point is just above the forward voltage. So for a red LED with a 2V forward voltage this is about 3V. The final sinusoid connected to the cathode of a modulated red LED should be from 0 3V at whatever desired frequency. On the detector side we can detect this signal and display the information as a function of the frequency. If we have modulated the red LED from the example above at 8kHz, then we will see a matching 8kHz sinusoid on the detection circuit minus any losses. Taking a look at the frequency spectrum we will see a spike at the 8kHz point with amplitude pertaining to the intensity of the detected signal. Use figure 1 below and the LF356N schematics to build the detecting circuit. The op-amp should have +9V and -9V at its corresponding rails and you will probably need to move the photodiode away from the op-amp so that there is enough room for the filter channel. Figure 1: Detection Circuit To construct the full experiment use the 3-color LED specification sheet from the reference section and the design process for a modulated light source above. The ANDY board will need to be used to generate a 5kHz sinusoid for the green LED and the Velleman oscilloscope should be use to generate an 8kHz sinusoid for the red LED. You will need to use the oscilloscope to verify the ANDY board signal generator s configuration. Be sure to leave sufficient room between the LED and the detector so that the filter channel will fit on the board. At this point you have a source emitting light of differing wavelengths with each being 34

3 modulated by a different carrier signal. By using modulation and different frequency of operations it is possible to transmit similar signals across the same channel and use a simple detector circuit with little interference. Had the signals not been modulated, a more complex detector circuit would have been necessary to isolate the red and green wavelengths of light. At this point, taking a reading from the detector circuit will show two peaks at 5kHz and 8kHz corresponding to the green and red light respectively. Now, suppose that we wanted to isolate either of these signals so that we could individually measure it. The choices available are to use either the modulating frequencies or the lights wavelength to isolate the signals. If we decided to use the lights wavelength as the deciding factor we would need to place an optical filter in between the source and detector. An optical filter works by passing through light of only a certain wavelength, or frequency, and blocking or absorbing all others. This is why a red colored filter appears red; all other colors have been blocked when passing through. To pass the green LED through we will need a matching green optical filter and respectively a matching red optical filter for the red LED. The LEDs are actually producing light at a very specific wavelength, about 630 nm for the red and 525 nm for the green. The filters, however, pass a range of wavelengths. So the red filter might pass wavelengths of nm while the green filter passes wavelengths at nm. This method works well for wavelengths far apart on the electromagnetic spectrum like green and red, but would not work well if we were using light sources that had wavelengths very close together, ie differing shades of green. On the other hand, if the wavelengths being detected were near each other and could not be easily filtered optically, a good method of isolating signals would be to electronically filter out unwanted signals. Since we can control the modulating frequency of each wavelength it is very easy to spread signals out by their frequency of operation. Assume that the red wavelength is being modulated at 8kHz while the green wavelength is being modulated at 5kHz. Knowing this we can pass the output of the detector circuit into either a passive or active filter circuit and isolate the desired signal. The drawbacks of using an electronic filter are increased detector complexity and increased cost if high quality filters are needed. Also, depending on the amount of bandwidth available for modulating and the non-ideality of filters it may be impractical to attempt filtering. However, for the modulating frequencies being used, we can easily design separate bandpass filters to isolate the desired signals. You will design an RLC bandpass filter centered at 5kHz and 8kHz with a bandwidth wide enough to pass the desired frequency and block the other. Figure 2 below shows the RLC to be used along with the relevant equations for designing component parameters. When using the bandpass filter, the detector output connects at Vin and the 10 kω resistor should be removed from the output. 35

4 Figure 2: RLC bandpass filter Using the above circuit and equations you should be able to design a bandpass filter to meet the specifications also stated above. In the analysis section you are asked to find the transfer function of the above circuit. Use Vin as the input and the voltage marker as the output in finding the transfer function. References Materials Procedure 3-Color LED Spec Sheet. Electronics Express [Viewed on May 10, 2011] The equipment and components required to perform this experiment are: ANDY Board Velleman Oscilloscope 1 ea. 3-color LED 1 ea. LF256N op-amp 1 ea. OP950 photodiode 1 ea. 1 MΩ resistor 1 ea. 10 kω resistor 1 ea. 100 mh Inductor Various capacitors and resistors 1 ea. Channel with slots for optical filters 1 ea. Optical filters: Diffuser, Red, and Green Analysis: 1. Using the equations with Figure 2, determine the RLC parameters that will give a 5 khz and 8 khz bandpass filter with a bandwidth that only passes the desired frequency. Hint: Start by using L=100 mh and designing with standard capacitors as this will make constructing the circuit much easier. 2. Using Figure 2 above, find the transfer function of the voltage where Vin is the input voltage and the output voltage is the node voltage at the voltage marker. 3. Using the transfer function and the calculated RLC values use Matlab to create a bode plot of the transfer function. It should show clearly that it passes the desired frequency and only that frequency. Show two plots using the 5 khz and 36

5 8 khz parameters. Note: Matlab uses rad/s when plotting, you may need to adjust your plot for this. 4. Using the calculated RLC values construct the filtering circuit in pspice. Show the frequency analysis with both the 5 khz and 8 khz components. The graph should mark the center frequency and the cut-off frequencies. 5. Using the process described above, build driving circuits for the red and green LEDs of the 3-color LED. The green LED should be modulated at 5 khz and the red LED at 8 khz. Use the oscilloscope to configure the ANDY board function generator. Remember to place an offset to limit the current and to make sure the LED does not turn off. Capture the voltage waveforms of the two driving circuits. Record which color LED is being modulated at which frequency. 6. Construct the detection circuit from Figure 1. Remember to include enough room so that the filter channel can fit between the LED and photodetector. Measurements: 7. Place only the diffuser in position in the filter holder and power both the red and green LEDs. View the waveform at the output to ensure that there is no saturation of the output signal. If saturation occurs (i.e., there is clipping of signal from the photodetector), adjust the driving circuit until there is none. The frequency spectrum should have peaks at 5 khz and 8 khz. Capture an image of the waveform and the frequency spectrum. 8. Place the corresponding optical filter for 5 khz in one of the channels of the filter holder. Aside from harmonics that result from driving a nonlinear device (the LED), only the 5 khz signal should be seen on the oscilloscope. Capture an image of the waveform with input and out signal. Capture an image of the spectrum analysis. Note: The color of the filter is an indication of what color light is passed through the filter unperturbed. The red color filter will allow red light to pass through it, but not green light. 9. Remove the 5 khz filter and place the corresponding optical filter for 8 khz in the channel. The only signal that should be seen is the 8 khz. Capture an image of the input and output waveform. Capture an image of the spectrum analysis. 10. Place the filters for both the 5 khz and 8 khz signals in the channel. No measurable signal should be observed at the output of the detector circuit. Capture an image of the input and output waveform. Capture an image of the spectrum analysis. 11. Remove all filters from the channel, leaving the diffuser in place. Construct the 5 khz and 8 khz RLC filter circuits designed above. Do not connect the filter circuit to the detector yet. 12. Use the Velleman oscilloscope to record the Bode plots of each RLC filter. Capture an image of the Bode plots. 13. Remove the 10 kω load resistor from the detector circuit and connect the detector output to the 5 khz bandpass filter input. Remember to take measurements from the filter output marked by the voltage marker. Capture an image of the input and output waveform. Capture an image of the spectrum 37

6 analysis. 14. Switch the detector circuit to the 8 khz bandpass filter circuit. Capture an image of the input and output waveform. Capture an image of the spectrum analysis. 15. Compare the spectrums of the bandpass filter circuits to the spectrum of the detector circuit collected in Step 7. 38