Analog and Digital Electronics
LAB 8: Signals & Filtering. April 11 & 18
I want you to drive an LED with the function generator and detect it with a photodetector (a photodiode). The goal is to see if you can detect the LED from across the room. (Pretend you are making a light tag toy or a voice communication device.)
Transmitter
I recommend you build the transmitter circuit first. Initially it will consist of the circuit at the right. Use one of the high intensity LED’s. At first the input will come from a function generator, but later you will add a multiplier circuit and an amplifier circuit to go with a microphone. Leave room for those! Put this on the right side of the breadboard. I recommend you start with an R=2.2k. You will probably lower it to 1.5k or 1.2k later. Check it out by giving it a ± 8V signal at 1Hz from the function generator. Can you see it dim and brighten? Show me what you have at this time.
Detector
Build the Photodetector circuit shown at the right. Put it on one of the “larger” breadboards. Note that the diode is reversed biased! For the resistor R, use something between 100kW and 220kW. If it is too large, ambient light may drive you “off scale”. You are going to drive the LED at between 25kHz and 60 kHz. This will be your carrier frequency. Eventually you may want to use a parallel LC circuit instead of R. L= 4.7mH and C =3.3nF would give a resonant frequency around 40kHz, (You could also try 4.7mH with a 2.2nF capacitor if you want a higher frequency.) If you do use the LC circuit, put a small resistor in series with the power supply to protect the diode from an accidental reversal of the diode. If you do use an LC circuit, choose your carrier frequency equal to the LC circuit’s resonant frequency. (It is easier to test the photodiode with the resistor instead of the LC circuit, so start out with that and then switch to the LC after you’ve verified that it’s working properly.)
For the op amp, I suggest a gain of 5 to 10. (You can make it slightly larger if you use the LC circuit.) Use a capacitor around the feedback resistor and choose RGC to correspond to about 10kHz above your chosen frequency. Again you will be adding to this, so leave room on the board! Align the LED and photodiode and drive the LED at between 25kHz and 60 kHz. Can you see the driving signal at the output of the amplifier? (Let me see your circuit at this time.)
After the first amplification you will need to filter the signal to remove both low and high frequency noise. I recommend the band pass filter at the right which I discussed in class. Try a Q=3 or 4 and a center frequency of equal to your operating frequency. (If you are operating at higher frequencies, I can give you a slightly faster op amp.) After adding this you should have a much clearer view of the signal. (Adjust the frequency of the function generator to give the maximum signal.) Again, let me see your circuit. You may need more amplification and filtering. If you need more filtering, use another band pass filter with the same resonant frequency as the band pass above. If you need more amplification, just put it in.
Adding Voice Modulation to the Transmitter
Now add the modulator and microphone to your transmitter. That circuit is shown at the right. The microphones are electret microphones. They have an FET inside to provide some buffering, since the electret itself is a high impedance source. The connections you make are the ones outside the circle. The op amp amplifies the signal by 11 (you may need to increase it so the voltage variation is about 1 to 2 V peak to peak) and provides an offset (the 15k resistor) so that the signal is always negative. (You may need to reduce the value of this 15k resistor slightly to produce enough offset.) This is important in the modulation scheme; you want the signal that goes to the modulator (multiplier) to always be of one sign, in this case negative. (It will make the demodulation on the other end easier.) Let me see your circuit at this time
To add the modulation you need to add a multiplier, a circuit that has two input voltages and multiplies them together. The AD633 does just that. The output of the amplifier above (microphone circuit) goes to pin 1, pins 2 and 4 are grounded, the carrier from the function generator goes to pin 3. Pin 8 is the positive power supply, pin 5 is the negative supply and pin 7 is the output, which goes to the transistor that drives the LED. Now when you talk into the microphone, the electrical signal it generates will modulate the amplitude of the signal from the function generator.
It is often helpful to replace the microphone by another function generator, especially if you have trouble producing a consistent signal from the microphone. If you do, try using a triangle wave of 200mV peak to peak at about 300Hz. (You may need to use the – 20dB attenuation button on the function generator to get a signal this small.)
Adding a Demodulation Circuit to the Detector.
To demodulate the AM signal and recover your voice, use the circuit at the right, which uses a diode and RC filter. Choose RC so that the carrier (25 to 60 kHz) is filtered out and the modulation signal (a few hundred Hz to 3kHz) gets through, but you want R/10 ≈ 1.5kW to 2.2kW. The signal and carrier are close together so you may need two RC sections or a Sallen Key low pass filter to get adequate filtering. Let me see your circuit at this time.
The final part is to move the transmitter and detector away from each other and see how far you can separate them and still detect the transmission, i.e. your voice.
Write-Up Instructions for Lab 8
The first thing to do is make schematics for the circuits you used and label their function, i.e. amplifies signal, or band pass filter etc. After that you should sketch or take pictures of the signal(s) at the following points in your circuits. (The run/stop button on the scope holds the screen for a stable display. It can come in handy for the instructions below.)
Transmitting Board
1. The “signal” from the microphone going to the multiplier. You might try to get a single frequency of around 300Hz.
2. The signal coming out of the multiplier. If you take a picture you can display both of these, 1 & 2, on the scope when you take the picture.
Receiving Board
1. The signal just after the first amplifier. (This one is not essential since it is probably very similar to #2 below.)
2. The signal just after the band pass filter.
3. The signal just after the rectifying diode.
4. The signal just after the low pass filter.
Again if you are taking pictures, display #2 from the transmitting board simultaneously when taking pictures of 2 & 3. For #4, simultaneously display #1 from the transmitting board on the scope when taking a picture of #4. Be sure to label what each trace represents and what the vertical scales are (Volts per division).