Transcript
Physics 196 Lab 15: AM Radio Receiver Equipment: Item
Part #
Oscilloscope Power cord for Oscilloscope Function Generator Power cord set for Function Generator Capacitor change-out box Oscilloscope probes Banana/Alligator cords red/black pair Electronics Breadboard 9 V battery with clip modified for board MCI 741 Operational Amplifier IC Capacitor Set (22 pF, 0.01μF, 22μF) 1 MΩ Resistor Wire and Form to make Inductor Stripped wires for breadboard, various Alligator/Alligator cords misc. colors Bare Speaker Amplified Speaker with cord
RIGOL DS1102E PASCO Extech 380405
Radio Shack
Qty per Team 1 1 1 1 1 1 2 pair 1 1 1 1 1 1 set 10 6 1 1
# of Teams
Total Qty Needed
8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
8 8 8 8 8 8 16 pair 8 8 8 8 8 8 set 80 48 8 8
Storage Location
Qty Set Out
Qty Put Back
Layouts:
Lab 15 equipment
Lab 15 Radio Receiver parts
Summary: Students will build a simple AM radio receiver and verify that it can be tuned to different stations. In the first experiment, the students will make their own inductor by winding wire into coils on a form such as a cut section of PVC pipe. This will be connected in series with a variable capacitor (provided by a switch-in box). A function generator will be used to excite the LC circuit with a 1 kHz square wave, and the voltage across the capacitor will be monitored with an oscilloscope. By measuring the oscillation frequency when the voltage switches, the circuit can be tuned to some known strong AM radio frequencies by varying the capacitance. Once the capacitance values have been selected, the LC circuit will be used as a front end filter between an antenna and an amplifier to select particular radio stations for monitoring. Using a voltage follower circuit and a speaker, it may be possible to detect the strongest stations. Further sound amplification will be provided with a second amplified speaker.
Prelab: Two of the stronger intensity AM radio stations which can be received at Miramar College are KCBQ (1170 kHz) and KFMB (760 kHz). We would like to tune to both of these stations (and more) by using a single inductor and varying the capacitance in a resonant LC circuit. The resonant frequency of an LC circuit is given by f=1/2π(LC)1/2. If we wish to receive KCBQ using a 700 pF capacitor, what should be the value of the inductance? What oscillation period would we try to observe on the oscilloscope to tune to this frequency (T=1/f)? If the inductor is wound on a 4 cm diameter tube, and the inductance is given approximately by L=NAμ0/d, and μ0 = 4π x 10-7 Tm/A, and the cladded wire diameter d is 2mm, about how many turns N are needed to achieve the calculated inductance? (Note, the A in the formula for L is the cross-sectional area of the tube, not to be confused with the units A in μ0, which are Amps.
Lab: In your lab notebook, please include a description and labelled diagram of each part of the experiment, sketches of input and output waveforms, calculations, a discussion of the results in terms of expectations, and a conclusion. Experiment 1, LC resonance: After checking your prelab calculations with your instructor, wind an inductor with the appropriate number of turns to achieve your desired inductance (to tune to 1170 kHz with a 7 pF capacitor). Attach your inductor in series with the capacitor switch-out box, excite the LC circuit with a function generator set to a 1 kHz, 1V peak square wave, and observe the voltage across the capacitor with the oscilloscope. You should be able to see voltage oscillations on the oscilloscope when the function generator switches. For instance, in the oscilloscope trace below the horizontal scale is 500 ns per division, and the observed oscillation period is slightly under 1 μs (so frequency is slightly greater than 1 kHz).
Record the actual capacitance values needed with your inductor to achieve resonances at 1170 kHz and 760 kHz. Note that these may not be exactly as you expected (because your inductance will differ from your prediction), but they should be close. The number of oscillations is a measure of how narrow the resonance is. The more oscillations, the more precisely the radio can be tuned. Experiment 2, AM Receiver: The following diagram and photograph show a layout in which the Op Amp can be used as a voltage follower to rectify the signal across the LC circuit when it is hooked to an antenna, without introducing a lot of resistance which would affect the selectivity of the receiver. The input voltage through the 0.01 μF capacitor is an AC signal at the resonant frequency, modulated at much slower audio frequencies. Since the OpAmp is being run only off of a positive voltage (9V battery), the output has a net positive voltage which accumulates on the 22 μF capacitor, and drives current through the speaker with audio frequency variations. Build this circuit, keeping the leads of your inductor and capacitor fairly short to avoid making an extra antenna between the LC circuit and the amplifier. When you are ready, have your instructor check your circuit.
Now hook up your circuit (with speakers) to the antenna and Earth ground in the room (teams will have to take turns). Tune the variable capacitance (in switch-out box) to the values measured in Experiment 1. If you are lucky, you will be able to hear a signal with just the bare speaker. By attaching the second (Radio Shack) speaker in parallel, you should get a louder signal. See how many stations you can tune to by switching the capacitance, and document the capacitance values where you received decent signals. (Today I was able to bring in about 8 stations clearly at my office, including KCBQ and KFMB).
