1. Each group will get one aluminum BUD chassis (also called BUD box ).

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1 I. INTRODUCTION At the beginning of this lab, each group will be given an aluminum box called a BUD box or a BUD chassis. ( BUD is just the name of a company that makes these boxes.) Each BUD box has a breadboard mounted in it so that you can easily build circuits inside the box. Each box also has two potentiometers mounted on the front bulkhead (the front surface) of the box, connectors for DC power on the rear bulkhead, and at least one BNC connector for signals. There are holes drilled to accomodate additional components that we will install as needed. You will be building circuits in these boxes for the next two weeks, probably. Our immediate goal is to build and investigate a Q multiplier (which can also be an oscillator, if the positive feedback is large enough). We will then add circuitry to the Q multiplier/oscillator to make it an AM receiver, and have some fun with this circuit. But these exercises are not just for fun, and they re NOT unrelated to electron spin resonance (ESR) and nuclear magnetic resonance (NMR). If you were to make certain minor modifications to the AM receiver circuit you will build, you would have a circuit identical to the one that you will use to detect ESR and NMR absorption signals a little later in the course. II. TUNING WITH VARACTORS AND THE BEGINNINGS OF POSITIVE FEEDBACK 1. Each group will get one aluminum BUD chassis (also called BUD box ). 2. You will need to solder leads onto binding posts, potentiometers (pots), and BNC connector(s). Also, put knobs on the pots. 3. Using the breadboard mounted in the BUD box, build the circuit of the tuned inductor shown in Figure Set the pot near the middle of its range. Observe the input and output signals on your scope (look at these two signals simultaneously). Sweep the frequency of the signal generator through some large range and look for evidence of a resonance. When you have found the resonance, record the resonant frequency in the space below. Resonant frequency ( f 0 ): C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 1

2 5. Measure the resonant frequency at a few pot settings (you could, for example, turn the pot all the way counterclockwise, then put it somewhere in the middle, then turn it all the way clockwise). Record the pot settings and the corresponding resonant frequencies in the space below. What happens to the resonant frequency as you increase the bias voltage (i.e., as you make the varactor more reverse biased)? What must have happened to the junction capacitance of the varactor (as you increased the bias voltage)? 6. Measure and record the lowest and highest frequencies you can tune to using only the varactor bias. (Don t change the inductor or any other components.) Lowest frequency: Highest frequency: 7. Compare the amplitudes of the input and output signals at the lowest and highest frequencies you can tune to. Then comment on your results: Which is larger? By what factor? C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 2

3 Figure 1. Setup for Part II. sinewave 200 mvp-p f =? (you choose) BNC OUT to scope (X10) IN to scope same coil as in Lab #5 (big one!) NO NODE HERE!! BUD box C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 3

4 III. THE SOURCE FOLLOWER 1. In the space just to the right of your tuned circuit on your breadboard, build the source follower shown in Figure Look at the input and output signals simultaneously on the scope. 3. Measure and record the voltage gain and phase shift at 500 khz, 1.5 MHz. (These frequencies are the lower and upper limits of the AM broadcast band.) Voltage gain (500 khz): Phase shift (500 khz): Voltage gain (1.5 MHz): Phase shift (1.5 MHz): Now make similar measurements of gain and phase shift at 4 MHz and 15 MHz. Voltage gain (4 MHz): Phase shift (4 MHz): Voltage gain (15 MHz): Phase shift (15 MHz): In the space below, comment on how the gain and phase shift depend on frequency. 4. Now take the output from the wiper of the 2.5-k pot. Adjust the pot through its entire range. Comment on how changes in the pot setting affect the gain. Move the RC coupling network to the new output terminals before you make these observations. C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 4

5 Figure 2. Source follower. 200 mvp-p (sinewave) Signal Generator IV. THE Q MULTIPLIER AND RINGDOWN REVISITED 1. Connect your tuned inductor circuit to your source follower as shown in Figure 3. This establishes (for the first time in this experiment) positive feedback, and you now have a Q multiplier. Take the output from the source of the MPF102 (RC coupled, as shown in Figure 3). 2. Now retrieve the small primary coil (with the 47-ohm resistor in series with it) that you used in Lab #5. Connect this coil to your function generator as you did in Lab #5, and shock excite the larger inductor (L2 in Figure 3). Set the function generator rep rate to a few hundred hertz. Look at the function generator output on one channel of your scope and the output of your Q multiplier (see Figure 3) on the other channel. Adjust the sweep speed of the scope so that you can see the damped sinusoidal oscillations in a single ringdown. 3. Vary the 2.5-k pot through its entire range repeatedly and watch the Q multiplier output. What happens to the half-amplitude decay time when you move the wiper of the pot toward the source of the MPF102 (i.e., when you increase the positive feedback)? C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 5

6 4. What happens to the half-amplitude decay time when you turn the 2.5-k pot all the way up (i.e., when you put the wiper at the source of the MPF102)? 5. Move the 2.5-k pot to its lowest setting. Measure the half-amplitude decay time. Record it below. Half-amplitude decay time (no positive feedback): 6. Switch the function generator to the sinewave mode and measure the resonant frequency of the circuit. Record it below. Resonant frequency f 0 (no positive feedback): 7. Calculate the Q of the circuit. Record it below. Q (no positive feedback): 8. Now set the 2.5-k pot at the highest setting for which you can still measure the half-amplitude decay time. 9. Measure and record the half-amplitude decay time. Then measure the resonant frequency and calculate Q. Half-amplitude decay time (maximum positive feedback): Resonant frequency f 0 (maximum positive feedback): Q (maximum positive feedback): 10. Compare your two measured values of Q and comment on how increasing the positive feedback affects Q. (Use the back of this sheet if you need to.) C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 6

7 Figure 3. Q Multiplier/Oscillator. BNC same coil as in Lab #5 (big one!) BUD box C:\TEACHING\PHYS426\SPRING00\LAB5.DOC Page 7