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TEL 444 Wireless Systems Fall 2007 WESTERN CAROLINA UNIVERSITY DEPARTMENT OF ENGINEERINIG AND TECHNOLOGY TEL 444 WIRELESS SYSTEMS LAB #4 – Pierce Crystal Oscillator Performed on Monday, Oct. 8, 2007 Report Due Friday, Nov. 2, 2007 Equipment list: One 10 k variable resistor One 10 M resistor A collection of various resistors One 1 nF capacitor One 2N5485 Field Effect Transistor (FET) Breadboard Small printed circuit board 8.00 MHz crystal Radio frequency choke Soldering iron and solder 8-pin wire wrap socket Wire wrap tools Wire cutters Long nose pliers Oscilloscope Spectrum analyzer DC power supply Connecting leads BMC and N-type cables Parts for the DC regulator circuit (Optional): TPS7230 Three Volt regulator One 250 k resistor One 0.1 F capacitor 9 volt battery Battery clip and holder Procedure: 1) Connect a Pierce crystal oscillator circuit on a breadboard using an 8 MHz crystal. Cut the leads short to reduce noise sensitivity. 2) Provide a 3 volt DC power supply to the crystal oscillator circuit. 3) Monitor the output sine wave on the oscilloscope. Verify that the frequency of the generated sine wave is 8 MHz and the output voltage is at least 1 volt peak-to-peak. 4) Disconnect the output sine wave from the oscilloscope. Monitor the output sine wave on the spectrum analyzer. The spectrum analyzer has a 50 input impedance, which will excessively load the output signal. So monitor the output signal with the spectrum analyzer ground unconnected. TEL 444 Wireless Systems Fall 2007 5) Set the center frequency to 50 MHz and the frequency span to 100 MHz. Insert a 10 k variable resistor between the crystal and the capacitor. Adjust the variable resistor so that the harmonics are at least 10 dB below the fundamental. Be sure to take your hand away fronm the PC board and the variable resistor, before taking measurements on the spectrum analyzer. A screen shot of the frequency spectrum of a 11 MHz Pierce oscillator circuit is shown in Figure 1. Notice that the power of the 2nd harmonic (22 MHz) is about 14 dB below the power of the fundamental frequency (11 MHz), and the power of the 3rd harmonic (33 MHz) is about 24 dB below the fundamental frequency. The more you can reduce the harmonics while keeping the power of the fundamental frequency unchanged, the less distorted the generated sine wave will be. Figure 1. Frequency Spectrum of a 11 MHz Pierce Oscillator. 6) When you have a desirable frequency spectrum, pull out the variable resistor and measure its resistance. Measured resistance = ___________ 7) Find a fixed resistor or several resistors in series that closely match the measured resistance. Place the fixed resistor(s) in the circuit, and observe the frequency spectrum on the spectrum analyzer. If you do not have a desirable frequency spectrum, try different values of fixed resistance. TEL 444 Wireless Systems Fall 2007 8) When you have a spectrum that exceeds the 10 dB specification described in part 5, disconnect the spectrum analyzer and connect the oscilloscope to the generated sine wave. Verify that the frequency is 8 MHz and the output voltage is at least 1 volt peak –to-peak. If these requirements are met, continue to step 9. If the frequency is 8 MHz, but the output voltage is less than 1 volt peak –to-peak, then change resistors to get at least 1 volt peak-to-peak and then go back to step 7. 9) Layout the circuit on a printed circuit board. Start with the wire wrap socket and solder it is place. Use the socket for the FET. 10) Next layout the capacitor, your chosen resistor(s), and the crystal all in series on the board. One good way to lay them out is place them all in a parallel pattern, but solder them all in series on the reverse side. Solder one end to the FET gate and the other end to the FET drain. Solder connecting wires as necessary to do this. 11) Decide on a place to put DC power (3 volts) and ground. You may want to soledr a wire wrap pin for the + terminal and another wire wrap pin for the – terminal (ground). 12) Layout the 10 M resistor on the board so that one end will be near the FET gate and the othe end near ground. Solder this end to the FET gate and one end to ground. Solder connecting wires as necessary to do this. 13) Layout the coil on the board so that one end is near the FET drain and the other end near the + DC power terminal. Solder one end to the FET drain the other end to the + DC power terminal. 14) Solder the FET source to ground. Solder connecting wires as necessary to do this. 15) Solder a output test wire to the drain. 16) Connect the oscilloscope to monitor the output sine wave. 17) Connect 3 volts DC power to the circuit. 18) Monitor the generated sine wave using the oscilloscope. 19) Monitor the frequency spectrum of the generated sine wave using the spectrum analyzer. 20) If the circuit generates a sine wave, but does not meet the design specifications, try adding extra capacitance a various points in the circuit. 21) When the circuit is working to specification, save a screen shot of the frequency spectrum of the generated sine wave on the spectrum analyzer. Make sure the screen shot shows that all harmonic are at least 10 dB below the power of the fundamental frequency. Have the instructor inspect this step. Instructor Initials________ 22) When the circuit is working to specification, save a screen shot of the generated sine wave on the oscilloscope. Make sure the screen shot shows at least a 1 vol peak-to-peak sine wave. Have the instructor inspect this step. Instructor Initials________ TEL 444 Wireless Systems Fall 2007 Optional regulated power supply (if time permits or do outside of lab time) As an optional endeavor for extra credit, solder the 3 volt regulated power supply shown in Figure 2 onto your printed circuit board. Solder a 9 volt battery clip to the input of the regulator (pins 4 and 5). The 3 volts output is at pins 3 and 7. Solder this 3 volt output to your pierce oscillator circuit. Verify correct operation, and have the instructor inspect. Figure 2 – TPS7230 3 volt regulated power supply Lab Report In the theory section, discuss how crystals work and how crystal circuits work. In the results/analysis section, discuss the circuit implementation (how you designed, tested and built it), and present and discuss the results. In this section, include figures of: an oscilloscope screen shot of the generated sine wave an spectrum analyzer screen shot of the generated sine wave a scanner image or a photograph of the final circuit board a schematic diagram of the final design