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UCSB ECE 145b - VCO DESIGN PROJECT

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VCO Design Project ECE218B Winter 2011 ________________________________________________________________________ Report due 2/18/2011 VCO DESIGN GOALS. Design, build, and test a voltage-controlled oscillator (VCO). 1. Design VCO for highest center frequency (< 400 MHz). 2. At least 10 MHz tuning range. In addition, you need to observe the following specs: Varactor tuning voltage range 1 to 5 volts Supply voltage +6 V Output power -3 to 0 dBm in 50 ohm load Second and third harmonic -25 dBc minimum 1. You will determine the oscillator type that you will design based upon achieving a center frequency as high as possible but below 400 MHz. You decide on either common gate/base or common drain/collector configuration. You can use either the J310 N-channel JFET or 2N5179 for the oscillator and the 2N5179 BJT for the buffer amp. Data sheets are on the course web page, and there are ADS models for these devices in the RF Transistor Library/Packaged BJTs (pb_mot_2N5179_19921211) and Analog Parts Library (ap_NJF_J310_19930601). 2. The electrical tuning of the oscillator will make use of the BB131 varactor diode. Connect two of these diodes back-to-back for improved harmonic distortion. Refer to the data sheet on the course web page. The varactor Q is typically 150 at 200 MHz and 5 volts reverse bias. You should avoid biasing the varactor under 1 volt reverse bias so that the Q remains high. Isolate the bias port with an RF choke, series resistor (to De-Q the choke), R1 to isolate CB from the tuning port, and bypass capacitor (choose value for series resonance at 135 MHz). The time constant of R1CB should be no larger than 10 S. The varactor diode TC is shown on the data sheet – roughly +250 ppm/oC. This is an unacceptably high TC for the entire resonator, however, the varactor provides only a small part of the total capacitance. You should implement the fixed capacitors in the resonator with zero TC (NP0) capacitors. The inductor core material has a TC of 0 ppm/oC. RFCCBR2RFCCBVtuneR1RFCCBR2RFCCBVtuneR12 Chip inductors are not recommended for the resonator because their unloaded Q is quite low, on the order of 20 to 30 at 200 MHz, therefore you will fabricate wirewound inductors on toroidal core material. The inductance of the toroidal inductor can be estimated by the equation below; use an AL = 3 for the type 0 material with 0.125 OD. Typical unloaded Q at 200 MHz is on the order of 100 and is better when turns are bunched together. # turns 100L( H)AL( H / 100turns) Verify that your inductance is correct with the network analyzer to avoid needless frustration. The formula is only approximate. A cylindrical wire coil can also be used, but will be sensitive to bending and position and will also have a temperature coefficient. 3. Do a detailed hand analysis of your oscillator, predicting startup conditions and the oscillation amplitude. Do not use high bias current for your oscillator – this is neither necessary nor desirable for good startup and stability and will increase power dissipation. Include the 5V voltage regulator to power your oscillator. The oscillator is intended to operate from a 6V battery, and you want the VCC to remain constant as the battery voltage drops. The hand analysis should be followed by ADS simulations before attempting to build the oscillator. A large-signal nonlinear analysis on the closed loop oscillator can be done by transient analysis or harmonic balance. Starting the oscillator in the transient simulator will require an impulse of current at the resonator since there is no naturally occurring noise to cause the oscillator to start. Harmonic Balance is the faster tool for oscillator simulations. Explore the bias current as a variable in the design. Make sure your design limits in cutoff (current limited) rather than saturation. Do a small-signal open loop AC analysis to find the loop gain vs. bias current. Compare analysis with the measured result. (note that the simulation will be optimistic unless you include estimates for the PC board parasitic capacitances). 4. Tuning range. Predict tuning range with hand analysis. Then, build an ADS diode model from the information on the BB131 data sheet. Use this model for simulation of the VCO tuning range. 4. The output from your oscillator must be buffered in order to avoid pulling the oscillator frequency with variations in the load impedance and to drive a low load impedance. Use a 2N5179 BJT as a buffer stage to provide this isolation and to drive the 50 output. Either a CE or CC stage can be made to work. The oscillator output amplitude will most likely be too high, so a series resistance between the oscillator output and the buffer amp input can be used to reduce the voltage swing and improve harmonic distortion. Make sure your bias conditions are within the acceptable peak current and voltage of the device according to the data sheet on the web site. The final application3 only requires -3 to 0 dBm (0.32V) into a 50 ohm load, so don’t overdesign the buffer amplifier. Design the buffer amplifier so that it will provide the required output drive power while using the least DC power. You should perform a small signal stability analysis and modify the basic amplifier circuit to assure stable operation at the expected source and load impedances. Note that the amplifier need not be unconditionally stable, since you can control the S and L. 5. Implementation. You can use the generic VCO PCB to implement your oscillator. A plot of the board top layer is attached. It is recommended that you sketch on the plot which components are to be installed at each needed location. Note that some locations may remain empty depending on which configuration you choose. Solder down your components, keeping leads short, and build the oscillator. You will need to use both leaded and chip capacitors and resistors. Remember to use the NP0 caps for resonator components. Some layout examples can be found on the course website. You will need some additional components beyond what is in the parts kit for implementation of your design. Prepare a parts list of what you will need beyond your parts kit, and take this to the electronics shop or check with the TA. Include the hand analysis, the ADS simulations, and well-documented final design in your report. The following measurements should be made and documented in the report: You can


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UCSB ECE 145b - VCO DESIGN PROJECT

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