Model parameters for the MV2101 Varactor Diode. Common Collector Colpitts OscillatorTemperature Compensation of Resonant CircuitsCapacitorsInductorsVCO Design Project ECE145B Winter 2011 ________________________________________________________________________ Report due 2/18/2011 VCO DESIGN. Design, build, and test a voltage-controlled oscillator (VCO). The intent is that the VCO can be used as part of a VHF radio. Because the radio will be battery powered, you must meet the specs with as low operating current as possible. Power dissipation will be considered as part of the grade for this lab. You need to meet the following specs: Center frequency 135.5 MHz +/- 10 MHz Output frequency tuning range 3 MHz 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 -20 dBc minimum 1. The oscillator type that you will design is the common collector Colpitts using the 2N5179. Data sheet is on the course web page, and there is an ADS model in the RF Transistor Library/Packaged BJTs (pb_mot_2N5179_19921211). The oscillator design is related to the design described in Stanford notes Sect. 4-7.4 and 4-9.1 included in the supplemental reading. 2. The electrical tuning of the oscillator will make use of the MV2101 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 250 at 150 MHz and –4 volts reverse. 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 10μS. RFCCBR2RFCCBVtuneR1RFCCBR2RFCCBVtuneR1 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. There is a limited supply of NPO chip caps. Ask the TA for your required values. The inductor core material has a TC of 50 ppm/oC. 1Chip inductors are not recommended for the resonator because their unloaded Q is quite low, on the order of 20 to 30 at 100 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 = 7 for the type 12 material with 0.125 OD. Typical unloaded Q at 150 MHz is on the order of 100 and is better when turns are bunched together. (See the inductor tutorial on the course web page.) # turns=100L(μH)AL(μH / 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, however, harmonic Balance is the a 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. Use the ADS diode model with the attached model parameters to represent the varactor. Do a small-signal open loop AC analysis to find the loop gain vs. bias current. Compare analysis with the measured result. 4. The output from your oscillator must be buffered to avoid pulling the oscillator frequency with variations in the load impedance. Use a 2N5179 BJT as a buffer stage to provide this isolation and to drive the 50Ω output. Refer to the class notes on LO buffer amplifiers. 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 application will be battery powered, so don’t overdesign the buffer amplifier. Adding series resistance between the oscillator output and the buffer input has helped to improve harmonic distortion. 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 ΓS and ΓL. It 2has been found in the past that including some resistance in series with the base of the buffer amplifier improves stability and reduces harmonic distortion. 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 is used. 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. Meeting specs with the lowest power dissipation will be your target. The