High Frequency VCO Design and Schematics
Iulian Rosu, YO3DAC / VA3IUL, http://www.qsl.net/va3iul/
This note will review the process by which VCO (Voltage Controlled Oscillator) designers choose their oscillator’s topology and devices based on performance requirements, components types and DC power requirements. Basic oscillator design specifications often require a given output power into aspecified load at the design frequency. The drive level and bias current set the fundamental output current and the oscillation frequency is set by the resonator components. Transistor selection of the transistor should consider noise, frequency, and power requirements. Based on the particular device, the design may account for parasitics of the device affecting resonator components as well asnonlinear performance specifications. All the VCO schematics presented below were practical build using the Infineon SiGe transistor BFP420, and any of them can be re-tuned for different frequency ranges changing varicaps and LC tank values.
• The VCO must exhibit a low Phase Noise in order to meet the Sensitivity, Adjacent Channel and Blocking requirements. In digitalmodulation scheme the VCO’s Phase Noise affects the Bit Error Rate requirements. High Pushing (change of the oscillation frequency with supply voltage) can cause Phase Noise degradation due to increased sensitivity to the power supply noise. Phase Noise varies typically by 3dB with temperature, in the –55ºC to +85ºC range. A buffer at the output is necessarily to isolate the VCO from any output loadvariations (Pulling) and to provide the required output power. Meeting simultaneously the output power and load pull specification directly with a stand-alone oscillator would be difficult. However, this buffer amplifier requires a higher supply current. Alternative would include to use at the output circulators, isolators or passive attenuators. VCO output power is usually measured into a 50 ohm load.Output power requirements specified in dBm, and tolerances vs tuning frequency in ± dB. The tuning slope is the slope of the frequency to voltage tuning characteristic at any point and is the same as modulation sensitivity. The slope could be positive or negative. For a positive slope, the output frequency. increases as the tuning voltage increases. Similarly for a negative slope, the outputfrequency decreases as the tuning voltage increases A monotonic tuning characteristic means that the frequency is single valued at any tuning voltage and that the slope has the same sign across the tuning range. Tuning sensitivity as a function of tuning voltage is a measure of tuning linearity. For any given application, have to specify the minimum and maximum of the tuning sensitivity. In the case ofa VCO, the frequency coverage is rather restricted since the influence of the feedback network is small compared to the active device itself. Conventional oscillator designs (with a LC circuit or transmission-line equivalent coupled to a negative-resistance active device will only provide a restricted frequency coverage and poor stability). A negative resistance can easily be obtained from mostmicrowave transistors when considering chip and package parasitics. Tuning flatness - As the VCO frequency range is increased, the difficulty to achieve a flat output power is increased. Adding an output filter to suppress harmonics may in some cases degrade power output flatness. The drive level should consider the trade-off between harmonic content, oscillator stability, and noise.
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In order to lower the VCO Phase Noise, a number of rules should be respected: • The active device has noise properties which generally dominate the noise characteristic limits of an oscillator. Since all noise sources, except thermal noise, are generally proportional to average current flow through the active device, it is logical that reducing the current flow through the device...
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