High Frequency VCO Design

Description: This note will review the process by which Voltage Controlled Oscillator (VCO) designers select their oscillator's topology and devices based on performance requirements, component types, and DC power requirements. Basic oscillator design specifications typically necessitate a specific output power into a designated load at the design frequency. The drive level and bias current establish the fundamental output current, while the oscillation frequency is determined by the resonator components. Transistor selection should consider noise, frequency, and power requirements. The design may need to account for the parasitics of the device that affect resonator components as well as nonlinear performance specifications. All the VCO schematics presented below were practically built using the Infineon SiGe transistor BFP420, and any of them can be re-tuned for different frequency ranges by changing varicaps and LC tank values. The VCO must exhibit low phase noise to meet sensitivity, adjacent channel, and blocking requirements. In a digital modulation scheme, the VCO's phase noise impacts the bit error rate requirements. High pushing (the change of the oscillation frequency with supply voltage) can lead to phase noise degradation due to increased sensitivity to power supply noise. A buffer at the output is necessary to isolate the VCO from any output load variations (pulling) and to provide the required output power. Meeting the output power and load pull specification directly with a stand-alone oscillator would be challenging. However, this buffer amplifier requires a higher supply current. Alternatives include using circulators, isolators, or passive attenuators at the output. The tuning slope is the slope of the frequency to voltage tuning characteristic at any point and is synonymous with modulation sensitivity. The slope may be positive or negative. For a positive slope, the output frequency increases with an increase in tuning voltage, while for a negative slope, the output frequency decreases with an increase in tuning voltage. A monotonic tuning characteristic indicates that the frequency is single-valued at any tuning voltage and that the slope maintains the same sign across the tuning range. Tuning sensitivity as a function of tuning voltage serves as a measure of tuning linearity. For any specific application, it is necessary to specify the minimum and maximum tuning sensitivity. In the case of a VCO, frequency coverage may be limited since the influence of the feedback network is minimal compared to the active device itself. Conventional oscillator designs (with an LC circuit or transmission-line equivalent coupled to a negative-resistance active device) typically provide restricted frequency coverage and poor stability. Negative resistance can be easily obtained from most microwave transistors when considering chip and package parasitics. Tuning flatness becomes increasingly challenging as the VCO frequency range expands, potentially degrading power output flatness when adding an output filter to suppress harmonics. The active device's noise properties generally dominate the oscillator's noise characteristic limits. Since all noise sources, except thermal noise, are generally proportional to the average current flow through the active device, it is logical that reducing the current flow will result in lower noise levels. Narrowing the current pulse width in the active device decreases the time that noise is present in the circuit, thereby further reducing phase noise. There exists a trade-off between the Q factor of the oscillator, its size, and its cost. The low Q factor of an LC tank and its component tolerances necessitate careful design for phase noise without the need for individual readjustment of the oscillators. A bipolar transistor biased at a low collector current will maintain flicker noise characteristics.

The design of a Voltage Controlled Oscillator (VCO) is a critical aspect of many electronic systems, particularly in communication and signal processing applications. The choice of topology and components is influenced by multiple factors, including the required frequency range, output power, phase noise specifications, and the type of modulation used in the system.

Oscillator topology can vary significantly, with common choices including Colpitts, Hartley, and Clapp oscillators, each presenting unique advantages and disadvantages. The selection of resonator components, such as inductors and capacitors, is crucial for achieving the desired oscillation frequency and stability. The use of varactor diodes (varicaps) allows for fine-tuning of the oscillation frequency, enabling adaptability in various applications.

Transistor selection is paramount in VCO design. The BFP420 SiGe transistor, for example, is favored for its low noise characteristics and ability to operate at high frequencies. Designers must account for the noise figure of the selected transistor, as it directly impacts the overall performance of the VCO. Additionally, the effect of parasitic elements, such as stray capacitance and inductance, must be minimized to maintain the integrity of the oscillation.

Power supply considerations are also critical, as fluctuations in supply voltage can introduce phase noise and affect the stability of the oscillator. Incorporating a buffer amplifier helps mitigate these issues by isolating the VCO from load variations and ensuring consistent output power. However, this adds complexity to the design and may require additional current, which must be managed to prevent overheating and ensure reliability.

Tuning characteristics are another essential aspect of VCO design. The tuning slope indicates how the output frequency responds to changes in tuning voltage, and achieving a monotonic response is vital for predictable performance. A well-designed VCO should exhibit a flat output power across its tuning range, minimizing variations that could affect system performance.

Finally, careful consideration of the oscillator's Q factor is necessary, as it influences both frequency stability and phase noise. A high-Q tank circuit is desirable for minimizing phase noise, but it may come at the expense of increased size and cost. Ultimately, the design of a VCO requires a delicate balance between performance, size, and cost, necessitating a thorough understanding of the underlying principles and trade-offs involved.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 a specified load at the design frequency.

The drive le vel 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 as nonlinear 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 digital modulation 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. A buffer at the output is necessarily to isolate the VCO from any output load variations (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. 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 output frequency 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 of a 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 most microwave 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 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 will lead to lower noise levels. Narrowing the current pulse width in the active device will decrease the time that noise is present in the circuit and therefore, decrease Phase Noise even further.

There is a trade-off between the Q factor of the oscillator, its size and its price. The low Q-Factor of an LC tank and its component tolerances needs careful design for phase noise without individual readjustment of the oscillators. A bipolar transistor biased at a low collector current will keep the flicker