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.
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 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.
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 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 drive level should consider the trade-off between harmonic content, oscillator stability, and noise.
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 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.
Maximize the loaded Q of the tuned circuit in the oscillator.
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.
Usually a larger resonator will have a higher Q (e.g. a quarter wavelength coaxial resonator).
Choose an active device that has a low flicker corner frequency.
A bipolar transistor biased at a low collector current will keep the flicker corner frequency to a minimum, typically around 6 to 15 KHz (Most semi-conductor manufacturers can provide the frequency corner (fc) of their devices as well as the 1/f characteristic.
Maximize the power at the output of the oscillator.
In order to increase the power at the input of the oscillator, the current has to be increased. However, a low current consumption is critical to preserving battery life and keeping a low fc. In a practical application, the current will be set based on output power required to drive the system (typically a mixer), and then the Phase Noise will need to be achieved through other means.
Choose a varactor diode with a low equivalent noise resistance.
The varactor diode manufacturers do not measure or specify this parameter. The best approach is then empirical; by obtaining varactors from several vendors and experimentally finding out which one yields the lowest phase noise in the VCO circuit and thus has the lowest equivalent noise resistance.
There are two basic types of varactors: Abrupt and Hyperabrupt.
– The abrupt tuning diodes will provide a very high Q and will also operate over a very wide tuning voltage range (0 to 60 V). The abrupt tuning diode provides the best phase noise performance because of its high quality factor.
The hyperabrupt tuning diodes, because of their linear voltage vs. capacitance characteristic, will provide a much more linear tuning characteristic than the abrupt diodes. These are the best choice for wide band tuning VCO’s. An octave tuning range can be covered in less than 20 V tuning range. Their disadvantage is that they have a much lower Q and therefore provide a phase noise characteristic higher than that provided by the abrupt diodes.
Keep the voltage tuning gain (Ko) to the minimum value required.
This is the most challenging compromise because the thermal noise from the equivalent noise resistance of the varactor works together with the tuning gain of the VCO to generate phase noise. This compromise will be the limiting factor determining the phase noise performance.
Noisy power supplies may cause additional noise. Power supply induced noise may be seen at offsets from 20 Hz to 1 MHZ from the carrier. If the VCO is powered from a regulated power supply, the regulator noise will increase depending upon the external load current drawn from the regulator. The phase noise performance of the VCO may degrade depending upon the type of regulator used, and also upon the load current drawn from the regulator. To improve the phase noise performance of the VCO under external load conditions it is always a good design philosophy to provide RF bypassing of power and DC control lines to the VCO. RF chokes and good bypassing capacitors (low ESR) is recommended at the DC supply lines. This will minimize the possibility of feedback between stages in a complex subsystem. Improved bypassing may be provided by incorporating an active filter circuit.