VCO Phase Noise
- an overview or tutorial about VCO phase noise and how to design a VCO for optimum phase noise performance.
VCO phase noise is a key parameter in the voltage controlled oscillator used for applications including use in frequency synthesizers for radio receivers, transmitters and RF signal generators. VCO phase noise is a key specification parameter for any VCO design as the phase noise performance of a VCO will affect the overall performance of the system in which the oscillator is located.
Poor levels of VCO phase noise can manifest themselves in different ways. For an analogue radio receiver a poor performance oscillator may result in poor reciprocal mixing performance. It may also raise the noise floor of the receiver. In a radio system relying on phase modulation, phase noise will degrade the bit error rate performance.
For transmitters, a poor level of phase noise performance will result in noise being transmitted beyond the required transmit band, causing interference to users on other frequencies. Again it can result in poor levels of bit error rate in a radio communications system.
Additional RF signal generators will look for as "clean" a signal as possible. Phase noise if a key parameter for the performance of the signal generator, and in turn the VCO phase noise performance is elemental in determining areas of the overall phase noise performance.
VCO phase noise basics
Phase noise is present on all signal sources to a greater or lesser degree. Some forms of oscillator are better than others, those that use higher Q circuits and have smaller tuning ranges tend to offer a better phase noise performance, however the requirements for VCOs tend to require wide tuning ranges, and this makes their design more challenging.
Note on Phase Noise:
Phase noise consists of small random perturbations in the phase of the signal, i.e. phase jitter. An ideal signal source would be able to generate a signal in which the phase advanced at a constant rate. This would produce a single spectral line on a perfect spectrum analyzer. Unfortunately all signal sources produce some phase noise or phase jitter, and these perturbations manifest themselves by broadening the bandwidth of the signal.
Click on the link for a Phase Noise tutorial
Low phase noise VCO design key points
To ensure that a design provides the optimum VCO phase noise performance, there are a number of key points that can be followed in the design. These will help ensure that the design is able to provide a high level of VCO phase noise performance, although even when the design ahs been realised, there is still likely to be some optimisation required to provide the best performance.
- High Q resonant circuit: One of the major factors in determining the VCO phase noise performance is the Q of the resonant circuit. Broadly, the higher the Q of the oscillator tuned circuit, the better the VCO phase noise performance. Thus inductors should be chosen to provide the highest Q, as should the capacitors. This is particularly true of voltage controlled oscillators, VCOs where the varactor diodes normally employed have a lower Q than other capacitors.
Typically high Q tuned circuits do not have the tuning range of lower Q circuits. This means that when wide tuning ranges are required, it becomes more difficult to obtain a high level of Q and hence the optimum phase noise.
As an illustration of the effect of having a high Q resonant circuit in an oscillator, crystal oscillators exhibit very low levels of phase noise as a result of the fact that the crystals used in them possess very high levels of Q.
- Choice of oscillator device: It is possible to use both bipolar devices and FETs within an RF oscillator, using the same basic circuit topologies. The bipolar transistor has a low input impedance and is current driven, while the FET has a high input impedance and is voltage driven. The high input impedance of the FET is able to better maintain the Q of the tuned circuit and this should give a better level of performance in terms of the phase noise performance where the maintenance of the Q of the tuned circuit is a key factor in the reduction of phase noise. That said, many bipolar transistor designs are able to offer excellent phase noise performance.
Another major factor is the flicker noise generated by the devices. Oscillators are highly non-linear circuits and as a result the flicker noise is modulated onto the oscillation as sidebands. This manifests itself as VCO phase noise. In general bipolar transistors offer a lower level of flicker noise and as a result oscillators based around them often offer a superior phase noise performance.
- Correct feedback level: A critical feature in any oscillator design is to ensure that the correct level of feedback is maintained. There should be sufficient to ensure that oscillation is maintained over the frequency range, over the envisaged temperature range and to accommodate the gain and parameter variations between the devices used. However if the level of feedback is too high, then the level of VCO phase noise will also be increased. Thus the circuit should be designed to provide sufficient feedback for reliable operation and little more.
- Sufficient oscillator power output: It is found that the noise floor of an oscillator is reasonably constant in absolute terms despite the level of the output signal. In some designs there can be improvements in the overall signal to noise floor level to be made by using a high level signal and applying this directly to the mixer or other circuit where it may be required. Accordingly some low noise circuits may use surprisingly high oscillator power levels.
- Power line rejection: It is necessary to ensure that any supply line or other extraneous noise is not presented to the oscillator. Supply line ripple, or other unwanted pickup can seriously degrade the performance of the oscillator. To overcome this, good supply smoothing and regulation is absolutely necessary. Additionally it may be advisable to place the oscillator within a screened environment so that it does not pick up any stray noise. It is worth remembering that the oscillator acts as a high gain amplifier, especially close to the resonant frequency. Any noise picked up can be amplified and will manifest itself as VCO phase noise.
There are many elements to ensuring that VCO phase noise performance can be optimised. With phase noise being a key parameter in many situations, it is essential that the VCO phase noise performance meets its requirements. In this way, the performance of the overall system will not be degraded by the VCO phase noise performance.
By Ian Poole
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