Oscillator design for low phase noise

- an overview of the design of radio frequency, RF, oscillators for low phase noise levels.

One of the key requirements for any oscillator used in a radio receiver, radio transmitter, or many other applications is for the oscillator to perform with low levels of phase noise. Whether the oscillator is used in a frequency synthesizer, or in any other application, the basic design principles for achieving low phase noise are the same.

Poor levels of oscillator can manifest themselves in slightly 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. Additionally transmitters exhibiting a poor phase noise performance will tend to transmit wide band noise, causing interference to users on other frequencies.

Key points for oscillator design
There are some areas points to address when designing an oscillator to ensure that it has a good phase noise performance. By addressing these and other relevant points, the performance of the oscillator meets its requirements.

• High Q resonant circuit



• Choice of oscillator device



• Correct feedback level



• Sufficient oscillator power output



• Power line rejection

Oscillator design methodology
In order that the oscillator is able to provide the optimum phase noise performance it is necessary to implement these elements into the design of the circuit from the outset. Some aspects may affect the basic design criteria, and therefore need to be included from the concept stage of the circuit:

Q of the resonant circuit:    One of the major factors in determining the phase noise performance of an oscillator is the Q of the resonant circuit. Broadly, the higher the Q of the oscillator tuned circuit, the better the phase noise performance. This 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 active 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.

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 phase noise. In general bipolar transistors offer a lower level of flicker noise and as a result oscillators based around them offer a superior phase noise performance.

Oscillator 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 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 phase noise.

Summary
There are many elements to ensuring that an oscillator circuit design meets its requirements for low phase noise. The points provider here give a start to some of the basic decisions that are needed. Once initially realised, some refinement is likely to be needed to ensure the optimum performance is obtained.