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.
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