Good Practice Guide to Phase Noise Measurement ... - Sequanux

Measurement Good Practice Guide No. 68
5 Quadrature method
In the Quadrature System, two oscillators at identical frequencies are used.
One of the oscillators will be the source being tested and the other will be a reference source
whose performance is known to be better than the source under test. The sources are
combined in a mixer and the resulting output signal is filtered and amplified by a Low Noise
Amplifier (LNA). A Fast Fourier Transform (FFT) Analyzer or a Spectrum Analyzer
typically measures the output from the mixer.
Source
under
test
Mixer
Reference
source
Low
pass
filter
LNA
Spectrum analyzer
or
FFT analyzer
Figure 14: Block diagram of a phase noise measurement system using the Quadrature method
The system behaves as a phase detector if the input signals to the mixer are adjusted to be in
phase quadrature. When they are in quadrature the nominal DC output voltage from the mixer
is 0 Volts. The output from the mixer is a direct measure of the phase difference between the
two signals, and so it is also a direct measure of the phase noise of the UUT (assuming the
reference source is better than the UUT).
Setting the sources to be in phase quadrature is not always very easy. If both frequency
sources are synthesizers with good long-term stability, then there is usually not a great
problem. Phase quadrature can usually be achieved by offsetting the frequency of one of the
sources, during which time the system will produce an AC voltage at the same frequency as
the offset frequency, and then resetting the frequency to be the same. The synthesizers will
settle to a new phase that can be checked to see if it is close enough to phase quadrature,
typically a voltage less than 5% of the peak voltage from the phase detector when the
frequencies are offset 9 . If one of the sources has fine frequency control (e.g. 0.1 Hz) the
frequency can be set to a small offset and then returned to the required frequency when the
correct phase is achieved. Alternatively, many modern synthesized sources include phase
adjustment controls that can be used to adjust the phase more easily.
However, in the many applications, measurements are undertaken under less than ideal
conditions, and a feedback system has to be used to maintain phase quadrature. The feedback
system forms a phase locked loop that controls the frequency/phase of one of the oscillators to
9 This assumes the signal is larger than any residual offsets from the phase detector. More on this can be found in the section on measurement
uncertainty.
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