Phase Noise Measurement

Wenzel Associates employs several in-house systems for measuring
phase noise, including our new Model 501-12922B that features superior noise
performance:

click on image for higher resolution

The measurement technique compares the phase of an oscillator
under test to a reference oscillator with similar or superior noise performance.
An ordinary PLL holds the relative phase of the two oscillators at quadrature so
that small phase variations convert into voltage variations at the output of the
mixer. Although the PLL is constantly working to eliminate these phase
variations, the time constant is set long enough to preserve the slowest phase
variations of interest. The mixer output is connected through a low pass filter
to block the RF frequencies to a phase lock amplifier and to a signal analyzer.
The signal analyzer gives the power spectral density of the audio noise which is
easily converted to phase noise by dividing by the phase slope of the mixer
(volts/radian).

The phase slope of the mixer may be determined in a number of ways
including the observation of a beat-note on the oscilloscope or applying a known
frequency modulation to the reference oscillator.

The noise output from the mixer can be quite low requiring
amplification before spectrum analysis. Our custom amplifier exhibits less than
1nV/root-Hz down to 10 Hz and our custom mixers exhibit similar performance with
phase slopes near 1 volt/radian or greater.

An early engineer's setup is shown below:

This bench features a low-noise amplifier/PLL (white box bottom
left) with easily changed loop components. A backboard includes special low
noise power supplies, a multi-purpose meter, electrical tuning gain
potentiometer, and a troubleshooting audio amplifier/speaker. A reference
oscillator and multiplier chain is on one side of the mixer and the other side
awaits a source to be measured. The analyzer is on the high shelf above to keep
the noisy monitor away from the oscillators. The power supply features shielded
transformers located on the floor to keep magnetic fields away from the
oscillators. When better screening is desired, the designer can retreat to one
of the "caves":

Another production system successfully incorporates the power
supply into the chassis with the amplifiers:

Special power transformers and careful grounding virtually
eliminates line-related spurs. Special setups are often required to test phase
noise under various environmental conditions including temperature, vibration
and even high vacuum for space simulation (two systems below).

A new vibration system allows the testing of large assemblies or
instruments. The photo below shows a tiny oscillator mounted on the vertical
test table and a special fixture on the slip table.

See Also: Low-Cost Phase
Noise Measurement