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8
Residual Measurement Fundamentals
Calibrating the Measurement
8-8
E5500 Phase Noise Measurement System Version A.02.00
3
When making low-level measurements, the best results will be
obtained from uncluttered setups. Soft foam rubber is very useful
for isolating the UUT and other phase-sensitive components from
mechanically-induced phase noise. The mechanical shock of
bumping the test set or kicking the table will often knock a
sensitive residual phase noise measurement out of quadrature.
4
When making an extremely sensitive measurement it is essential
to use semi-rigid cable between the components. The bending of a
flexible cable from vibrations and temperature variations in the
room can cause enough phase noise in flexible connecting cables
to destroy the accuracy of a sensitive measurement. The
connectors also must be tight; a torque wrench is the best tool.
5
When measuring a low-noise device, it is important that the
source and any amplification, required to achieve the proper
power at the phase detector, be placed before the splitter so it
will be correlated out of the measurement. In cases where this is
not possible; remember that any noise source, such as an
amplifier, placed after the splitter in either phase detector path,
will contribute to the measured noise.
6
An amplifier must be used in cases where the signal level out of
the UUT is too small to drive the phase detector, or the drive level
is inadequate to provide a low enough system noise floor. In this
case the amplifier should have the following characteristics:
a
It should have the lowest possible noise figure, and the
greatest possible dynamic range.
b
The signal level must be kept as high as possible at all points
in the setup to minimize degradation from the thermal noise
floor.
c
It should have only enough gain to provide the required signal
levels. Excess gain leads to amplifiers operating in gain
compression, making them very vulnerable to multiplicative
noise problems. The non-linearity of the active device
produces mixing which multiplies the baseband noise of the
active device and power supply noise around the carrier.
d
The amplifier's sensitivity to power supply noise and the
power supply noise itself must both be minimized.
Document Part No. E5500-90024 Ed. 1.0
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