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Residual Measurement Fundamentals
Calibrating the Measurement
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.
8-8 Agilent Technologies E5500 Phase Noise Measurement System
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