User`s guide

12-2

Agilent Technologies E5500 Phase Noise Measurement System

AM Noise Measurement Fundamentals

AM-Noise Measurement Theory of Operation

Basic Noise

Measurement

The Agilent E5500A phase noise measurement software uses the following

process to measure carrier noise by:

• Calibrating the noise detector sensitivity.

• Measuring the recovered baseband noise out of the detector.

• Calculating the noise around the signal by correcting the measured data

by the detector sensitivity.

• Displaying the measured noise data in the required format.

Given a detector calibration, the system looks at the signal out of the

detector as just a noise voltage which must be measured over a band of

frequencies regardless of the signal’s origin.

The detector calibration is accomplished by applying a known signal to the

detector. The known signal is then measured at baseband. Finally, the

transfer function between the known signal and the measured baseband

signal is calculated.

Phase Noise

Measurement

In the case of small angle phase modulation (<0.1 rad), the modulation

sideband amplitude is constant with increasing modulation frequency. The

phase detector gain can thus be measured at a single offset frequency, and

the same constant will apply at all offset frequencies.

• In the case of calibrating with phase modulation sidebands, the system

requires the carrier-to-sideband ratio and the frequency offset of the

sidebands. The offset frequency is equal to the baseband modulation

frequency. The ratio of the baseband signal voltage to the

carrier-to-sideband ratio is the sensitivity of the detector.

• In the case of calibrating with a single-sided spur, it can be shown that a

single-sided spur is equal to a PM signal plus an AM signal. The

modulation sidebands for both are 6 dB below the original single-sided

spur. Since the phase detector attenuates the AM by more than 30 dB,

the calibration constant can be measured as in the previous case, but

with an additional 6 dB correction factor.