User`s guide

286 Agilent E5505A User’s Guide
11 AM Noise Measurement Fundamentals
AM-Noise Measurement Theory of Operation
Basic noise measurement
The 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.