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12
AM Noise Measurement Fundamentals
AM-Noise Measurement Theory of Operation
AM-Noise Measurement Theory of Operation
Basic Noise
Measurement
Phase Noise
Measurement
12-2
E5500 Phase Noise Measurement System Version A.02.00
The Agilent E5500A phase noise measurement software uses the
following process to measure carrier noise by:
G
Calibrating the noise detector sensitivity.
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Measuring the recovered baseband noise out of the detector.
G
Calculating the noise around the signal by correcting the
measured data by the detector sensitivity.
G
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.
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.
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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.
G
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.
Document Part No. E5500-90024 Ed. 1.0
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