N5511A Phase Noise Test System User’s Guide 57
Absolute Measurement Fundamentals
The Phase-Lock-Loop Technique
Let the input signal from the DUT be denoted by
Let the signal from the reference be denoted by
-recall the
reference is locked in quadrature.
Let the noise floor of the system be denoted by
Consider the sum and difference trigonometric formula:
1.
The output of the phase detector therefore is:
2.
The output of the phase detector therefore is:
Consider the small angle theorem sin(A)=A. The filtered output will be
represented as:
3.
Noise contributed from the channel internally is added to the overall
output. The FFT output can be represented as a sum:
When using the PLL technique in a single channel configuration, the overall
output will include the phase noise of reference and the noise floor of the
system. If the phase noise of the DUT is better than the phase noise of the
reference, the phase noise of the reference will limit the dynamic range. If the
DUT is better than the noise floor of the system, the measurement will also not
result in the true performance of the device under test. A method to
significantly increase dynamic range is to perform a dual-channel
measurement instead of single channel.
Summary of Contents for N5511A
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