Agilent Technologies E5503A Instrukcja Użytkownika Pobierz pdf (Strona 2)

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Agilent E5500 Theory of Operation

The “phase detector with reference

source” technique is the most general-

purpose and cost-effective measure-

ment approach to measure the single

sideband (SSB) phase noise charac-

teristic of oscillators. This technique

demodulates the noise sidebands of

the device-under-test (DUT) oscillator

to a baseband signal for a fast, easy

measurement using off-the-shelf base-

band analyzers.

With the reference source and the

DUT in steady state quadrature, the

phase detector generates a baseband

signal proportional to the phase dif-

ference of the two sources. When the

noise characteristics of the reference

source signal are less than the DUT

signal noise characteristics, the

resulting baseband noise signal is

effectively that of the DUT.

A narrow-band phase-lock-loop forces

the two phase detector input signals

into quadrature and allows the refer-

ence source to track DUT frequency

drift for drift rates within the loop

bandwidth.

Noise fluctuations within the loop

bandwidth are suppressed by the

operation of the phase-lock-loop. This

suppression effect can be independ-

ently measured and the noise data

corrected automatically by the meas-

urement software. Noise fluctuations

outside of the phase-lock-loop band-

width are unaffected.

When the oscillator-under-test has a

frequency output in the microwave

frequency range, it is difficult to find

suitable, low noise, microwave fre-

quency sources to use as the neces-

sary reference signal. In this case, the

solution is to use a low-noise down-

converter to translate the DUT

microwave output frequency to a

lower RF/IF frequency, allowing the

use of the same low-noise RF refer-

ence sources described previously.

The phase noise of the RF reference

or the microwave downconverter usu-

ally dominates effective noise floor of

this phase noise measurement. The

noise contribution of the baseband

test set is much lower than either and

typically does not factor into the

overall measurement noise floor.

Oscillator

under test

RF out

Tuning voltage

Baseband test set

Phase

detector

Signal conditioning

Baseband output

signal

Phase-lock

loop

Reference

source

Microwave

downconverter

Baseband

analysis

hardware

Simplified phase noise measurement diagram

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