Agilent Technologies E8267C PSG Manual de usuario descargar pdf (Pagina 7)

Because worst-case peak-to-average ratio does not imply worst-case IMD levels and

IMD varies considerably depending upon the phase relationships of the test stimulus, to

truly characterize IMD performance, statistical sampling analysis should be applied to a

collection of IMD test results that are gathered using multiple test signals with random

phase sets. The central limit theorem can be used to determine how many individual test

results from random phase sets need to be averaged to obtain a composite test result

that adequately represents realistic operating conditions within some reliable confidence

interval. As discussed later, vector stimulus techniques can be used to generate multi-

tone signals with random phase sets to facilitate this test approach. For additional

information on using statistical techniques to analyze IMD performance, see the paper

created by Agilent Technologies in the references section.

Analog Test Stimulus

A traditional test setup to measure IMD is shown in Figure 5. In this setup, two signals

from independent continuous wave (CW) signal generators are summed together using a

combiner to create a two-tone test stimulus. Multiple isolators are used to ensure that

signal generators do not interfere with each other, thus minimizing the IMD contributed

by the test equipment. Typical isolators provide 20 dB of isolation in the reverse path at

the cost of .5 to 1 dB of loss in the forward path. The amplifier at the output of each signal

generator is used to fine-tune the relative tone powers without changing the signal

generator level. As a result, the signal source IMD is held constant. The low pass filter is

used to minimize the out-of-band spectral components of the test stimulus. The combiner

used to sum the signals from the generators should have low loss, sufficient isolation,

and add minimal distortion to the test stimulus. The variable attenuator at the output of

the combiner is used to change the level of the composite signal at the input of the DUT

without manually adjusting each signal generator; again, attempting to hold the source

IMD constant.

Figure 5. Conventional analog setup to perform a two-tone IMD measurement.

Conventional multitone signal generation methods used to perform IMD measurements

are equipment intensive, particularly as the number of tones increases. CW signals

(i.e. tones) from multiple signal generators are summed together to create the desired

test stimulus. Figure 6 illustrates a multitone IMD measurement setup using CW

signal generators.

Isolator

AMP

LPF

PSG CW signal generator

Attenuator

DUT

PSA spectrum analyzer

Combiner

1 2 3 4 5 6 7 8 9 10 11 12 ... 19 20

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Agilent Technologies E8267C PSG Manual de usuario Pagina 7