Agilent Technologies E8267C PSG Manual de usuario descargar pdf (Pagina 3)

What is IMD?

Two-tone and multitone signals are used extensively in the communications industry to

test for nonlinear distortion at the component, device, sub-system, and system level.

Intermodulation distortion (IMD) is a particular type of nonlinear distortion; other types

include harmonic distortion and cross modulation. IMD is the primary cause of in-band

and out-of-band spectral regrowth (i.e. distortion) and results from unwanted intermodula-

tion between the multiple frequency components that comprise a signal. Intermodulation

occurs as a result of the signal traversing components and devices with nonlinear

transfer functions. Figure 1 illustrates some of the IMD products generated when two

tones at frequencies f

and f

are presented to the input of a nonlinear device.

Figure 1. Two-tone IMD products commonly generated by nonlinear devices.

Nonlinear distortion cannot be corrected using equalization techniques (that only

works with linear phenomena). As a result, quantitatively characterizing the effect of

nonlinear behavior from the component to the system level is an important part of

the transmitter/receiver design and verification process. High levels of IMD indicate

significant nonlinearities that not only degrade the system performance, but also

interfere with neighboring channels. In-band IMD refers to the intermodulation products

that fall within the channel bandwidth of the component, device, or system under test.

This type of distortion is particularly undesirable since it cannot be filtered and directly

interferes with the signal of interest. Although generating out-of-band distortion is also

objectionable behavior that must be characterized and addressed (ask your spectral

neighbor), simple filtering can typically be applied to correct the problem.

Amplitude

Frequency

2nd order

IMD

-f

)

5th order

IMD

(3f

-2f

)

3rd order

IMD

(2f

-f

)

5th order

IMD

(3f

-2f

)

3rd order

IMD

(2f

-f

)

2nd order

IMD

)

2nd harmonics

3rd harmonics

Typical channel

bandwidth

1 2 3 4 5 6 7 8 ... 19 20

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