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1、56High Frequency Electronics High Frequency Design IMD ISSUES Intermodulation Distortion Performance and Measurement Issues By Gary Breed Editorial Director I ntermodulation dis- tortion (IMD) is a part of all communi- cations systems,with each componentinclud- ing passive components, connectors,cab
2、les and antennascapable of adding significantly to the total distortion. This tutorial will present some of the issues regarding the effects of IMD on system performance and in the accu- rate and repeatable measurement of IMD in various circuits and systems. Third Order IMD IMD testing is usually do
3、ne by delivering multiple signals to the device under test, then measuring energy at the output at frequencies other than those signalsthe new spurious signals generated by the non-linearities of the DUT. By far, the most common IMD measure- ment is third order IMD. It is a convenient measurement be
4、cause it requires only two test signals, and if the test signals are close in frequency to one another, third order products fall close enough to the test signals to be with- in the passband of the DUT. The figure of merit associated with third order IMD is the third order intercept point (IP3), whi
5、ch is the amplitude at which the third order distortion products are equal to the input signals.This is an imaginary point, because the DUT will go into saturation before that ampli- tude is reached. For more information, see the References. With this basic overview, we can now look at some specific
6、 issues. Second Order IMD In the not-too-distance past, most commu- nications systems were relatively narrow- band, including bandpass filtering early in the signal chain. Since second order IMD products involve the sum or difference of two signals, or the second harmonic of a single signal, the ban
7、dpass filters effectively removed the sig- nals that could generate second order IMD products. Many modern systems are broadband, and this preselection filtering is not present.Thus, second order IMD is a “new” issue for many engineers. As pointed out by Hart 1, second order IMD is potentially more
8、troublesome than third order, because second order prod- ucts increase more rapidly with increased sig- nal levels than third order products (by a fac- tor of 4/3). Test Signal Quality To achieve reliable test results, the test sig- nals must be equal in amplitude, and having low sideband noise. Som
9、e years ago, it was not possible to make accurate IMD measurements using spectrum analyzers, because the side- band noise of the sweep generators and early synthesizers affected the measurement. For third order testing, two quality signal generators and a modern spectrum analyzer are quite sufficien
10、t, but attention must also be paid to the accuracy of the network that com- bines the two signals for presentation to the DUT 2. Any imbalance will result in unequal amplitudes of the various third order products. Full-System Concerns Each individual circuit element has an associated IMD performance
11、, which must be combined with the rest of circuit to obtain the required overall performance 3. Conversely, when overall system testing does not meet specifications, the contributions of individual Intermodulation distortion performance and mea- surement is a critical part of modern communication sy
12、stem and circuit design From May 2003 High Frequency Electronics Copyright 2003 Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 High Frequency Design IMD ISSUES parts must be evaluated to identify the source of the problem. Cascaded performance is well- understood, and was one of the first RF ap
13、plications of computer-aided design. Today, the performance of individual circuits can be simulated, then incorporated into the high-level “block diagram,” where its effects on the full system gain, noise, IMD etc. can be evaluated. Linearity Modern complex modulation schemes require highly linear t
14、rans- mission and reception systems for reliable communications, both to achieve low bit-error rate (BER) and to minimize interference. Although noise, compression and harmonic dis- tortion are also contributors to lin- earity performance, IMD remains the most critical metric since it (and its measu
15、rement) is, in part, dependent on these other non-linear functions. While linearity can be defined mathematically 4, there are suffi- cient unknown variables, such as the degree of non-linearity of a given device, that mathematical simulation can be complex (and computationally intensive). Measureme
16、nt remains important to verify the accuracy of simulation, especially at maximum performance (lowest levels of IMD). High-Order IMD High-order IMD is a significant factor in todays wireless communica- tions systems.With each cell site hav- ing multiple transmitters and receivers, system IMD performa
17、nce must be evaluated in the laboratory using multiple signal sources. Complex modulation can approxi- mate the randomness of noise, which- has a higher peak-to-average ratio than multiple sine waves. Standard IMD testing must be replaced with noise power testing, using a noise source with a notched
18、-out region, with a measurement of how much that region is “filled” with IMD products. Transmitter IMD In todays critical performance systems, the high order distortion products generated in the transmit- ter are especially important. These products can affect other receivers (and transmitters) oper
19、ating near the transmitter frequency. Most wire- less standards include a spectral “mask” showing the maximum allow- able power in transmitter sidebands at various frequency offsets. Theoretical analysis and design techniques to reduce transmitter IMD is complicated by the relatively strong non-line
20、ar performance of power devices. This has resulted in development of distortion-reducing feed-forward and predistortion signal processing techniques. References 1. W. E. Sabin and E. O. Schoen- ike, editors, Single Sideband Systems & Circuits, 2nd ed., Mc-Graw-Hill, 1995.Ch.9,“Preselectors and Posts
21、electors,” by Bill D. Hart. 2. W. Hayward, Introduction to Radio Frequency Design, American Radio Relay League, 1994, Sec. 8.2. 3. U. L. Rohde and T. T. N. Bucher, Communications Receivers: Principles & Design, McGraw-Hill, 1988, Ch. 3. 4. K. McClaning and T.Vito, Radio Receiver Design, Noble Publishing Corp., 2000, Chs. 6 and 7. RadioFans.CN 收音机爱 好者资料库
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