However, the third-order products 2f2–f1 and 2f1–f2 are close to the original signals, and are almost impossible to filter. But in all cases, the intermod products caused are close to the HF tone and are liable to be masked. The second IMD can be found by analyzing the third term of Equation 3. The fourth-order intermodulation products are, The fifth-order intermodulation products are. The exact level at which this occurs is dependent on the particular ADC under consideration—some ADCs may not exhibit significant increases in the IMD products over their full input range, however most will. If the system nonlinearity is approximated by a power series expansion, it can be shown that second-order IMD amplitudes increase 2 dB for every 1 dB of signal increase, as represented by the slope = 2 curve in the diagram. Knowing the levels of the discrete IM products the transmission characteristic of passive RF components can be calculated. This is in contrast to IM2 where the frequency of the intermodulation was at a lower and higher frequency. The second (and much more popular method) is to find the ratio of the output power in a smaller bandwidth around the center of the carrier to the power in the adjacent channel. Second-order intermodulation products are − f 1 − f 2 = f 2 f 1 + f 2 = f 3 f 2 − f 1 = f 4 f 1 − f 2 = f 5 , where frequencies f 3 , f 4 , and f 5 are the undesirable … COMPOSITE SECOND ORDER INTERMODULATION DISTORTION Composite Second Order distortion (CSO) is a result of one or two carriers experiencing a second order non-linearity.
second-order IM product suddenly as an additional interferer in the receive band. The formulas that describe the frequencies of the intermodulation products are well known. But the second- and third-order intercept lines may be extended to intersect the extension of the output signal line. These components may be inside or outside the frequency band of interest for the device. The internal amplifier’s open-loop gain is much higher than the gain externally, and the amplifier is compensated to push out the overall loop gain roll-off to higher frequencies. These sidebands are the intermod products, 60Hz (or whatever was the LF) either side of 7kHz, in this instance at 6940 and 7060Hz. Figure 6-140:. By continuing you agree to the use of cookies. The output signal power of a single tone (in dBm) as well as the relative amplitude of the third-order products (referenced to a single tone) is plotted as a function of input signal power. The IM3 level was approximately 9 dB lower. The second-order intermodulation products have frequencies ranging from (⍵1 - ⍵2) to (⍵1 + ⍵2). This discussion already shows two things: The power levels of the second- and third-order IM products are absolutely independent from each other and only related to their respective transmission characteristics. Degree in electrical engineering in 1987 from Fachhochschule Augsburg. IP2 as deduced from fig.6 is +86 dBm. This is shown in Figure 6-139 within the region where the ideal slope = 1 line becomes dotted, and the actual response exhibits compression (solid). John Price, Terry Goble, in Telecommunications Engineer's Reference Book, 1993. These possible intermodulation products were produced by just two input signals. IMD products are of special interest in the RF area, and a major concern in the design of radio receivers. intermodulation products, particularly the third-order inter-modulation (IM3) products . This is the point where the output signal is compressed 1 dB from an ideal input/output transfer function. The IMD curve in Figure 6-140 is divided into three regions. The minimum may be taken to be the rated figure, i.e. If the original frequencies are in a similar band, the four above terms will be situated relatively far away and, thus, easy to eliminate (even with inexpensive filters). For most analogue systems with many multiplexed channels, the addition of the very large combination of signals results in an output noise spectrum which is approximately flat with a frequency across a narrow band of about 4kHz. The output signal power in a single tone (in dBm) as well as the relative amplitude of the third-order products (referenced to a single tone) is plotted as a function of input signal power. The reason lies in the topology. Interestingly, the frequency sum products in the last four columns fall in at the same frequency as second-order intermodulation (composite second order) products at +1.25 MHz offset form the carrier. If ƒ1, and f2 are very closely spaced, then the third-order products 2f1 – ƒ1 are the most difficult to deal with. In conventional transmitters, the intermodulation distortion (IMD) is often used to characterize the nonlinearities of PAs. In the SMPTE standard, this signal is 25% of the LF component, i.e. In simple terms, a power law is a relationship which does not relate one set of values (the input voltage) to another (the output voltage) by a constant, which would yield a straight-line relationship – and incidentally a distortionless amplifier, but by a function which is made up of both constant and multiplication factor which is related to itself. It has been shown that a complete evaluation of a passive RF device requires a specification for the second-order IM product effects when used in a dual-band system. Norwood, MA 02062 USA
Definition of intercept points and 1 dB compression points for amplifiers. It is shown that the second intercept point rises by an amount equal to the circuit’s out-of-channel rejection. The almost flat curves are typical of linear, transformerless, electronic paths. The theoretical approach showed that no relationship or equation for calculating the power levels of the particular IM products can be given because the transmission characteristics of galvanic contacts are never … One of the possible sources of intermodulation has been referred to in Section 8.7.1 of this chapter. Unlike traditional “open-loop” RF/IF amplifiers, where there is very little or no feedback used in the amplifier circuit, the LTC6400 contains an internal differential op amp with the gain set using a feedback network. The ADC does not gradually begin to compress signals approaching full-scale (there is no 1 dB compression point); it acts as a hard limiter as soon as the signal exceeds the ADC input range, thereby suddenly producing extreme amounts of distortion because of clipping. to produce a series of second order sum and difference products, third order products etc. Third-order IMD is often specified in terms of the third-order intercept point (IP3) as shown in Figure 1-60. Tel: (781) 769-9750
The value of the IMD product is expressed in dBc relative to the value of either of the two original tones, and not to their sum. The non-linearities cause each signal to combine with the other signals in the set. This results in the chord in Figure 4.2. The mixtures between the original frequencies, ω a + ω b, ω a - ω b, and ω b - ω a are also called second-order intermodulation products (IM2). Similarly, the third-order IMD amplitudes increase 3 dB for every 1 dB of signal increase. However, it is important to note that the LTC6400 does not maintain great distortion performance all the way up to its actual −3dB bandwidth. Intermodulation is a scenario where signals outside the monitored channel combine nonlinearly to produce a frequency of monitored channel. Notice also that both of the intermodulation plots intersect the output level because they are offset from and at different slopes than the output level. With respect to a single-band system, this spectrum shows the third-order IM product as the one nearest to the generating frequencies and under some circumstances falling into the receive band. Once the input reaches a certain level, however, the output signal begins to soft-limit, or compress. about 0.01% SMPTE @ 1dB below clip. The TOI products are usually within or close to the use-band, i.e. The problem caused by the generation of intermodulation products within modern mobile communication systems where nonlinearities are present is already well known and discussed. The points where these graphs cross are called 3rd order intercept point IP 3 and 2nd order intercept point IP 2, respectively. Several comparable measurements of IM2 and IM3 were made to prove the conclusions of the theoretical approach and to provide additional information. These power level values are usually referenced to the output power of the device delivered to a matched load (usually, but not necessarily 50 Ω) expressed in dBm. Intermodulation (IM) distortion and Second order intermodulation products or IP2 discussion. Figure 7.23. The absolute value of f1 – 2f2 is the same as the absolute value of 2f2 – f1.It is common to talk about the third order intermodulation products as being The logarithm of the products of the rows for the transmission characteristic and the IM products provides the power levels of IM2 and IM3, depending on the included orders in the transmission characteristics. A parameter of interest here is the 1 dB compression point. Figure 6-139:. Figure 25.4 shows the 3rd order intermodulation distortion (IMD) products from a 2-tone signal test of the LTC6400-20.2 At low frequencies, the distortion products approach −100dBc. As an example of the production of intermodulation, consider four frequencies a, b, c and d. Third order intermodulation products can be produced when the frequencies bear the following particular relationships to each other: Then a + b – c = 456.175 MHz, which is again the Channel 24 fixed station transmit frequency. The characteristic of a galvanic contact is unknown. Intercept points for data converters have no practical significance. High SFDR increases the receiver's ability to capture small signals in the presence of large ones, and prevent the small signals from being masked by the intermodulation products of the larger ones. As the input signal continues to increase beyond full-scale, the ADC should as an ideal limiter, and the IMD products become very large. Reinaldo Perez, in Wireless Communications Design Handbook, 1998. will generate its own second-harmonic output. To be able to select five channels without any third order intermodulation products being produced, it is necessary to have 12 regularly spaced (e.g., 25 kHz spaced) channels available. 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