The second-order intermodulation products correspond to (⍵1 ∓ ⍵2) terms. Similarly, higher-order terms give rise to higher-order harmonic and intermodula-tion distortion. One second order product is the 2A term. If ƒ1, and f2 are very closely spaced, then the third-order products 2f1 – ƒ1 are the most difficult to deal with. The first is by finding 10*log of the ratio of the total output power to the power in adjacent channel. they are in a 4:1 amplitude ratio. Second and third-order intermodulation products for f1 = 5 MHz, f2 = 6 MHz. An argument similar to the third order distortion can be made here. Now the crucial concept here is multiplication. IP 3 is the point where the intermodulation product is equal to the fundamental signal. 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 two tones are close to fs/3, the aliased second harmonics may interfere with the measurement. Calculate intermodulation products from 2 and 3 frequencies. 685 Canton St. The points where these graphs cross are called 3rd order intercept point IP 3 and 2nd order intercept point IP 2, respectively. Tel: 978-671-0449 The wide bandwidth of the LTC6400 results in flat gain to hundreds of Megahertz; Table 25.4 summarizes the gain flatness specifications in the data sheets. 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. Operating Frequency Interactions: Intermodulation A single wireless microphone system can theoretically be used on any open operating frequency. Reinaldo Perez, in Wireless Communications Design Handbook, 1998. For these reasons, the second- and third-order IMD intercept points are not specified for ADCs. The values are usually referenced to the output power of the device expressed in dBm. To examine intermodulation products we will consider two frequencies f1 and f2 and some of the orders of intermodulation products. Werner Wild received his Dipl.-Ing. ACPR is desired to be as low as possible: a high ACPR indicates that significant spectral spreading has occurred. it can hit a neighboring channel in a communication system. The questions of what power level the second-order IM product appears at and whether there is any relationship between the levels of other-order IM products is analyzed from a theoretical standpoint. This intermodulation occurs because of the nonlinearity in the amplification process. When a second system is added it must be on a different operating frequency in order to be used at the same time as the first. This is shown graphically in Figure 6-140. The performed measurements lead to the same results as the theoretical conclusions. Fax: (781) 769-5037, For magazine subscriptions and newsletter customer service: As estimated, the quadratic shape of the transmission characteristics of the diode is dominant. This product is at two times each These sidebands are the intermod products, 60Hz (or whatever was the LF) either side of 7kHz, in this instance at 6940 and 7060Hz. However, the performance is still good even up to 250MHz-300MHz, which makes the LTC6400 suitable for even mid-to-high IF systems. With a low level two-tone input signal, and two data points, one can draw the second- and third-order IMD lines as they are shown in Figure 6-139 (using the principle that a point and a slope define a straight line). It should be noted that IP2, IP3, and the 1 dB compression point are all a function of frequency, and as one would expect, the distortion is worse at higher frequencies. Intercept points for data converters have no practical significance. This is because the third harmonic of fs/4 is 3fs/4, and the alias occurs at fs− 3fs/4 = fs/4. We use cookies to help provide and enhance our service and tailor content and ads. So SMPTE test signals, without any loss of validity, are not helpful for corroboration with what is heard. In other applications, the high bandwidth of the LTC6400 can be a significant advantage. Multitone SFDR is often measured in communications applications. It is generated when two signals are applied to a nonlinear device. The IM2 level of the jumper cable with diode was measured at ­150 dBc. Tel: (781) 769-9750 Once the input reaches a certain level, however, the output signal begins to soft limit, or compress due to things like power supply limits, output drive maximums, and the like. But with a well adjusted modern transistor amplifiers with high NFB, it discloses little more than %THD figures in most circumstances. Therefore, with a nearly quadratic characteristic (diode or transistor), the second-order IM product is dominant; with its cubic characteristic, the third-order IM product is also dominant. In the 3 Hz to 3 GHz band, a filter is used to attenuate signal generator second harmonic product (2*F1). The formulas that describe the frequencies of the intermodulation products are well known. Note, however, that if the two tones are close to fs/4, the aliased third harmonics of the fundamentals can make the identification of the actual 2f2–f1 and 2f1–f2 products difficult. Hank Zumbahlen, with the engineering staff of Analog Devices, in Linear Circuit Design Handbook, 2008. Figure 1-61:. W. Chen, ... S. Zhang, in Microwave Wireless Communications, 2016. The system sensitivity or self-intermodulation of the test bench was ­180 dBc for IM2 and ­178 dBc for IM3. Table 1 lists the test results. The IM3 measurement was performed in the GSM band with f1 = 936 MHz and f2 = 958 MHz at 2 * 20 W (+43 dBm). second-order IM product suddenly as an additional interferer in the receive band. In either case, the single- or multi-tone SFDR specification is the most accepted way to measure data converter distortion. If you consider a simple case in which two perfect sine tones a major third apart are applied to an amplifier with a square law characteristic, referring to Table 2.3 in Chapter 2, it is possible to calculate that this will result in two intermodulation products: one a major ninth above the root and another two octaves below the root. The effect of the second Therefore, neither the power levels of the several IM products nor the relations between them can be calculated. I. The slope of the third intermodulation product is three times that of the output signals. In the example shown in Figure 3 , the transmit frequencies are f1 = 936 MHz and f2 = 1850 MHz, and the IM2 product is fIM2 = 914 MHz. Two spectrally pure tones are applied to the system. The jumper cables create neither IM2 nor IM3 in a measureable level. Two pieces (each) of the following passive RF components were selected as devices under test (DUT): * a jumper cable HCF 1/2" assembled with 2 * 7-16 male connectors; the connectors are soldered directly to the cable and consist of solid inner and outer conductors, *          the same cable as above, but equipped with an adjustable diode in one of the connectors; the IM2 level was tuned to ­150 dBc, *          a 40 dB/100 W power attenuator. The larger number of tones more closely simulates the wideband frequency spectrum of cellular telephone systems such as AMPS or GSM. Additional measurements for IM2 and IM3 with different materials and RF devices are necessary. Then a + b – c = 461.550 MHz, which is Channel 21A mobile transmitter frequency. 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. (57) [Summary] There is provided a homodyne radio receiving apparatus for mitigating a fixed or variable DC offset generated by a residual second-order intermodulation term caused by an unnecessary signal. The same reasoning applies here; the second harmonic of fs/3 is 2 fs/3, and its alias occurs at fs− 2 fs/3 = fs/3. The test usually comprises a low frequency. Figure 7.24. However, it is important to note that the LTC6400 does not maintain great distortion performance all the way up to its actual −3dB bandwidth. However it should be noted that if fixed stations operate in the duplex mode, third order intermodulation could arise when two adjacent fixed stations are transmitting and a handportable on one of the channels is also transmitting. Second-order intermodulation products are, where frequencies f3, f4, and f5 are the undesirable distortion products that show up at the output. about 0.01% SMPTE @ 1dB below clip. The difference between +36 dBm and +4 dBm is 32 dB. For low level input signals, the IMD products remain relatively constant regardless of signal level. the second term is the fundamental signal(s). These intersections are called the second- and third-order intercept points, respectively, or IP2 and IP3. This is the output stage of the transmitter. A first common node signal is generated between a first resistor and a second resistor coupled across the mixer's differential output terminals. 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. The factors z2 and z3 are the respective quadratic cubic elements of the transmission characteristic. Second-Order Intermodulation in Current-Commutating Passive FET Mixers Abstract: Amplitude-modulation detection in the mixer plagues the performance of the zero-intermediate-frequency receiver by downconverting the envelope of amplitude modulated blockers to baseband where the desired channel is after downconversion. One of the possible sources of intermodulation has been referred to in Section 8.7.1 of this chapter. IMD products are of special interest in the RF area, and a major concern in the design of radio receivers. The characteristic of a galvanic contact is unknown. By continuing you agree to the use of cookies. It is shown that the second intercept point rises by an amount equal to the circuit’s out-of-channel rejection. A technique for cancelling out target IM2 components in a wireless receiver's mixer output is disclosed. Figure 4 shows a graphical comparison of IM2 and IM3 performance. Third-order IMD is often specified in terms of the third-order intercept point (IP3) as shown in Figure 1-60. These are then expressed at % of the original, usually in rms terms. Definition of intercept points and 1 dB compression points for amplifiers. The concept of second- and third-order intercept points is not valid for an ADC, because the distortion products don't vary predictably (as a function of signal amplitude). This application note gives an overview which Stephen Harman, Aveillant Ltd., Cambridge, U.K. Jason Zhang, Jonas Urbonas and Giampiero Esposito, Maury Microwave, Ontario, Calif. Andrea Arias-Purdue and Petra Rowell, Teledyne Technologies, Thousand Oaks, Calif. With respect to a dual-band system, this spectrum shows the second-order IM product suddenly as an additional interferer in the receive band. 3IM is the "stray image signal" produced when the radio frequency (RF) signals from 2 transmitters combine in the input stage of a receiver. If a number of signals are passed through a non-linear device the result will be intermodulation products that are spurious frequency components. Figure 1-61 shows the third order intercept value as a function of frequency for a typical VFB amplifier. However, it is the absolute value of these calculations that is of concern. 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). Third-order IMD products can mask out small signals in the presence of larger ones. SMPTE intermodulation distortion swept against frequency. The amplitude of each tone is set slightly more than 6 dB below full scale so that the ADC does not clip when the two tones add in-phase. ACPR measurement template for modulated signals. Nevertheless, both the second- and third-order intercept lines may be extended, to intersect the (dotted) extension of the ideal output signal line. Based on two pure sinusoidal signals S1 cos(T1 t) and S2 cos(T2 t) with the amplitudes S1 = S2 = 1, by developing into potential rows up to the ninth order, the expressions for the second- and third-order IM products are determined to be, Again, developed up to the ninth order, the transmission characteristics of a component can be described by the polynomial, Pol(z) = A0 + A1 z + A2 z2 + A3 z3 + A4 z4             + A5 z5 + A6 z6 + A7 z7 + A8 z8 + A9 z9        (3). This characteristic is curved which, to the mathematically minded of you, will immediately imply a power law. will generate its own second-harmonic output. The Timeout markers (T) confirm that the measured data was largely noise. Now, suppose a second harmonic from the RF generators is 180° out-of-phase with the mixer’s second harmonic. The LTC6400 has a very wide bandwidth (approaching 2GHz), but the vast majority of applications will not require frequencies beyond a few hundred Megahertz. Each distortion sum product in table 1 lies within a channel in a typical cable TV system, and will interfere with desired channels. But the second- and third-order intercept lines may be extended to intersect the extension of the output signal line. Second– and third-order product locations are shown in Figure 3-20. Norwood, MA 02062 USA The ADC doesn't 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 input range, producing extreme distortion due to clipping. The second order intermodulation product is given by + + = ( ) ( ) ( ) ( ) 20log 1 1 2 2 0 i j i j i j j H H H IMP m p ω ω ω ω ω ω ω (13) For an SCM-FM system the signal-to-noise ratio (SNR) will be related to the carrier-to-noise It should be noted that essentially the same arguments apply to DACs. The performed measurements lead to the third order products etc of Analog devices, in Op Amp Handbook. 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