SECOND AND THIRD-ORDER INTERMODULATION PRODUCTS FOR f,= 5MHz and f,= 6MHz 2=SECOND ORDER IMD PRODUCTS (3=THIRD ORDER IMD PRODUCTS NOTE: f1=5MHz, f2=6MHz 101112 FREQUENCY: MHz Figure 8.4 Intermodulation distortion products are of special interest in the rF area, and a major concern in the design of radio receivers. Third-order IMd products can mask out small signals in the presence of larger ones. Third order IMD is often specified in terms of the third order intercept point as shown in Figure 8.5 two spectrally pure ones are applied to the system. 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. If the system non-linearity is approximated by a power series expansion, the second-order IMd amplitudes increase 2dB for every 1dB of signal increase. Similarly, the third-order IMD ldB of sig With a low level two-ton input signal, and two data points, draw the second and third order Imd lines as are shown in Figure 8.5, because one point and a slope determine each straight line Once the input reaches a certain level, however, the output signal begins to soft limit, or compress. But the second and third-order intercept lines may be extended to intersect the extension of the output signal line. These intersections are called the second-and third order intercept points, respectively. The values are usually referenced to the output power of the device expressed in dBm. Another parameter which may be of interest is the 1dB compression point. This is the point at which the output signal is compressed by 1dB from the ideal input/output transfer function This point is also shown in Figure 8.5
6 SECOND AND THIRD-ORDER INTERMODULATION PRODUCTS FOR f1 = 5MHz and f2 = 6MHz Figure 8.4 Intermodulation distortion products are of special interest in the RF area, and a major concern in the design of radio receivers. Third-order IMD products can mask out small signals in the presence of larger ones. Third order IMD is often specified in terms of the third order intercept point as shown in Figure 8.5. Two spectrally pure tones are applied to the system. 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. If the system non-linearity is approximated by a power series expansion, the second-order IMD amplitudes increase 2dB for every 1dB of signal increase. Similarly, the third-order IMD amplitudes increase 3dB for every 1dB of signal increase. With a low level two-tone input signal, and two data points, draw the second and third order IMD lines as are shown in Figure 8.5, because one point and a slope determine each straight line. Once the input reaches a certain level, however, the output signal begins to softlimit, or compress. But the second and third-order intercept lines may be extended to intersect the extension of the output signal line. These intersections are called the second- and third order intercept points, respectively. The values are usually referenced to the output power of the device expressed in dBm. Another parameter which may be of interest is the 1dB compression point. This is the point at which the output signal is compressed by 1dB from the ideal input/output transfer function. This point is also shown in Figure 8.5. I
NTERCEPT POINTS, GAIN COMPRESSION, AND IMD SECOND ORDEI INTERCEPT OUTPUT r THIRD OHDER 1 6B COMPRESSION FUNDAMENTA oFn INPUT POWER (PER TONE, dBm Figure 8.5 Knowing the third order intercept point allows calculation of the approximate level of the third-order Imd products as a function of output signal level Figure 8.6 shows the third order intercept value as a function of frequency for the AD9622 voltage feedback amplifier
7 NTERCEPT POINTS, GAIN COMPRESSION, AND IMD Figure 8.5 Knowing the third order intercept point allows calculation of the approximate level of the third-order IMD products as a function of output signal level. Figure 8.6 shows the third order intercept value as a function of frequency for the AD9622 voltage feedback amplifier
AD9622 THIRD ORDER IMD INTERCEPT VERSUS FREQUENCY to。UT +-au 10 100 FREQUENCY-MHz Figure 8.6 Assume the op amp output signal is 5MHz and 2v peak-to-peak into a 100ohm load (50ohm source and load termination). The voltage into the 50ohm load is therefore 1V peak-to-peak, corresponding to +4dBm. The value of the third order intercept at 5MHz is 36dBm. The difference between +36dBm and +dbM is 32dB. This value is then multiplied by 2 to yield 64db (the value of the third-order intermodulation products referenced to the power in a single tone). Therefore, the intermodulation products should be -64dBe(dB below carrier frequency), or at a level of-60dBm Figure 8.7 shows the graphical analysis for this example
8 AD9622 THIRD ORDER IMD INTERCEPT VERSUS FREQUENCY Figure 8.6 Assume the op amp output signal is 5MHz and 2V peak-to-peak into a 100ohm load (50ohm source and load termination). The voltage into the 50ohm load is therefore 1V peak-to-peak, corresponding to +4dBm. The value of the third order intercept at 5MHz is 36dBm. The difference between +36dBm and +4dBm is 32dB. This value is then multiplied by 2 to yield 64dB (the value of the third-order intermodulation products referenced to the power in a single tone). Therefore, the intermodulation products should be –64dBc (dB below carrier frequency), or at a level of –60dBm. Figure 8.7 shows the graphical analysis for this example