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SFH628A-3

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File Size : 116 KB Manufacturer:VISHAY Description: Method 1 In military standard (1131 A-2204B), the distortion products are referenced to one of the two tones of the test signal. The maximum permissible IMD is not specified but, numbers like C 35 dB are not uncommon in some equipment specifications. However, when this measuring system is employed in industrial applications, the IMD requirement (d3) is usually relaxed to C 30 dB. Figure 3 shows the frequency spectrum of IM distortion products and their relative amplitudes for a typical class AB linear amplifier. Biasing the amplifier more toward class B will cause the lower order distortion products to go down and the amplitudes of the higher order products to increase. There is a bias point where the d3 and d5 products become equal resulting in 2 C 5 dB improvement in the lower order IMD readings. Method 2 ln the proposed EIA standard, the amplitude of the distortion products is referenced to the peak envelope power, which is 6 dB higher in power than that represented by one of the two tones. The amplifier or device indicating a maximum distortion level of C 30 dB in Method 1 represents C 36 dB with the EIA proposed standard. Conversely, a C 30 dB reading with ElAs PEP reference would be C 24 dB when measured with the more conservative military method. In practical measurements, the two tones can be adjusted 6 dB down from the zero dB line, and direct IMD readings can be obtained on the calibrated scale of the analyzer. Alternatively, the tone peaks can be set to the zero dB level and 6 dB deducted from the actual reading. The military standard, with the relaxed C30 dB IMD specification, is employed by most manufacturers of high power commercial transmitters and marine radio base stations. *The EIA measuring method is used by the majority of ham radio equipment and CB radio manufacturers. It is also used to measure IMD in various mobile radio applications operating from a 12.5 V nominal dc supply. Because of the importance to your design, data sheets of the newer generation Motorola devices specify linearity tests appropriate to the expected application of the particular device and test conditions are always indicated.


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