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ADL5590 Datasheet(PDF) 5 Page - Analog Devices |
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ADL5590 Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page Application Note AN-1039 Rev. 0 | Page 5 of 8 The second alternative presented previously is to perform factory calibration. To do this, the popular single sideband spectrum can be used as a simple but valuable diagnosis tool during factory calibration. To create a single sideband spectrum, the I and Q inputs are driven by low frequency (typically 1 MHz) sine and cosine signals, that is, the base- band signals are in quadrature. Figure 3 shows the spectrum that results when these baseband signals are mixed with the LO. The primary components of the single sideband spectrum are: • Lower sideband: If the IQ modulator has no imperfections, this is the only spectral component observed, the result of multiplication and summing of the baseband sine and cosine signals with the two in-quadrature LO signals. • Undesired upper sideband: This undesired component results from gain and phase imbalances between the I and Q signal paths along with LO quadrature imbalance. • Undesired LO leakage: As discussed earlier, the LO leakage results from I and Q offsets and/or parasitic leakage of the LO directly to the IQ modulator’s output. 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 CENTER 899.9334MHz 333kHz/ SPAN 3.33MHz Figure 3. Single Sideband Spectrum Figure 4 shows a plot that can be used to relate sideband suppression to I/Q gain mismatch and quadrature mismatch. From the plot, it can be noted that a quadrature phase error of 1°, coupled with an I/Q gain mismatch of 0.5 dB, results in −30 dB of sideband suppression. It is notable in this example that improving the quadrature phase mismatch has no effect on the sideband suppression unless the gain mismatch is also improved. 0dB 0.0125dB 0.025dB 0.05dB 0.125dB 0.25dB 0.5dB 1.25dB 2.5dB 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 0.01 0.1 1 10 100 PHASE ERROR (Degrees) Figure 4. Plot Showing the Relationship Between Modulator Errors and Sideband Suppression By using a directional coupler and a power splitter (as shown in Figure 1), it is quite simple to add an auxiliary output to the transmitter that can be used during factory calibration. A spectrum analyzer is connected to this port. Another alternative would be to connect the spectrum analyzer at the antenna connector (after the signal has been adequately attenuated). |
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