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ADL5385 Arkusz danych(PDF) 7 Page - Analog Devices |
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ADL5385 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 8 page Application Note AN-1039 Rev. 0 | Page 7 of 8 Once the LO leakage and quadrature error have been calibrated, all that remains is for the calibration coefficients to be stored in nonvolatile RAM so that they are available when the equipment is turned on in the field. To recap, the four calibration coefficients are • I channel offset voltage • Q channel offset voltage • I channel vs. Q channel gain imbalance • Quadrature phase imbalance FREQUENCY VARIATIONS Calibrating at multiple frequencies within a band adds time to the factory calibration, requires more nonvolatile memory for the larger look up table, and is more cumbersome during field operation as calibration coefficients have to be swapped out as the frequency changes. Now, consider what happens to the quality of calibration as the frequency changes. In Figure 7, sideband suppression and LO leakage have been nulled to −60 dBc and −74 dBm, respectively, at 1900 MHz. Figure 7 also shows how the uncompensated sideband suppression and LO leakage vary with frequency (the flatter green and red traces a the top of the plot). Next, adjust the frequency over a range of ±30 MHz (the typical width of a cellular telephony band) without recalibration. The LO leakage quickly loses its null and at some frequencies is only around 8 dB better than the uncompensated value. In the case of the sideband suppression, the difference between the compensated and uncompensated values becomes as low as around 1 dB. Figure 7 suggests that factory calibration be performed at multiple frequencies within a band to maintain nulled performance across the band. –80 –75 –70 –65 –60 –55 –50 –45 –40 –35 –30 1870 1880 1890 1900 1910 1920 1930 OUTPUT FREQUENCY (MHz) UNADJUSTED SIDEBAND SUPPRESSION (dBc) UNADJUSTED LO LEAKAGE (dBm) POST (MIDBAND) NULLING SIDEBAND SUPPRESSION (dBc) POST (MIDBAND) NULLING LO LEAKAGE (dBm) Figure 7. Variation of LO Leakage and Sideband Suppression vs. Frequency after Nulling Midband POST CALIBRATION TEMPERATURE DRIFT Factory calibration at multiple temperatures is even more difficult and expensive than calibration at multiple frequencies. As a result, it is generally only practical to perform factory adjustment of LO leakage and sideband suppression at an ambient temperature. Thus, what happens to post-calibration performance as temperature varies? In Figure 8, the LO leakage and sideband suppression have again been nulled midband. After nulling, the device is cycled over temperature. This again has the effect of moving sideband suppression and LO leakage off their nulled levels. However, notice that the performance at temperature is quite flat across frequency and it is no longer clear at which frequency the nulling was performed. The net improvement over temperature is approximately 15 dB compared to the unadjusted LO leakage. CALIBRATION VS. TIME In the set-and-forget factory calibration scheme that has just been described, the question of long-term drift arises because the equipment may never be recalibrated in the field. Experi- ments have shown that it is very difficult, if not impossible, to measure the degradation of nulled sideband suppression and LO leakage over time. Very mild changes in environmental conditions tend to quickly move the device off its null. This makes it impossible to determine whether the environment and the test equipment are altering the experiment or if genuine device drift over time is taking place. However, Figure 8 shows that the question of drift over time is less important. This is because the effect of temperature drift is much more significant. Thus, in a system that experiences reasonable temperature fluctuations, whatever drift over time takes place is completely masked by the temperature drift. –80 –75 –70 –65 –60 –55 –50 –45 –40 –35 –30 1802 1812 1822 1832 1852 1872 1842186218 LO FREQUENCY (MHz) 82 +25°C UNADJUSTED +25°C NULLED +85°C UNADJUSTED +85°C NULLED –40°C UNADJUSTED –40°C NULLED Figure 8. Variation of LO Leakage vs. Frequency and Temperature After Nulling Midband |
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