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AD8345 データシート(PDF) 6 Page - Analog Devices |
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AD8345 データシート(HTML) 6 Page - Analog Devices |
6 / 8 page AN-1039 Application Note Rev. 0 | Page 6 of 8 CALIBRATION PROCEDURE Correcting all of the modulator’s imperfections is a multistep process. Start by looking at the procedure for LO leakage cor- rection which results in a constellation that is offset from the origin. A single sideband spectrum is applied to the transmitter and is monitored on the spectrum analyzer. Next, small diffe- rential offset voltages are applied to the I and Q inputs. Applying differential offset voltages to the I and Q inputs should not be confused with changing the dc bias levels (also referred to as the common-mode level) on these pins, which has no effect. This is done as an I offset sweep followed by a Q offset sweep (or vice versa). Returning briefly to Figure 1, note that the AD9788 (a 16-bit, 800 MSPS dual DAC) conveniently includes two aux- iliary DACs that can be used to couple differential dc offset voltages on I and Q lines. This coupling is performed externally using resistor dividers. Figure 5 shows how sweeping the I and Q offset voltages alters the LO leakage. Start by sweeping the I offset voltage around 0 V while holding the Q offset voltage at 0 V. With modern IQ modulators exhibiting unadjusted LO leakage in the −40 dBm range and having voltage gains in the −5 dB to +5 dB range, an offset voltage sweep range of ±5 mV is more than adequate to identify the location of the null (in this example, ±2 mV is adequate to identify a nulling voltage somewhere between 100 μV and 200 μV). Note, however, that the first pass (black trace) only manages to reduce the LO leakage to just under −40 dBm. This clearly indicates that the Q offset needs correction. The second pass (blue trace) involves sweeping the Q offset around 0 V with the I offset held at the value that yielded the first I null. Note that a Q offset of 400 μV reduces the LO leakage a further 10 dB to around −50 dBm. However, a third pass is required. The trough from the first pass is quite shallow because the Q channel had not yet been adjusted. This makes it difficult to identify the ideal I nulling voltage. A third pass (red trace) that involves again sweeping the I offset while holding the Q offset at 400 μV, identifies the optimum I nulling voltage to be 150 μV. –80 –70 –60 –50 –40 –30 –20 –10 0 –2 –1 0 1 2 I AND Q DIFFERENTIAL OFFSET VOLTAGES (mV) FIRST PASS – I OFFSET ADJUST SECOND PASS – Q OFFSET ADJUST THIRD PASS – I OFFSET ADJUST Figure 5. Multipass LO Leakage Compensation Sweeps CORRECTING FOR QUADRATURE AND I/Q GAIN ERRORS A similar procedure can be used to correct quadrature and I/Q gain mismatch. IQ modulator family data sheets typically specify the quadrature phase mismatch and I/Q gain imbalance in degrees and decibels, respectively, along with the sideband suppression (also in decibels). Using this information, it is advisable to perform the first optimization pass on the weaker of the two specifications, that is, the specification which most contributes to the sideband suppression. For example, assume that the device data sheet specifies a sideband suppression of −40 dBc, comprising of 1 degree of phase imbalance and 0.1 dB of gain imbalance amplitude. In this case, it is advisable to first try to adjust phase because making a gain adjustment has almost no effect as long as the 1 degree of phase error is present (see Figure 4). Figure 6 shows the results of a gain sweep followed by a phase sweep. In the first pass, the gain delta between I and Q is adjusted over a range of approximately ±2 dB. The TxDAC® in Figure 1 facilitates this adjustment by providing internal gain adjust auxiliary DACs. The sweep yields a null of around −57 dBc for a gain difference of approximately −0.1 dB (gain is scaled on the top axis). Next, adjust the skew between I and Q. This drives the null down further to −60 dBc for a phase adjust of −0.05°. –70 –60 –50 –40 –30 –20 –10 –0.4 –0.3 –0.2 –0.1 0 0.1 0.2 0.3 0.4 PHASE ADJUST (Degrees) –2 –1 0 1 2 GAIN ADJUST (dB) SECOND PASS (PHASE ADJUST) FIRST PASS (GAIN ADJUST) Figure 6. Multipass Sideband Suppression Compensation Sweeps In this case, a third pass is not necessary and does not yield further improvement. This stems from the fact that the unadjusted phase error is very close to the optimized value (~0.05°). Thus, the first-pass gain adjust yields a deep trough that is only slightly improved during the phase sweep. This contrasts to the LO leakage nulling where a third pass yielded further improvement. |
同様の部品番号 - AD8345 |
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同様の説明 - AD8345 |
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