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ADL5370 Datasheet(PDF) 3 Page - Analog Devices |
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ADL5370 Datasheet(HTML) 3 Page - Analog Devices |
3 / 8 page Application Note AN-1039 Rev. 0 | Page 3 of 8 After low-pass filtering, the two word streams are applied to a pair of digital-to-analog converters (DAC). The DAC outputs drive two low-pass filters whose primary role is to remove Nyquist images. The outputs of these filters then drive the baseband inputs of the IQ modulator. The local oscillator (LO) input of the modulator is driven by a relatively pure CW signal generated by a phase-locked loop (PLL) such as the ADF4106 from Analog Devices, Inc. Now, take a closer look at the operation of the IQ modulator. The LO signal is split into two signals, equal in amplitude but with a phase difference of exactly 90°. These two quadrature signals drive the inputs of the two mixers that, for the purposes of this application note, are viewed as analog multipliers. The outputs of these two multipliers are added together (in the Σ block of the IQ modulator) to provide the IQ modulator’s output. While it is apparent that the baseband data streams have been filtered, instead briefly consider them as the original bit streams. Instead of a stream of 1s and 0s, think of them as two streams switching between a value of +1 and –1. So, the output of the I multiplier consists of a vector which is flipping in-phase between 0° and 180°as the bit stream alternates. Likewise, the output of the Q multiplier is a vector that flips between +90° and –90° as the bit stream modulates the original 90° vector. Thus, if at a particular instant, both the I and Q bit streams are equal to +1, the result at the output of the IQ modulator is the sum of the 90° and 0° vectors, that is, a +45° vector. Likewise, I and Q bit combinations of −1/+1, −1/−1, and +1/−1 produce vectors (commonly called symbols) all of equal amplitude at +135°, −135°, and −45°, respectively. If these vectors were plotted, observe the constellation of the modulated carrier (see Figure 2A). (A) (B) (C) (D) (E) (F) Figure 2. Error Vector Magnitude Constellations that Result from Various Modulator Imperfections MODULATOR IMPERFECTIONS Contrary to the previous hypothetical situation, in a real IQ modulator, things do not look so perfect. A series of effects in the IQ modulator conspire to create QPSK (or QAM) vectors that are neither equal in amplitude nor separated by exactly 45°. Consider first what happens if for some reason the gain of the I path is greater than that of the Q channel; this could be caused by a DAC gain mismatch, low-pass filter insertion loss, mismatch, or gain imbalance inside the IQ modulator. Regardless of where this gain imbalance comes from, its effect is the same. Because the 0°/180° vectors at the output of the I multiplier are larger than the +90°/−90° vectors from the Q multiplier, the shape of the constellation becomes rectangular (see Figure 2B). This degrades signal integrity at the receiver because the receiver is expecting a perfectly square constellation. In the QPSK example shown in Figure 2B, a slight gain imbalance is unlikely to result in an incorrect bit decision in the receiver unless the received signal is very small. However, in higher order modulation schemes such as 16 QAM or 64 QAM (see Figure 2E and Figure 2F), the increased density of the constellation points could easily combine with an IQ gain imbalance to produce an incorrect symbol decision in the receiver. In most IQ modulators, the 90° phase split of the LO is achieved using either a polyphase filter or a divide-by-two flip-flop circuit (which requires an external LO that is twice the desired output frequency). In either circuit, the 90° phase split or quadrature is never perfect. For example, if there is a 1° quadrature error, the shape of the resulting constellation is slightly trapezoidal (see Figure 2C). Just like IQ gain imbalance, this can result in incorrect bit decisions in the receiver. Now consider what happens if either the I or Q paths have unwanted dc offset errors. This results in the +1/−1 multipli- cation being skewed. For example, an offset that is equal to 1% of the baseband signal amplitude causes the +1/−1 multipliers to be modified to +1.01/−0.99. This has the effect of shifting the center of the constellation off the origin, on either the I or Q axis, most likely in both (see Figure 2 D). In the frequency domain, this manifests itself as a small portion of the unmodu- lated carrier appearing at the output of the modulator. In the frequency domain, this LO leakage (also referred to as LO feedthrough) appears at the center of the modulated spectrum. Because of parasitic capacitances within the silicon die and bond-wire to bond-wire coupling, the signal that is applied to the LO port of the IQ modulator may also couple directly to the RF output. This leakage is independent of the offset multiplication effect that was described previously. However, its manifestation, that is, the presence of the unmodulated carrier in the output spectrum, is exactly the same. Thus, the net LO leakage seen at the output of the IQ modulator is the vector sum of these two components. Fortunately, as discussed in the Correcting Modulator Imperfections section, the com- posite LO leakage at the output can be mitigated by a single compensation technique. |
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