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LT5571 Datasheet(PDF) 8 Page - Linear Technology |
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LT5571 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LT5571 8 5571f BLOCK DIAGRAM APPLICATIONS INFORMATION The LT5571 consists of I and Q input differential voltage- to-current converters, I and Q up-conversion mixers, an RF signal combiner/balun, an LO quadrature phase generator and LO buffers. External I and Q baseband signals are applied to the dif- ferential baseband input pins, BBPI, BBMI, and BBPQ, BBMQ. These voltage signals are converted to currents and translated to RF frequency by means of double-balanced up-converting mixers. The mixer outputs are combined in an RF output balun, which also transforms the output impedance to 50Ω. The center frequency of the resulting RF signal is equal to the LO signal frequency. The LO input drives a phase shifter which splits the LO signal into in- phase and quadrature LO signals. These LO signals are then applied to on-chip buffers which drive the up-conversion mixers. Both the LO input and RF output are single-ended, 50Ω-matched and AC-coupled. Baseband Interface The baseband inputs (BBPI, BBMI), (BBPQ, BBMQ) present a differential input impedance of about 90kΩ. At each of the four baseband inputs, a capacitor of 1.8pF to ground and a PNP emitter follower is incorporated (see Figure 1), which limits the baseband bandwidth to approximately 200MHz (–1dB point), if driven by a 50Ω source. The circuit is optimized for a common mode voltage of 0.5V which should be externally applied. The baseband input pins should not be left floating because the internal PNP’s base current will pull the common mode voltage higher than the 0.6V limit. This condition may damage the part. The PNP’s base current is about 24µA in normal opera- tion. On the LT5571 demo board, external 50Ω resistors to ground are added to each baseband input to prevent this condition and to serve as a termination resistance for the baseband connections. It is recommended that the I/Q signals be DC-coupled to the LT5571. An applied common mode voltage level at the I and Q inputs of about 0.5V will maximize the LT5571’s dynamic range. Some I/Q generators allow setting the common mode voltage independently. For a 0.5V com- mon mode voltage setting, the common-mode voltage of those generators must be set to 0.5V to create the desired 0.5V bias, when an external 50Ω is present in the setup (See Figure 2). The part should be driven differentially; otherwise, the even- order distortion products will degrade the overall linearity severely. Typically, a DAC will be the signal source for the LT5571. A reconstruction filter should be placed between the DAC output and the LT5571’s baseband inputs. In Figure 3 a typical baseband interface is shown, includ- ing a fifth-order low-pass ladder filter. For each baseband pin, a 0 to 1V swing is developed corresponding to a DAC output current of 0mA to 20mA. The maximum sinusoidal single side-band RF output power is about +5.8dBm for 90 ° 0 ° V-I V-I BALUN VCC RF LO 5571 BD 11 EN 1 3 9 6 GND 4 2 5 7 16 14 8 13 BBPI BBMI BBPQ BBMQ 17 15 GND 12 10 |
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