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LT5558 Datasheet(PDF) 9 Page - Linear Technology |
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LT5558 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LT5558 9 5558fa APPLICATIONS INFORMATION BALUN GND 1.3k 1.8P 1.8P 1.3k RF VCC = 5V BBPI BBMI LOMI VREF = 0.5V LOPI C CM 200 200 FROM Q LT5558 5558 F01 5558 F02 2.1VDC 1.05VCC GENERATOR 50 Ω 50 Ω + – 2.1VDC 2.1VDC GENERATOR 50 Ω + – 2.1VDC LT5558 1.5k Ω + – 5558 F03 2.1VDC 2.1VDC 2.1VDC 2.1VDC VCC RF LT5558 EN BBPI BBPQ BBMI BBMQ GND LO BB SOURCE BB SOURCE 2, 4, 6, 9, 10, 12, 15, 17 14 8, 13 11 1 16 7 5 C1 C2 RF OUT 4.5V TO 5.25V 3 C5 C3 C4 VCC If the I/Q signals are DC-coupled to the LT5558, it is important that the applied common-mode voltage level of the I and Q inputs is about 2.1V in order to properly bias the LT5558. Some I/Q generators allow setting the common-mode voltage independently. In this case, the common-mode voltage of those generators must be set to 1.05V to match the LT5558 internal bias where the internal DC voltage of the signal generators is set to 2.1V due to the source-load voltage division (See Figure 2). The LT5558 baseband inputs should be driven differen- tially, otherwise, the even-order distortion products will degrade the overall linearity severely. Typically, a DAC will be the signal source for the LT5558. A pulse-shaping filter should be placed between the DAC outputs and the LT5558’s baseband inputs. An AC-coupled baseband interface with the LT5558 is drawn in Figure 3. Capacitors C1 to C4 will introduce a low-frequency high-pass corner together with the LT5558’s differential input impedance of 3k Ω. Usually, capacitors C1 to C4 will be chosen equal and in such a way that the –3dB corner frequency f–3dB = 1/(π • RIN,DIFF • C1) is much lower than the lowest baseband frequency. DC coupling between the DAC outputs and the LT5558 baseband inputs is recommended, because AC coupling will introduce a low-frequency time constant that may affect the signal integrity. Active level shifters may be required to adapt the common mode level of the DAC outputs to the common mode input voltage of the LT5558. Such circuits may, however, suffer degraded LO leakage performance as small DC offsets and variations over temperature accumulate. A better scheme is shown in Figure 16, where feedback is used to track out these variations. LO Section The internal LO input amplifier performs single-ended to differential conversion of the LO input signal. Figure 4 shows the equivalent circuit schematic of the LO input. The internal, differential LO signal is split into in-phase and quadrature (90° phase shifted) signals that drive LO buffer sections. These buffers drive the double bal- anced I and Q mixers. The phase relationship between Figure 1. Simplifed Circuit Schematic of the LT5558 (Only I-Half is Drawn) Figure 2. DC Voltage Levels for a Generator Programmed at 1.05VDC for a 50 Ω Load and the LT5558 as a Load Figure 3. AC-Coupled Baseband Interface LO INPUT 20pF ≈ 50Ω 5558 F04 VCC Figure 4. Equivalent Circuit Schematic of the LO Input |
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Similar Description - LT5558 |
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