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LT5575EUFTRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT5575EUFTRPBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT5575 11 5575f APPLICATIONS INFORMATION The LT5575 is a direct I/Q demodulator targeting high linearity receiver applications, such as RFID readers and wireless infrastructure. It consists of RF transconductance amplifiers, I/Q mixers, a quadrature LO phase shifter, and bias circuitry. The RF signal is applied to the inputs of the RF transconductance amplifiers and is then demodulated into I/Q baseband signals using quadrature LO signals which are internally generated from an external LO source by precision 90° phase-shifters. The demodulated I/Q signals are single-pole low-pass filtered on-chip with a –3dB bandwidth of 490MHz. The differential outputs of the I-channel and Q-channel are well matched in amplitude; their phases are 90° apart. Broadband transformers are integrated on-chip at both the RF and LO inputs to enable single-ended RF and LO interfaces. In the high frequency band (1.5GHz to 2.7GHz), both RF and LO ports are internally matched to 50Ω. No external matching components are needed. For the lower frequency bands (800MHz to 1.5GHz), a simple network with series and/or shunt capacitors can be used as the impedance matching network. RF Input Port Figure 4 shows the demodulator’s RF input which con- sists of an integrated transformer and high linearity transconductance amplifiers. The primary side of the transformer is connected to the RF input pin. The second- ary side of the transformer is connected to the differential inputs of the transconductance amplifiers. Under no cir- cumstances should an external DC voltage be applied to the RF input pin. DC current flowing into the primary side of the transformer may cause damage to the integrated transformer. A series blocking capacitor should be used to AC-couple the RF input port to the RF signal source. The RF input port is internally matched over a wide fre- quency range from 1.5GHz to 2.7GHz with input return loss typically better than 10dB. No external matching network is needed for this frequency range. When the part is operated at lower frequencies, however, the input return loss can be improved with the matching network shown in Figure 4. Shunt capacitor C10 and series capacitor C11 can be selected for optimum input impedance matching at the desired frequency as illustrated in Figure 5. For lower fre- quency band operation, the external matching component C11 can serve as a series DC blocking capacitor. Figure 5. RF Input Return Loss with External Matching Figure 4. RF Input Interface 5575 F04 2 3 EXTERNAL MATCHING NETWORK FOR LOW BAND AND MID BAND RF INPUT RF C10 C11 TO I-MIXER TO Q-MIXER –30 –25 –20 –15 –10 –5 0 0.5 1.0 1.5 2.0 2.5 3.0 FREQUENCY (GHz) 5575 F05 NO EXTERNAL MATCHING C11 = 3.9pF; NO SHUNT CAP C11 = 5.6pF; C10 = 4.7pF |
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