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LT5520EUF Datasheet(PDF) 7 Page - Linear Technology |
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LT5520EUF Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LT5520 7 5520f Table 1 lists the differential IF input impedance and reflec- tion coefficient for several frequencies. A 4:1 balun can be used to transform the impedance up to about 50 Ω. Table 1. IF Input Differential Impedance Frequency Differential Input Differential S11 (MHz) Impedance Mag Angle 10 10.1 + j0.117 0.663 180 44 10.1 + j0.476 0.663 179 70 10.1 + j0.751 0.663 178 140 10.2 + j1.47 0.663 177 170 10.2 + j1.78 0.663 176 240 10.2 + j2.53 0.663 174 360 10.2 + j3.81 0.663 171 500 10.2 + j5.31 0.663 167 LO Input Port The simplified circuit for the LO buffer input is shown in Figure 4. The LO buffer amplifier consists of high-speed limiting differential amplifiers, optimized to drive the mixer quad for high linearity. The LO+ and LO– ports can be driven differentially; however, they are intended to be driven by a single-ended source. An internal resistor connected across the LO+ and LO– inputs provides a broadband 50 Ω impedance match. Because of the resis- tive match, a DC voltage at the LO input is not recom- mended. If the LO signal source output is not AC coupled, then a DC blocking capacitor should be used at the LO input. Figure 3. IF Input with External Matching C1 C2 C3 IFIN 50 Ω T1 4:1 2 3 100 Ω 0.1% 100 Ω 0.1% VCC 18mA 18mA 5520 F03 LT5520 LOIN 50 Ω 14 15 VCC 5520 F04 85 Ω LO– LO+ 5pF 5pF 220 Ω 220 Ω LT5520 Figure 4. LO Input Circuit Though the LO input is internally 50 Ω matched, there may be some cases, particularly at higher frequencies or with different source impedances, where a further optimized match is desired. Table 2 includes the single -ended input impedance and reflection coefficient vs frequency for the LO input for use in such cases. Table 2. Single-Ended LO Input Impedance Frequency Input S11 (MHz) Impedance Mag Angle 1300 62.8 – j9.14 0.139 –30.9 1500 62.2 – j11.4 0.148 –37.1 1700 61.5 – j13.4 0.157 – 42.4 1900 60.0 – j15.2 0.164 – 48.9 2100 58.4 – j16.9 0.172 –54.7 2300 56.5 – j17.9 0.176 –60.4 2500 54.9 – j18.8 0.182 –65.1 2700 53.7 – j18.8 0.182 –68.5 RF Output Port An internal RF transformer, shown in Figure 5, reduces the mixer-core impedance to provide an impedance of 50 Ω across the RF+ and RF– pins. The LT5520 is designed and tested with the outputs configured for single-ended opera- tion, as shown in the Figure 5; however, the outputs can be used differentially as well. A center-tap in the transformer provides the DC connection to the mixer core and the transformer provides DC isolation at the RF output. The RF+ and RF– pins are connected together through the secondary windings of the transformer, thus a DC voltage should not be applied across these pins. APPLICATIO S I FOR ATIO |
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