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LT5519EUF Datasheet(PDF) 7 Page - Linear Technology |
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LT5519EUF Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LT5519 7 5519f APPLICATIO S I FOR ATIO The LT5519 consists of a double-balanced mixer, a high performance LO buffer and bias/enable circuits. The RF and LO ports may be driven differentially; however, they are intended to be used in single-ended mode by connect- ing one input of each pair to ground. The IF input ports must be DC-isolated from the source and driven differen- tially. The IF input should be impedance-matched for the desired input frequency. The LO input has an internal broadband 50 Ω match with return loss better than 10dB at frequencies up to 1800MHz. The RF output band ranges from 700MHz to 1400MHz, with an internal RF trans- former providing a 50 Ω impedance match across the band. Low side or high side LO injection can be used. IF Input Port The IF inputs are connected to the emitters of the double- balanced mixer transistors, as shown in Figure 3. These pins are internally biased and an external resistor must be connected from each IF pin to ground to set the current through the mixer core. The circuit has been optimized to work with 100 Ω resistors, which will result in approxi- mately 18mA of DC current per side. For best LO leakage performance, the resistors should be well matched; thus resistors with 0.1% tolerance are recommended. If LO leakage is not a concern, then lesser tolerance resistors can be used. The symmetry of the layout is also important for achieving optimum LO isolation. The capacitors shown in Figure 3, C1 and C2, serve two purposes. They provide DC isolation between the IF+ and IF – ports, thus preventing DC interactions that could cause unpredictable variations in LO leakage. They also improve the impedance match by canceling excess induc- tance in the package and transformer. The input capacitor value required to realize an impedance match at desired frequency, f, can be estimated as follows: CC fL L IN EXT 12 2 1 2 == π+ () ( ) where; f is in units of Hz, LIN and LEXT are in Henry, and C1, C2 are in Farad. LIN is the differential input inductance of the LT5519, and is approximately 1.67nH. LEXT represents the combined inductances of differential external compo- nents and transmission lines. For the evaluation board shown in Figure 10, LEXT = 4.21nH. Thus, for f = 140MHz, the above formula gives C1 = C2 = 220pF. 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 DIFFERENTIAL S11 (MHz) INPUT 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 LT5519 IF+ IF– 18mA 18mA 5519 F03 |
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