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

driven differentially; however, they are intended to be

driven by a single-ended source. An internal resistor

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

50

Ω

T1

4:1

2

3

100

Ω

0.1%

100

Ω

0.1%

18mA

18mA

5520 F03

LT5520

50

Ω

14

15

5520 F04

85

Ω

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

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

secondary windings of the transformer, thus a DC voltage

should not be applied across these pins.

APPLICATIO S I FOR ATIO