LT5519

8

5519f

APPLICATIO S I FOR ATIO

Though the LO input is internally matched to 50

Ω, there

may be some cases, particularly at higher frequencies or

with different source impedances, where a further opti-

mized 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

200

72.3 – j16.1

0.223

–28.4

400

63.3 – j11.3

0.153

–34.7

600

61.6 – j7.5

0.124

– 29.2

800

61.9 – j6.0

0.119

– 23.6

1000

62.7 – j6.1

0.125

–22.7

1200

63.2 – j7.4

0.134

–25.5

1400

63.3 – j9.5

0.144

–30.8

1600

62.8 – j12.0

0.155

–37.1

1800

61.6 – j14.2

0.163

–43.4

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.

The impedance data for the RF output, listed in Table 3, can

be used to develop matching networks for different load

impedances.

Table 3. Single-Ended RF Output Impedance

FREQUENCY

OUTPUT

S11

(MHz)

IMPEDANCE

MAG

ANGLE

700

27.6 + j32.0

0.465

103

800

39.7 + j32.1

0.354

88.1

900

50.9 + j23.5

0.227

74.7

1000

53.5 + j10.3

0.105

65.5

1100

48.3 + j1.3

0.022

143

1200

42.0 – j3.1

0.093

–157

1300

36.6 – j3.4

0.159

–164

1400

33.0 – j2.0

0.207

–172

Operation at Different Input Frequencies

On the evaluation board shown in Figure 10, the input of

the LT5519 can be easily matched for different frequencies

by changing the capacitors, C1, C2 and C3. Capacitors C1

and C2 set the input matching frequency while C3 im-

proves the LO to RF leakage performance. Decreasing the

value of C3 at higher input frequencies reduces its impact

on conversion gain. Table 4 lists some actual values used

at selected frequencies.

Figure 4. LO Input Circuit

50

Ω

14

15

5519 F04

85

Ω

LT5519

5pF

5pF

220

Ω

220

Ω

Figure 5. RF Output Circuit

50

Ω

11

10

10pF

LT5519

5519 F05

8