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LT5575

5575f

APPLICATIONS INFORMATION

The RF input impedance and S11 parameters (without

external matching components) are listed in Table 1.

LO Input Port

The demodulator’s LO input interface is shown in Fig-

ure 6. The input consists of an integrated transformer and a

precision quadrature phase shifter which generates 0° and

90° phase-shifted LO signals for the LO buffer ampliﬁers

driving the I/Q mixers. The primary side of the transformer

is connected to the LO input pin. The secondary side of

the transformer is connected to the differential inputs of

the LO quadrature generator. Under no circumstances

should an external DC voltage be applied to the input pin.

DC current ﬂowing into the primary side of the transformer

may damage the transformer. A series blocking capacitor

should be used to AC-couple the LO input port to the LO

signal source.

Table 1. RF Input Impedance

FREQUENCY

(GHz)

INPUT

IMPEDANCE (

Ω)

S11

MAG

ANGLE (°)

0.8

8.1 +j 21.3

0.760

133.0

0.9

10.5 +j 24.9

0.715

125.4

1.0

13.8 +j 28.8

0.660

117.2

1.1

18.6 +j 32.5

0.595

108.6

1.2

25.2 +j 35.5

0.521

99.6

1.3

33.6 +j 36.8

0.441

90.3

1.4

43.1 +j 34.6

0.355

80.8

1.5

51.4 +j 28.4

0.270

71.6

1.6

55.8 +j 19.3

0.188

1.7

55.4 +j 10.4

0.110

56.9

1.8

51.8 +j 3.9

0.042

1.9

46.9 +j 0.4

0.032

172.7

2.0

42.3 +j –0.8

0.084

–173.9

2.1

38.4 +j –0.3

0.131

–178.2

2.2

35.4 +j 1

0.172

175.3

2.3

33 +j 2.9

0.207

168.4

2.4

31.5 +j 4.9

0.235

161.9

2.5

30.4 +j 7

0.258

155.4

2.6

29.9 +j 9.1

0.274

149.2

2.7

29.7 +j 11.1

0.287

143.4

The LO 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 a lower frequency, the input return loss can

be improved with the matching network shown in Figure

6. Shunt capacitor C12 and series capacitor C13 can be

selected for optimum input impedance matching at the

desired frequency as illustrated in Figure 7. For lower

frequency operation, external matching component C13

can serve as the series DC blocking capacitor.

Figure 7. LO Input Return Loss with External Matching

Figure 6. LO Input Interface

5575 F06

EXTERNAL

MATCHING

NETWORK FOR

LOW BAND AND

MID BAND

INPUT

C12

C13

LO QUADRATURE

GENERATOR AND

BUFFER AMPLIFIERS

–30

–25

–20

–15

–10

–5

0.5

1.0

1.5

2.0

2.5

3.0

FREQUENCY (GHz)

5575 F07

NO EXTERNAL

MATCHING

C13 = 5.6pF;

NO SHUNT CAP

C13 = 5.6pF;

C12 = 3.9pF

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