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LT5575

5575f

APPLICATIONS INFORMATION

The LO input impedance and S11 parameters (without

external matching components) are listed in Table 2.

Table 2. LO Input Impedance

FREQUENCY

(GHz)

INPUT

IMPEDANCE (

Ω)

S11

MAG

ANGLE (°)

0.8

9.6 +j 23.7

0.731

127.9

0.9

13 +j 27.1

0.669

120.4

1.0

17.9 +j 30

0.592

113.2

1.1

24.1 +j 31.7

0.508

106.1

1.2

31.2 +j 31.4

0.421

99.8

1.3

37.5 +j 28.9

0.341

95.1

1.4

41.9 +j 24.6

0.272

93.4

1.5

43.4 +j 20

0.221

96.2

1.6

42.9 +j 16.4

0.189

103.5

1.7

41.2 +j 14.1

0.18

113.1

1.8

39.5 +j 13.1

0.186

120.3

1.9

37.8 +j 13.1

0.201

124.5

2.0

36.6 +j 13.6

0.217

125.6

2.1

35.6 +j 14.6

0.236

125

2.2

35.1 +j 15.7

0.25

123.1

2.3

34.9 +j 17.1

0.264

120.1

2.4

35.1 +j 18.5

0.272

116.6

2.5

35.5 +j 19.9

0.281

113

2.6

36.3 +j 21.2

0.284

109

2.7

37.2 +j 22.5

0.287

105.1

I-Channel and Q-Channel Outputs

Each of the I-channel and Q-channel outputs is internally

connected to VCC through a 65Ω resistor. The output DC

bias voltage is VCC – 1.1V. The outputs can be DC-coupled

or AC-coupled to the external loads. Each single-ended

output has an impedance of 65Ω in parallel with a 5pF

internal capacitor, forming a low-pass ﬁlter with a –3dB

corner frequency at 490MHz. The loading resistance

on each output, RLOAD (single-ended), should be larger

than 300Ω to assure full gain. The gain is reduced by

20 • log10(1 + 65Ω/RLOAD) in dB when the output port is

terminated by RLOAD. For instance, the gain is reduced

by 7.23dB when each output pin is connected to a

50Ω load (or 100Ω differentially). The output should be

taken differentially (or by using differential-to-single-

ended conversion) for best RF performance, including

NF and IM2.

The phase relationship between the I-channel output signal

and the Q-channel output signal is ﬁxed. When the LO

input frequency is larger (or smaller) than the RF input

frequency, the Q-channel outputs (QOUT

+, Q

OUT

–) lead (or

lag) the I-channel outputs (IOUT

+, I

OUT

–) by 90°.

When AC output coupling is used, the resulting high-

pass ﬁlter’s –3dB roll-off frequency is deﬁned by the RC

constant of the blocking capacitor and RLOAD, assuming

RLOAD >> 65Ω.

Figure 8. I/Q Output Equivalent Circuit

VCC

IOUT

–

5575 F08

QOUT

–

Ω

5pF

Ω

5pF

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