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LT5579

5579f

APPLICATIONS INFORMATION

C9 to step the impedance up to about 12.5Ω. At lower

frequencies additional series inductance may be required

between the IF ports and C9. The position of C9 may vary

with the IF frequency due to the different series inductance

requirements. The 4:1 impedance ratio of transformer T1

completes the transformation to 50 ohms. Table 1 lists the

differential IF input impedances and reﬂection coefﬁcients

for several frequencies.

Table 1. IF Input Differential Impedance

FREQUENCY

(MHz)

IF INPUT

IMPEDANCE

REFLECTION COEFFICIENT

MAG

ANGLE

8.8+j1.3

0.70

177

140

8.7+j2.3

0.70

175

170

9.0+j2.8

0.70

174

190

8.9+j3.0

0.70

173

240

9.0+j4.0

0.70

170

380

9.7+j4.9

0.68

168

450

10.0+j5.2

0.67

167

750

10.8+j9.4

0.65

158

1000

11.8+j13.8

0.64

148

The purpose of capacitor C3 is to improve the LO-RF

leakage in some applications. This relatively small-valued

capacitor has little effect on the impedance match in most

cases. This capacitor should typically be located close to

the IC, however, there may be cases where re-positioning

the capacitor may improve performance.

The measured return loss of the IF input is shown in

Figure 3 for application frequencies of 70MHz, 240MHz

and 456MHz. Component values are listed in Table 2. (For

70MHz matching details, refer to Figure 8.)

Table 2. IF Input Component Values

FREQUENCY

(MHz)

C1, C2

(pF)

L1, L2

(nH)

R1, R2

(Ω)

MATCH BW

(at 12dB RL)

140

1000

120

(1)

100

9.1

<50 to 158

240

(1)

174 to 263

450

(1)

330 to 505

Note: (1) Depends on RF, (2) T1 = M/A-Com MABAES0061

FREQUENCY (MHz)

–10

–5

300

500

800

5579 F03

–15

–20

–25

100 200

400

600 700

Figure 3. IF Input Return Loss with 70MHz (a),

240MHz (b) and 456MHz (c) Matching

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