LTC5598

13

5598f

Table 7. RF Output Impedance vs Frequency for EN = High

FREQUENCY

(MHz)

RF OUTPUT

IMPEDANCE

REFLECTION COEFFICIENT

MAG

ANGLE

0.1

59.0 – j0.6

0.083

–3.6

1

58.5 – j2.1

0.081

–12.7

2

57.3 – j3.5

0.076

–23.6

4

54.6 – j4.5

0.061

–41.6

8

51.9 – j3.6

0.040

–60.8

16

50.5 – j2.1

0.022

–74.8

30

50.2 – j1.1

0.011

–80

60

50 – j0.5

0.005

–86.5

100

50 – j0.2

0.002

–84.9

200

49.7 + j0

0.003

177.4

400

48.9 + j0.3

0.011

162

800

46.1 + j0.4

0.041

173.3

1000

44.5 + j0.2

0.058

178

1250

42.8 + j0

0.077

–179.7

1500

41.2 – j0.1

0.097

–179.4

1800

39.9 + j0.4

0.113

177.4

The RF port output impedance for EN = Low is given in Table

8. It is roughly equivalent to a 1.3pF capacitor to ground.

Table 8. RF Output Impedance vs Frequency for EN = Low

FREQUENCY

(MHz)

LO INPUT

IMPEDANCE

REFLECTION COEFFICIENT

MAG

ANGLE

100

82.3 – j1223

0.995

–4.6

200

51.1 – j618

0.987

–9.2

400

35.3 – j310

0.965

–18.1

800

24.4 – j148

0.906

–36.6

1000

20.4 – j114

0.878

–46.4

1250

17 – j87

0.847

–58.4

1500

14.7 – j68

0.818

–70.7

1800

13.1 – j54

0.785

–84.3

In Figure 7 the simplified circuit schematic of the RF

output buffer is drawn. A plot of the RF port return loss vs

frequency is drawn in Figure 8 for EN = High and Low.

Enable Interface

Figure 9 shows a simplified schematic of the EN pin

interface. The voltage necessary to turn on the LTC5598

is 2V. To disable (shut down) the chip, the enable voltage

must be below 1V. If the EN pin is not connected, the chip

is enabled. This EN = High condition is assured by the 125k

on-chip pull-up resistor. It is important that the voltage at

APPLICATIONS INFORMATION

RF

1k

1.8V

4.6V

48Ω

48Ω

1k

1V

2.8V

FROM

INTERNAL

MIXERS

INTERNAL

BIAS

5598 F07

Figure 7. Simplified Circuit Schematic of the RF Output

FREQUENCY (MHz)

1

–10

–30

–20

–40

100

1000

10

–60

–50

0

5598 F08

EN = LOW

EN = HIGH

C6 = 220nF, SEE FIGURE 10

Figure 8. RF Port Return Loss vs Frequency

EN

125k

50k

2V

3V

INTERNAL

ENABLE

CIRCUIT

5598 F09

Figure 9: EN Pin Interface