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 simplifi ed 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 simplifi ed 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
the EN pin does not exceed V
CC
by more than 0.3V. Should
APPLICATIONS INFORMATION
RF
1k
1.8V
4.6V
48Ω
48Ω
1k
1V
2.8V
FROM
INTERNAL
MIXERS
INTERNAL
BIAS
5598 F07
V
CC2
Figure 7. Simplifi ed Circuit Schematic of the RF Output
FREQUENCY (MHz)
1
RETURN LOSS (dB)
–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
V
CC1
125k
50k
2V
3V
INTERNAL
ENABLE
CIRCUIT
5598 F09
Figure 9: EN Pin Interface
