lt5560 Linear Technology Corporation, lt5560 Datasheet - Page 19

• Manufacturer Part Number: lt5560
• Description: 0.01mhz To 4ghz Low Power Active Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Signal Output Port
A simplifi ed schematic of the output circuit is shown in
Figure 9. The output pins, OUT
connected to the collectors of the mixer transistors. These
pins must be biased at the supply voltage, which can be
applied through a transformer center-tap, impedance
matching inductors, RF chokes, or pull-up resistors. With
external resistor R1 = 3Ω (Figures 1 to 3), each OUT pin
draws about 4.5mA of supply current. For optimum per-
formance, these differential outputs should be combined
externally through a transformer or balun.
An equivalent small-signal model for the output is shown
in Figure 10. The output impedance can be modeled as
a 1.2kΩ resistor in parallel with a 0.7pF capacitor. For
low frequency applications, the 0.7nH series bond-wire
inductances can be ignored.
The external components, C2, L3 and L4, form a lowpass
impedance transformation network to match the mixer
output impedance to the input impedance of transformer
T2. The values for these components can be estimated
LT5560
Figure 9. Output Port Schematic
U
1.2k
0.7pF
U
+
V
CC
and OUT
W
OUT
OUT
5560 F09
–
+
–
, are internally
6
5
U
using the impedance parameters listed in Table 6 along
with similar equations as used for the input matching net-
work. As an example, at an output frequency of 140MHz
and R
LT5560
C2 = C – C
X
n =
Q =
C
L
X
R
1.2k
L
3
INT
C
=
= R
L
=
=
R
R
ω •
= 200Ω (using a 4:1 transformer for T2),
L
S
L
R
(
L
Figure 10. Output Port Small-Signal Model
with External Matching
Q
4
n 1
1
• Q = 420Ω
S
=
X
=
0.7pF
−
C
INT
= 515Ω
C
INT
1082
2
X
200
)
ω
=
0.7nH
0.7nH
L
=
= 1.51pF
2 21
=
2 10
.
OUT
=
239
OUT
.
5560 F10
pF
5 41
+
–
.
6
5
nH
C2
L3
L4
V
CC
T2
LT5560
19
C10
5560f
OUT