LT5526 LINER [Linear Technology], LT5526 Datasheet - Page 10

• Manufacturer Part Number: LT5526
• Description: High Linearity, Low Power Downconverting Mixer
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT5526
The external inductance is split in half (1.4nH), with each
half connected between the pin and C1 as shown in
Figure 4. The inductance may be realized with short, high
impedance printed transmission lines, as in Figure 3,
which provides a compact board layout and reduced
component count. A 1:1 transformer (T1 in Figure 3)
converts the 50Ω differential impedance to a 50Ω single-
ended input.
Table 1. RF Input Differential Impedance
An alternative method of driving the RF input is to use a
lumped-element balun configuration, as shown in Fig-
ure 5. This type of network may provide a more cost-
effective solution for narrow band applications (fractional
bandwidths < 30%). The actual balun is composed of
components C7, C9, L1 and L4, and their values may be
estimated as follows:
10
FREQUENCY
(MHz)
1500
1900
140
240
360
450
750
900
70
Figure 4. RF Input Impedance Matching Topology
R
50Ω
S
C1
IMPEDANCE
28.2 + j2.46
28.0 + j1.34
28.4 + j3.30
28.4 + j4.75
28.6 + j5.42
29.9 + j7.39
31.3 + j8.41
38.3 + j17.9
42.5 + j24.6
U
1/2 X
1/2 X
INPUT
EXT
EXT
U
2
3
RF
RF
+
–
REFLECTION COEFFICIENT
W
1/2 X
1/2 X
0.282
0.280
0.278
0.282
0.280
0.268
0.251
0.237
0.269
MAG
RF
RF
LT5526
R
RF
5526 F04
U
ANGLE
92.2
176
172
169
164
162
155
150
112
Where R
mixer input resistance from Table 1.
The computed values are only approximate, as they don’t
factor in the effects of X
components. Actual component values for several fre-
quencies are listed in Table 2, and measured return loss
vs. frequency is plotted for each example in Figure 6.
L
C
Figure 6. Input Return Loss with Lumped Element Baluns
Using Values from Table 2
1
7
Figure 5. Schematic of Lumped Element Input Balun
=
=
RF
50Ω
L
C
S
IN
4
9
–10
–15
–20
–25
is the source resistance (50Ω) and R
–5
=
0
=
100
L5
ω
R
S
300
R
ω
•
S
R
1
C7
L4
•
RF
500
FREQUENCY (MHz)
R
RF
RF
or the parasitics of the external
700
L1
C9
2
3
900
RF
RF
+
–
1100 1300
1/2 X
1/2 X
5526 F06
RF
RF
LT5526
R
5526 F05
RF
RF
is the
5526f