LT5526 LINER [Linear Technology], LT5526 Datasheet - Page 12

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

APPLICATIO S I FOR ATIO
LT5526
through impedance-matching inductors. Each IF pin draws
about 7.5mA of supply current (15mA total). For optimum
single-ended performance, these differential outputs must
be combined externally through an IF transformer or
balun.
An equivalent small-signal model for the output is shown
in Figure 10. The output impedance can be modeled as a
575Ω resistor in parallel with a 0.7pF capacitor. For most
applications, the bond-wire inductance (0.7nH per side)
can be ignored.
The external components, C3, L2 and L3 form an imped-
ance transformation network to match the mixer output
impedance to the input impedance of transformer T2. The
values for these components can be estimated using the
same equations that were used for the input matching
12
LT5526
Figure 9. IF Output with External Matching
Figure 10. IF Output Small-Signal Model
0.7pF
575Ω
R
574Ω
IF
U
V
CC
C
0.7pF
IF
U
0.7nH
0.7nH
LT5526
IF
IF
IF
IF
+
–
+
–
11
10
11
10
W
5526 F09
5526 F10
C3
C3
L3
L2
L3
L2
V
CC
U
R
200Ω
4:1
T2
L
IF
OUT
network, along with the impedance values listed in Table
4. As an example, at an IF frequency of 140MHz and R
200Ω (using a 4:1 transformer for T2),
Table 4. IF Differential Impedance (Parallel Equivalent)
Low Cost Output Match
For low cost applications in which the required fractional
bandwidth of the IF output is less than 25%, it may be
possible to replace the output transformer with a lumped-
element network similar to that discussed earlier for the
RF input. This circuit is shown in Figure 11, where L11,
L12, C11 and C12 form a narrowband bridge balun. These
element values are selected to realize a 180° phase shift at
the desired IF frequency and can be estimated by using the
equations below. In this case, R
resistance and R
n = R
Q = √(n – 1) = 1.368
X
C = 1/(ω • X
C3 = C – C
X
L2 = L3 = X
FREQUENCY
C
L
(MHz)
= R
= R
1000
1250
1500
140
240
450
750
860
70
IF
L
IF
/R
/Q = 420Ω
• Q = 274Ω
L
IF
= 574/200 = 2.87
L
C
/2ω = 156nH
= 2.01pF
) = 2.71pF
L
575|| – j3.39k
574|| – j1.67k
IMPEDANCE
572|| – j977
561|| – j519
537|| – j309
525|| – j267
509|| – j229
474|| – j181
435|| – j147
is the load resistance (50Ω).
OUTPUT
REFLECTION COEFFICIENT
IF
0.840
0.840
0.840
0.838
0.834
0.831
0.829
0.822
0.814
MAG
is the mixer output
ANGLE
–11.1
–18.6
–21.3
–24.8
–31.3
–38.0
–1.8
–3.5
–5.9
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L
=