lt5525 Linear Technology Corporation, lt5525 Datasheet - Page 10

• Manufacturer Part Number: lt5525
• Description: High Linearity, Low Power Downconverting Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5525
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
574Ω resistor (R
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
equations below, along with the impedance values listed in
Table 3. As an example, at an IF frequency of 140MHz and
R
50Ω load),
Table 3. 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-
10
L
n = R
Q = √(n – 1) = 1.368
X
C = 1/(ω • X
C3 = C – C
X
L2 = L3 = X
FREQUENCY
= 200Ω (using a 4:1 transformer for T2 with an external
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
IF
575|| – j3.39k
574|| – j1.67k
IMPEDANCE
572|| – j977
561|| – j519
537|| – j309
525|| – j267
509|| – j229
474|| – j181
435|| – j147
) in parallel with a 0.7pF capacitor. For
U
OUTPUT
U
REFLECTION COEFFICIENT
W
0.840
0.840
0.840
0.838
0.834
0.831
0.829
0.822
0.814
MAG
U
ANGLE
–11.1
–18.6
–21.3
–24.8
–31.3
–38.0
–1.8
–3.5
–5.9
element network. 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
using the equations below. In this case, the load resis-
tance, R
Inductor L13 or L14 provides a DC path between V
the IF
cost multilayer chip inductors are adequate for L11, L12
and L13. If L14 is used instead of L13, a larger value is
usually required, which may require the use of a wire-
wound inductor. Capacitor C13 is a DC block which can
also be used to adjust the impedance match. Capacitor
C14 is a bypass capacitor.
Actual component values for IF frequencies of 240MHz,
360MHz and 450MHz are listed in Table 4. Typical IF port
return loss for these examples is shown in Figure 12.
L
C
11
11
+
=
=
LT5525
pin. Only one of these inductors is required. Low
L
L
, is 50Ω.
C
12
12
Figure 10. IF Output Small Signal Model
Figure 11. Narrowband Bridge IF Balun
=
R
574Ω
=
IF
V
ω
IF
IF
CC
+
–
R
IF
R
ω
IF
•
1
L14
OPT
L12
C
0.7pF
R
C12
IF
•
L
R
0.7nH
0.7nH
L
C11
L11
IF
IF
+
–
11
10
L13
OPT
5525 F10
C13
C14
C3
L3
L2
5525 F11
IF
50Ω
OUT
R
200Ω
L
CC
and
5525f