lt5557 Linear Technology Corporation, lt5557 Datasheet - Page 10

• Manufacturer Part Number: lt5557
• Description: 400mhz To 3.8ghz 3.3v High Linearity Downconverting Rf Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5557
series transmission line/shunt capacitor matching topol-
ogy allows the LT5557 to be used for multiple frequency
standards without circuit board layout modifications. The
series transmission line can also be replaced with a series
chip inductor for a more compact layout.
Input return losses for the 450MHz, 900MHz, 2.6GHz and
3.6GHz applications are plotted in Figure 4b. The input
return loss with no external matching is repeated in Figure
4b for comparison. The 2.6GHz RF input match uses the
high-pass matching network shown in Figures 1 and 3
with C5 = 3.9pF and L5 = 3.6nH. The high-pass input
matching network is also used to create a wideband or
dual-band input match. For example, with C5 = 3.3pF and
L5 = 10nH, the RF input is matched from 800MHz to
2.2GHz, with optimum matching in the 800MHz to 1.1GHz
and 1.6GHz to 2.2GHz bands, simultaneously.
RF input impedance and S11 versus frequency (with no
external matching) are listed in Table 1 and referenced to
Pin 3. The S11 data can be used with a microwave circuit
simulator to design custom matching networks and simu-
late board-level interfacing to the RF input filter.
Table 1. RF Input Impedance vs Frequency
10
FREQUENCY
(MHz)
1300
1700
1950
2200
2450
2700
3000
3300
3600
3900
300
450
600
900
50
IMPEDANCE
U
12.0 + j14.5
14.7 + j17.4
20.5 + j23.3
34.4 + j30.3
59.6 + j23.8
59.2 – j18.1
41.5 – j24.5
28.3 – j21.3
19.0 – j13.5
9.1 + j11.2
69.2 + j2.8
13.9 – j5.1
10.8 + j3.4
9.4 + j12.3
4.6 + j2.3
INPUT
U
W
0.832
0.706
0.639
0.588
0.506
0.380
0.229
0.163
0.184
0.274
0.374
0.481
0.568
0.645
0.700
MAG
S11
U
ANGLE
–120.3
–145.5
–167.3
174.7
153.8
145.8
138.7
123.4
–53.5
–94.2
171.9
151.4
97.5
55.8
6.9
LO Input Port
The mixer’s LO input, shown in Figure 5, consists of an
integrated transformer and high speed limiting differential
amplifiers. The amplifiers are designed to precisely drive
the mixer for the highest linearity and the lowest noise
figure. An internal DC blocking capacitor in series with the
transformer’s primary eliminates the need for an external
blocking capacitor.
The LO input is internally matched from 1 to 5GHz. The
input match can be shifted down, as low as 750MHz, with
a single shunt capacitor (C4) on Pin 15. One example is
plotted in Figure 6 where C4 = 2.7pF produces a 750MHz
to 1GHz match.
LO input matching below 750MHz requires the series
inductor (L4)/shunt capacitor (C4) network shown in
Figure 5. Two examples are plotted in Figure 6 where L4 =
2.7nH/C4 = 3.9pF produces a 650MHz to 830MHz match
and L4 = 10nH/C4 = 8.2pF produces a 460MHz to 560MHz
match. The evaluation boards do not include pads for L4,
so the circuit trace needs to be cut near Pin 15 to insert L4.
A low cost multilayer chip inductor is adequate for L4.
LO
IN
FOR LO < 1GHz
EXTERNAL
MATCHING
C4
L4
–10
–20
–30
0
Figure 6. LO Input Return Loss
Figure 5. LO Input Schematic
0.3
15
C4 = 8.2pF
L4 = 10nH
L4 = 2.7nH
C4 = 3.9pF
LO
V
CC2
LO FREQUENCY (GHz)
L4 = 0
C4 = 2.7pF
1
REGULATOR
NO EXT
MATCH
5557 G06
LIMITER
5
V
REF
5557 F05
TO
MIXER
5557fa