LT5557_11 Datasheet, PDF (10/18 Pages) –

LT5557
applications information
This series transmission line/shunt capacitor matching to-
pology 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 highpass matching network shown in Figures 1
and 3 with C5 = 3.9pF and L5 = 3.6nH. The highpass in-
put 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 simulate board level interfacing to the RF input filter.
Table 1. RF Input Impedance vs Frequency
FREQUENCY
(MHz)
INPUT
IMPEDANCE
S11
MAG
ANGLE
50
4.6 + j2.3
0.832
174.7
300
9.1 + j11.2
0.706
153.8
450
12.0 + j14.5
0.639
145.8
600
14.7 + j17.4
0.588
138.7
900
20.5 + j23.3
0.506
123.4
1300
34.4 + j30.3
0.380
97.5
1700
59.6 + j23.8
0.229
55.8
1950
69.2 + j2.8
0.163
6.9
2200
59.2 – j18.1
0.184
–53.5
2450
41.5 – j24.5
0.274
–94.2
2700
28.3 – j21.3
0.374
–120.3
3000
19.0 – j13.5
0.481
–145.5
3300
13.9 – j5.1
0.568
–167.3
3600
10.8 + j3.4
0.645
171.9
3900
9.4 + j12.3
0.700
151.4
10
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 1GHz to 5GHz.
The input match can be shifted down, as low as 750MHz,
with a single shunt capacitor (C4) on Pin 15. One exam-
ple 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.
EXTERNAL
MATCHING
FOR LO < 1GHz
LO_IN
L4
LO
15
C4
TO
MIXER
LIMITER
V_CC2
REGULATOR
V_REF
5557 F05
Figure 5. LO Input Schematic
0
–10
L4 = 10nH
–20 ^{C4 = 8.2pF}
NO EXT
MATCH
L4 = 2.7nH
C4 = 3.9pF
–30
L4 = 0
C4 = 2.7pF
0.3
1
5
LO FREQUENCY (GHz)
5557 G06
Figure 6. LO Input Return Loss
5557fc