lt5519 Linear Technology Corporation, lt5519 Datasheet - Page 7

• Manufacturer Part Number: lt5519
• Description: 0.7ghz To 1.4ghz High Linearity Upconverting Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The LT5519 consists of a double-balanced mixer, a high
performance LO buffer and bias/enable circuits. The RF
and LO ports may be driven differentially; however, they
are intended to be used in single-ended mode by connect-
ing one input of each pair to ground. The IF input ports
must be DC-isolated from the source and driven differen-
tially. The IF input should be impedance-matched for the
desired input frequency. The LO input has an internal
broadband 50 match with return loss better than 10dB
at frequencies up to 1800MHz. The RF output band ranges
from 700MHz to 1400MHz, with an internal RF trans-
former providing a 50
band. Low side or high side LO injection can be used.
IF Input Port
The IF inputs are connected to the emitters of the double-
balanced mixer transistors, as shown in Figure 3. These
pins are internally biased and an external resistor must be
connected from each IF pin to ground to set the current
through the mixer core. The circuit has been optimized to
work with 100 resistors, which will result in approxi-
mately 18mA of DC current per side. For best LO leakage
performance, the resistors should be well matched; thus
resistors with 0.1% tolerance are recommended. If LO
leakage is not a concern, then lesser tolerance resistors
can be used. The symmetry of the layout is also important
for achieving optimum LO isolation.
The capacitors shown in Figure 3, C1 and C2, serve two
purposes. They provide DC isolation between the IF
IF
cause unpredictable variations in LO leakage. They also
–
50
IF
ports, thus preventing DC interactions that could
IN
Figure 3. IF Input with External Matching
4:1
T1
C1
C2
U
C3
100
0.1%
100
0.1%
U
impedance match across the
2
3
LT5519
IF
IF
+
–
18mA
18mA
W
V
CC
U
5519 F03
+
and
improve the impedance match by canceling excess induc-
tance in the package and transformer. The input capacitor
value required to realize an impedance match at desired
frequency, f, can be estimated as follows:
where; f is in units of Hz, L
C2 are in Farad. L
the LT5519, and is approximately 1.67nH. L
the combined inductances of differential external compo-
nents and transmission lines. For the evaluation board
shown in Figure 10, L
the above formula gives C1 = C2 = 220pF.
Table 1 lists the differential IF input impedance and reflec-
tion coefficient for several frequencies. A 4:1 balun can be
used to transform the impedance up to about 50 .
Table 1. IF Input Differential Impedance
LO Input Port
The simplified circuit for the LO buffer input is shown in
Figure 4. The LO buffer amplifier consists of high speed
limiting differential amplifiers, optimized to drive the mixer
quad for high linearity. The LO
driven differentially; however, they are intended to be
driven by a single-ended source. An internal resistor
connected across the LO
broadband 50 impedance match. Because of the resis-
tive match, a DC voltage at the LO input is not recom-
mended. If the LO signal source output is not AC coupled,
then a DC blocking capacitor should be used at the LO
input.
FREQUENCY
C
1
(MHz)
140
170
240
360
500
10
44
70
C
2
2 ( ) (
INPUT IMPEDANCE
f L
IN
DIFFERENTIAL
2
10.1 + j0.117
10.1 + j0.476
10.1 + j0.751
10.2 + j1.47
10.2 + j1.78
10.2 + j2.53
10.2 + j3.81
10.2 + j5.31
is the differential input inductance of
EXT
IN
1
= 4.21nH. Thus, for f = 140MHz,
IN
L
+
and L
EXT
and LO
)
+
EXT
and LO
–
are in Henry, and C1,
0.663
0.663
0.663
0.663
0.663
0.663
0.663
0.663
inputs provides a
MAG
DIFFERENTIAL S11
–
EXT
LT5519
ports can be
represents
ANGLE
180
179
178
177
176
174
171
167
7
5519f