LT5519

7

5519f

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

Ω 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

Ω impedance match across the

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

Ω 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

cause unpredictable variations in LO leakage. They also

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:

CC

fL

L

IN

EXT

12

2

1

2

==

π+

() (

)

the combined inductances of differential external compo-

nents and transmission lines. For the evaluation board

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

Ω.

Table 1. IF Input Differential Impedance

FREQUENCY

DIFFERENTIAL

DIFFERENTIAL S11

(MHz)

INPUT IMPEDANCE

MAG

ANGLE

10

10.1 + j0.117

0.663

180

44

10.1 + j0.476

0.663

179

70

10.1 + j0.751

0.663

178

140

10.2 + j1.47

0.663

177

170

10.2 + j1.78

0.663

176

240

10.2 + j2.53

0.663

174

360

10.2 + j3.81

0.663

171

500

10.2 + j5.31

0.663

167

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

driven differentially; however, they are intended to be

driven by a single-ended source. An internal resistor

broadband 50

Ω 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.

Figure 3. IF Input with External Matching

C1

C2

C3

50

Ω

T1

4:1

2

3

100

Ω

0.1%

100

Ω

0.1%

LT5519

18mA

18mA

5519 F03