LT5557 datasheet(9/16 Pages) LINER

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LT5557

5557fa

APPLICATIO S I FOR ATIO

Introduction

The LT5557 consists of a high linearity double-balanced

mixer, RF buffer amplifier, high speed limiting LO buffer

amplifier and bias/enable circuits. The RF and LO inputs

are both single ended. The IF output is differential. Low

side or high side LO injection can be used.

Two evaluation circuits are available. The standard evalu-

ation circuit, shown in Figure 1, incorporates transformer-

based IF matching and is intended for applications that

require the highest dynamic range and the widest IF

bandwidth. The second evaluation circuit, shown in Fig-

ure 2, replaces the IF transformer with a discrete IF balun

for reduced solution cost and size. The discrete IF balun

delivers higher conversion gain, but slightly degraded IIP3

and noise figure, and reduced IF bandwidth.

RF Input Port

The mixer’s RF input, shown in Figure 3, consists of an

integrated transformer and a high linearity differential

amplifier. The primary terminals of the transformer are

connected to the RF input (Pin 3) and ground. The second-

ary side of the transformer is internally connected to the

amplifier’s differential inputs. The DC resistance of the

primary is 4.2

Ω. If the RF source has DC voltage present,

then a coupling capacitor must be used in series with the

RF input pin.

The RF input is internally matched from 1.6GHz to 2.3GHz,

requiring no external components over this frequency

range. The input return loss, shown in Figure 4a, is typi-

cally 12dB at the band edges. The input match at the lower

band edge can be optimized with a series 3.9pF capacitor

at Pin 3, which improves the 1.6GHz return loss to greater

than 25dB. Likewise, the 2.3GHz match can be improved

to greater than 25dB with a series 1.5nH inductor. A series

2.7nH/2.2pF network will simultaneously optimize the lower

and upper band edges and expand the RF input bandwidth

to 1.2GHz-2.5GHz. Measured RF input return losses for

these three cases are also plotted in Figure 4a.

Alternatively, the input match can be shifted as low as

400MHz or up to 3800MHz by adding a shunt capacitor (C5)

to the RF input. A 450MHz input match is realized with C5

= 12pF, located 6.5mm away from Pin 3 on the evaluation

board’s 50

Ωinputtransmissionline.A900MHzinputmatch

requires C5 = 3.9pF, located at 1.7mm. A 3.6GHz input

match is realized with C5 = 1pF, located at 2.9mm. This

Figure 3. RF Input Schematic

Figure 4. RF Input Return Loss With

and Without External Matching

(4b) Series Shunt Matching

(4a) Series Reactance Matching

FREQUENCY (GHz)

0.2

–30

–25

–20

–15

–10

1.2

2.2

3.2

4.2

5557 F04a

–5

0.7

1.7

2.7

3.7

SERIES 2.7nH

AND 2.2pF

NO EXT MATCH

SERIES 1.5nH

SERIES 3.9pF

FREQUENCY (GHz)

0.2

–30

–25

–20

–15

–10

1.2

2.2

3.2

4.2

5557 F04b

–5

0.7

1.7

2.7

3.7

450MHz

L = 6.5mm

C5 = 12pF

2.6GHz

SERIES 3.9pF

SHUNT 3.6nH

3.6GHz

L = 2.9mm

C5 = 1pF

900MHz

L = 1.7mm

C5 = 3.9pF

NO EXT

MATCH

RFIN

ZO = 50Ω

L = L (mm)

5557 F03

RFIN

LOW-PASS MATCH

FOR 450MHz, 900MHz

and 3.6GHz RF

HIGH-PASS MATCH

FOR 2.6GHz RF

AND WIDEBAND RF

MIXER

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