LT5525EUF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT5525EUF

High Linearity, Low Power Downconverting Mixer

Linear Technology 

LT5525

APPLICATIO S I FOR ATIO

through impedance-matching inductors. Each IF pin draws

about 7.5mA of supply current (15mA total). For optimum

single-ended performance, these differential outputs must

be combined externally through an IF transformer or balun.

An equivalent small-signal model for the output is shown

in Figure 10. The output impedance can be modeled as a

574Ω resistor (RIF) in parallel with a 0.7pF capacitor. For

most applications, the bond-wire inductance (0.7nH per

side) can be ignored.

The external components, C3, L2 and L3 form an imped-

ance transformation network to match the mixer output

impedance to the input impedance of transformer T2. The

values for these components can be estimated using the

equations below, along with the impedance values listed in

Table 3. As an example, at an IF frequency of 140MHz and

RL = 200Ω (using a 4:1 transformer for T2 with an external

50Ω load),

n = RIF/RL = 574/200 = 2.87

Q = √(n – 1) = 1.368

XC = RIF/Q = 420Ω

C = 1/(ω • XC) = 2.71pF

C3 = C – CIF = 2.01pF

XL = RL • Q = 274Ω

L2 = L3 = XL/2ω = 156nH

Table 3. IF Differential Impedance (Parallel Equivalent)

FREQUENCY

(MHz)

OUTPUT

IMPEDANCE

REFLECTION COEFFICIENT

MAG

ANGLE

70

575|| – j3.39k

0.840

–1.8

140

574|| – j1.67k

0.840

–3.5

240

572|| – j977

0.840

–5.9

450

561|| – j519

0.838

–11.1

750

537|| – j309

0.834

–18.6

860

525|| – j267

0.831

–21.3

1000

509|| – j229

0.829

–24.8

1250

474|| – j181

0.822

–31.3

1500

435|| – j147

0.814

–38.0

Low Cost Output Match

For low cost applications in which the required fractional

bandwidth of the IF output is less than 25%, it may be

possible to replace the output transformer with a lumped-

10

LT5525

RIF

574Ω

0.7nH IF+

11

CIF

0.7pF

0.7nH

IF– 10

L3

C3

RL

200Ω

L2

5525 F10

Figure 10. IF Output Small Signal Model

element network. This circuit is shown in Figure 11, where

L11, L12, C11 and C12 form a narrowband bridge balun.

These element values are selected to realize a 180° phase

shift at the desired IF frequency, and can be estimated

using the equations below. In this case, the load resis-

tance, RL, is 50Ω.

L11= L12 = RIF • RL

ω

C11= C12 =

1

ω RIF • RL

Inductor L13 or L14 provides a DC path between VCC and

the IF+ pin. Only one of these inductors is required. Low

cost multilayer chip inductors are adequate for L11, L12

and L13. If L14 is used instead of L13, a larger value is

usually required, which may require the use of a wire-

wound inductor. Capacitor C13 is a DC block which can

also be used to adjust the impedance match. Capacitor

C14 is a bypass capacitor.

IF+

C12 L11

L14

OPT C11

IF–

L12

VCC

C13

L13

OPT

C14

IFOUT

50Ω

5525 F11

Figure 11. Narrowband Bridge IF Balun

Actual component values for IF frequencies of 240MHz,

360MHz and 450MHz are listed in Table 4. Typical IF port

return loss for these examples is shown in Figure 12.

5525f

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