LT5526EUF Просмотр технического описания (PDF) - Linear Technology

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LT5526EUF

High Linearity, Low Power Downconverting Mixer

Linear Technology 

LT5526

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.

575Ω

0.7pF

VCC

LT5526

IF+

11

IF– 10

L3

T2 IFOUT

4:1

C3 VCC

L2

5526 F09

Figure 9. IF Output with External Matching

An equivalent small-signal model for the output is shown

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

575Ω resistor in parallel with a 0.7pF capacitor. For most

applications, the bond-wire inductance (0.7nH per side)

can be ignored.

LT5526

RIF

574Ω

0.7nH IF+

11

CIF

0.7pF

0.7nH

IF– 10

L3

C3

RL

200Ω

L2

5526 F10

Figure 10. IF Output Small-Signal Model

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

same equations that were used for the input matching

network, along with the impedance values listed in Table

4. As an example, at an IF frequency of 140MHz and RL =

200Ω (using a 4:1 transformer for T2),

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 4. IF Differential Impedance (Parallel Equivalent)

FREQUENCY

(MHz)

70

140

240

450

750

860

1000

1250

1500

OUTPUT

IMPEDANCE

575|| – j3.39k

574|| – j1.67k

572|| – j977

561|| – j519

537|| – j309

525|| – j267

509|| – j229

474|| – j181

435|| – j147

REFLECTION COEFFICIENT

MAG

ANGLE

0.840

–1.8

0.840

–3.5

0.840

–5.9

0.838

–11.1

0.834

–18.6

0.831

–21.3

0.829

–24.8

0.822

–31.3

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-

element network similar to that discussed earlier for the

RF input. 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 by using the

equations below. In this case, RIF is the mixer output

resistance and RL is the load resistance (50Ω).

5526f

12

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