LTC5591 datasheet(16/20 Pages) LINER

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LTC5592

5592f

Values of L1A and L2A are tabulated in Figure 1 for vari-

ous IF frequencies. The measured IF output return loss

for bandpass IF matching is plotted in Figure 9.

Table 3. IF Output Impedance vs Frequency

FREQUENCY (MHz)

DIFFERENTIAL OUTPUT

IMPEDANCE (RIF || XIF (CIF))

403 || – j610 (2.9pF)

140

384 || – j474 (2.4pF)

190

379 || – j381 (2.2pF)

240

380 || – j316 (2.1pF)

300

377 || – j253 (2.1pF)

380

376 || – j210 (2.0pF)

450

360 || – j177 (2.0pF)

APPLICATIONS INFORMATION

Figure 9. IF Output Return Loss with Bandpass Matching

board (see Figure 2) has been laid out to accommodate

this matching topology with only minor modifications.

IF Amplifier Bias

The IF amplifier delivers excellent performance with VCCIF

= 3.3V, which allows a single supply to be used for VCC and

VCCIF. At VCCIF = 3.3V, the RF input P1dB of the mixer is

limited by the output voltage swing. For higher P1dB, in

this case, resistor R2A (Figure 7) can be used to reduce

the output impedance and thus the voltage swing, thus

improving P1dB. The trade-off for improved P1dB will be

lower conversion gain.

With VCCIF increased to 5V the P1dB increases by over 3dB,

at the expense of higher power consumption. Mixer P1dB

performanceat1950MHzand2350MHzistabulatedinTable

4 for VCCIF values of 3.3V and 5V. For the highest conversion

gain, high-Q wire-wound chip inductors are recommended

for L1A and L2A. Low cost multilayer chip inductors may

be substituted, with a slight reduction in conversion gain.

Figure 10. IF Output with Lowpass Matching

Figure 11. IF Output Return Loss with Lowpass Matching

Lowpass IF Matching

For IF frequencies below 90MHz, the inductance values

become unreasonably high and the lowpass topology

shown in Figure 10 is preferred. This topology also can

provide improved RF to IF and LO to IF isolation. VCCIFA

is supplied through the center tap of the 4:1 transformer.

A lowpass impedance transformation is realized by shunt

elements R2A and C9A (in parallel with the internal RIF

and CIF), and series inductors L1A and L2A. Resistor

R2A is used to reduce the IF output resistance for greater

bandwidth, or it can be omitted for the highest conver-

sion gain. The final impedance transformation to 50Ω is

realized by transformer T1A. The measured return loss

is shown in Figure 11 for different values of inductance

(C9A = open). The case with 82nH inductors and a 1k

load resistor (R2A) is also shown. The LTC5592 demo

4:1

T1A

IFA

50Ω

VCCIFA

3.1 TO 5.3V

C5A

IFA–

IFA+

C9A

R2A

L1A

L2A

LTC5592

5592 F10

FREQUENCY (MHz)

–25

–20

–15

–10

–5

100

300

5592 F09

150 200 250

350 400 450 500

270nH

150nH

100nH

56nH

33nH

22nH

–20

–25

–15

–10

–5

FREQUENCY (MHz)

100

150

5592 F11

200

250

68nH

100nH

180nH

82nH + 1k

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