LTC6948 datasheet(17/26 Pages) AD

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LTC5556

Rev 0

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APPLICATIONS INFORMATION

The standard evaluation board is built with 100Ω dif-

ferential IF outputs, but also has pads which allow the

use of IF transformers to provide 50Ω single-ended out-

puts. To implement this, it is recommended to use the

200Ω matching shown in Table 1 and 4:1 IF transform-

ers. Figure 16 shows the circuit schematic and measured

performance using this approach.

Table 1. IF Output Matching Element Values

DIFFERENTIAL

ZOUT

L15, L17

9dB RETURN LOSS

BANDWIDTH

200Ω*

—

10nH

23MHz to 440MHz

100Ω

3.9pF

47nH

70MHz to 242MHz

2.2pF

33nH

87MHz to 352MHz

1.2pF

20nH

115MHz to 495MHz

0.6pF

16nH

155MHz to 610MHz

—

12nH

190MHz to 810MHz

*200Ω differential output return loss measured with 4:1 transformer.

The differential IF output impedance vs frequency is listed

in Table 2. The impedances are at the package pins with

no external components. Measured IF output return loss

vs frequency is shown in Figure 8.

Figure 8. IF Output Return Loss (100Ω Differential)

Table 2. Differential IF Output Impedance vs Frequency

IF FREQUENCY (MHz)

DIFFERENTIAL IMPEDANCE (RIF || CIF)

210 || 1.10pF

209 || 1.09pF

100

209 || 1.04pF

150

208 || 0.97pF

200

207 || 0.94pF

300

206 || 0.92pF

400

203 || 0.93pF

500

200 || 0.91pF

600

196 || 0.91pF

700

192 || 0.91pF

800

186 || 0.91pF

900

179 || 0.90pF

1000

172 || 0.89pF

Mixer Output to IF DVGA Interface

The mixer’s 200Ω differential output impedance matches

the IF DVGA’s 200Ω differential input impedance, even

over normal process variation due to the monolithic imple-

mentation. This assures minimal and repeatable DNL and

INL over the full IF attenuation range. Furthermore, the

mixer output and DVGA input include integrated matched

capacitors, which simplify the realization of a lowpass

filter between the mixer and DVGA. This filter attenuates

undesired high frequency mixing products and LO leakage

before entering the DVGA.

A simplified schematic of the interface for channel 1 is

shown in Figure 9 (channel 2 is identical and not shown).

L5 and L7 connect the mixer output to the DVGA input,

while forming a 1GHz 3rd-order, 0.2dB ripple Chebyshev

lowpass filter. L1 and L3 supply DC current to the mixer

and C5 and C7 are DC-blocking capacitors.

3.9p/47n

2.2p/33n

1.2p/20n

0.6p/16n

0p/12n

IF FREQUENCY (MHz)

50 150 250 350 450 550 650 750 850 950

–30

–27

–24

–21

–18

–15

–12

–9

–6

–3

5556 F08

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