LTC5566 datasheet(20/28 Pages) LINER

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LTC5566

5566f

For more information www.linear.com/LTC5566

applicaTions inForMaTion

Mixer Output to IF DVGA Interface

The mixer’s 300Ω differential output impedance matches

the IF DVGA’s 300Ω differential input impedance, even

overnormalprocessvariationduetothemonolithicimple-

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 650MHz 3rd-order, 0.2dB ripple Cheby-

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

mixer and C5 and C7 are DC-blocking capacitors.

Figure 9. Mixer to IF DVGA Interface

Figure 11. 3rd-Order Bandpass Filter Realization

It’s also possible to implement a bandpass filter between

the mixer and DVGA. An example is shown in Figure 11,

where a 3rd-order bandpass filter is realized by changing

the values of the reactive components and adding C21,

C23 and L19. Figure 19 shows measured conversion gain

vs IF output frequency using this bandpass topology.

Figure 10. Equivalent Lowpass Filter Schematic

An equivalent AC schematic of the lowpass filter is shown

in Figure 10, where the mixer output and DVGA input are

modeled as 300Ω in parallel with 1pF. The mixer supply

chokes and series DC blocking capacitors are ignored in

this schematic.

IF DVGA Phase vs IF Attenuation

Ideally, the phase of the IF output would be constant over

the full IF attenuation range. Practically, there is some

phase shift due to circuit parasitics in the attenuator. The

LTC5566’s IF DVGA is optimized for the lowest possible

phase variation (or phase error) over the full IF attenuation

range. Phase error vs IF attenuation for the complete IF

section is listed in Table 3.

Table 3. IF Phase Error vs IF Attenuation

ATT (dB)

150MHz

250MHz

350MHz

REF

–1.1°

–1.4°

–3.0°

–1.6°

–2.1°

–4.2°

–2.0°

–2.8°

–5.1°

–2.2°

–3.2°

–5.3°

–2.4°

–3.0°

–5.5°

5566 F11

MO1–

MIXER OUT

DVGA IN

MO1+

AI1–

AI1+

1pF

300

VCC

L19

C21

C23

5566 F09

MO1–

MIX OUT

(CH 1)

IF ATT IN

(CH 1)

MO1+

AI1+

AI1–

VCC

150

2pF

150

VCC

1.65V

VCC

150

2pF

150

C5, 1nF L5, 47nH

680nH

C7, 1nF L7, 47nH

27mA

5566 F10

MO1–

MO1+

AI1–

AI1+

1pF

300

L5, 47nH

L7, 47nH

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