ADL5387(RevB) Просмотр технического описания (PDF) - Analog Devices

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ADL5387
(Rev.:RevB)

30 MHz to 2 GHz Quadrature Demodulator

Analog Devices 

Data Sheet

CIRCUIT DESCRIPTION

The ADL5387 can be divided into five sections: the local

oscillator (LO) interface, the RF voltage-to-current (V-to-I)

converter, the mixers, the differential emitter follower outputs,

and the bias circuit. A detailed block diagram of the device is

shown in Figure 49.

BIAS

IHI

RFIP

RFIN

DIVIDE-BY-TWO

QUADRATURE

PHASE SPLITTER

ILO

LOIP

LOIN

QHI

QLO

Figure 49. Block Diagram

The LO interface generates two LO signals at 90° of phase

difference to drive two mixers in quadrature. RF signals are

converted into currents by the V-to-I converters that feed into

the two mixers. The differential I and Q outputs of the mixers

are buffered via emitter followers. Reference currents to each

section are generated by the bias circuit. A detailed description

of each section follows.

LO INTERFACE

The LO interface consists of a buffer amplifier followed by a

frequency divider that generate two carriers at half the input

frequency and in quadrature with each other. Each carrier is

then amplified and amplitude-limited to drive the double-

balanced mixers.

ADL5387

V-TO-I CONVERTER

The differential RF input signal is applied to a resistively

degenerated common base stage, which converts the differential

input voltage to output currents. The output currents then

modulate the two half-frequency LO carriers in the mixer stage.

MIXERS

The ADL5387 has two double-balanced mixers: one for the

in-phase channel (I channel) and one for the quadrature channel

(Q channel). These mixers are based on the Gilbert cell design

of four cross-connected transistors. The output currents from

the two mixers are summed together in the resistive loads that

then feed into the subsequent emitter follower buffers.

EMITTER FOLLOWER BUFFERS

The output emitter followers drive the differential I and Q

signals off-chip. The output impedance is set by on-chip 25 Ω

series resistors that yield a 50 Ω differential output impedance

for each baseband port. The fixed output impedance forms a

voltage divider with the load impedance that reduces the effective

gain. For example, a 500 Ω differential load has 1 dB lower

effective gain than a high (10 kΩ) differential load impedance.

BIAS CIRCUIT

A band gap reference circuit generates the proportional-to-

absolute temperature (PTAT) as well as temperature-independent

reference currents used by different sections. The mixer current

can be reduced via an external resistor between the BIAS pin

and ground. When the BIAS pin is open, the mixer runs at

maximum current and hence the greatest dynamic range. The

mixer current can be reduced by placing a resistance to ground;

therefore, reducing overall power consumption, noise figure,

and IIP3. The effect on each of these parameters is shown in

Figure 14, Figure 17, and Figure 18.

Rev. B | Page 15 of 28

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