ADL5802ACPZ-R7 datasheet(24/32 Pages) AD

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ADL5802

Rev. 0 | Page 24 of 32

CIRCUIT DESCRIPTION

The ADL5802 provides two double-balanced active mixers.

These mixers are designed for a 50 Ω input impedance and a

200 Ω output impedance. Both are driven from a common local

oscillator (LO) amplifier. The RF inputs and LO outputs are

differential, providing maximum usable bandwidth at the input

and output ports. The LO also operates with a 50 Ω input

impedance and can, optionally, be operated differentially or

single-ended. The input, output, and LO ports can be operated

over an exceptionally wide frequency range. The ADL5802 can

be configured as a downconvert mixer or as an upconvert mixer.

The ADL5802 can be divided into the following sections: the

local oscillator (LO) amplifier and splitter, the RF voltage-to-

current (V-to-I) converter, the mixer cores, the output loads,

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

shown in Figure 67. The LO block generates a pair of differential

LO signals to drive two mixer cores. The RF input is converted

into current by the V-to-I converters that then feed into the two

mixer cores. The internal differential load of the mixers is

designed for a wideband 200Ω output impedance from the

mixer. Reference currents to each section are generated by the

bias circuit, which can be enabled or disabled using the ENBL

pin. A detailed description of each section of the ADL5802

follows.

GND

ENBL

LOIP LOIN

VPOS

GND

OP2+

RF1–

RF1+

VPOS

ADL5802

RF2–

GND

GND VSET

GND

RF2+

OP2–

VPOS

GND

OP1+

OP1–

IP3

BIAS

Figure 67. ADL5802 Block Diagram

LO AMPLIFIER AND SPLITTER

The LO input is amplified using a broadband LNA and is then

split and followed by separate LO limiting amplifiers. The LNA

input impedance is nominally 50 Ω. The LO is designed to

accommodate a wide range of LO input power levels. The LO

input is conditioned by the series of amplifiers to provide a well

controlled and limited LO swing to the mixer core, resulting in

excellent IP3. The LO circuit exhibits low additive noise,

resulting in an excellent mixer noise figure and output noise

under RF blocking. For optimal performance, the LO inputs

should be driven differentially but at lower frequencies; single-

ended drive is acceptable.

RF VOLTAGE TO CURRENT (V-TO-I) CONVERTER

The differential RF input signal is applied to a voltage-to-current

converter that converts the differential input voltage to output

currents. The V-to-I converter provides a 50 Ω input

impedance. The V-to-I section bias current can be adjusted up

or down using the VSET pin. Adjusting the current up improves

IP3 and P1dB input but degrades SSB NF. Adjusting the current

down improves SSB NF but degrades IP3 and P1dB input. The

conversion gain remains nearly constant over a wide range of

VSET pin settings, allowing the part to be adjusted dynamically

without affecting the conversion gain. The current adjustment

can be made by connecting a resistor from the VSET pin to the

positive supply to increase the bias current or from the VSET

pin to ground to decrease the bias current. The VSET pin

impedance is approxi-mately 675 Ω in series with two diodes

and an internal current source.

MIXER CORES

The ADL5802 has two double-balanced mixers that use high

performance SiGe NPN transistors. These mixers are based on

the Gilbert cell design of four cross-connected transistors.

MIXER LOAD

Each mixer load is designed to use a pair of 100 Ω resistors con-

nected to the positive supply. This provides a 200 Ω differential

output resistance. The mixer output should be pulled to the

positive supply externally using a pair of RF chokes or using an

output transformer with the center tap connected to the positive

supply. It is possible to exclude these components when the mixer

core current is low, but both P1dB and IP3 are then reduced.

The mixer load output can operate from direct current (dc) up to

approximately 500 MHz into a 200 Ω load. For upconversion

applications, the mixer load can be matched using off-chip

matching components. Transmit operation up to 2 GHz is

possible. See the Applications Information section for matching

circuit details.

BIAS CIRCUIT

A band gap reference circuit generates the reference currents

used by the mixers. The bias circuit can be enabled and disabled

using the ENBL pin. If the ENBL pin is grounded or left open,

the part is enabled. Pulling the ENBL pin high shuts off the bias

circuit and disables the part. However, the ENBL pin does not

alter the current in the LO section and, therefore, does not

provide a true power-down feature. Certain configurations may

require the VSET pin to be connected to the positive supply

through a resistor. This will result in an increased mixer core

current. Unless this resistor to positive supply is removed, bias

current will continue to be supplied to the mixer core.

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