ADL5350

Preliminary Technical Data

Rev. PrC | Page 18 of 24

FUNCTIONAL DESCRIPTION

CIRCUIT DESCRIPTION

The ADL5350 is a GaAs MESFET, single-ended passive mixer

with an integrated LO buffer amplifier. The device relies on the

varying drain to source channel conductance of a FET junction

to modulate an RF signal. A simplified schematic is shown in

Figure 57.

RF

GND

GND

LO

LO

INPUT

VPOS

GC

RF

INPUT

OR OUTPUT

IF

IF

OUTPUT

OR INPUT

Figure 57. Simplified Schematic

The LO signal is applied to the gate contact of a FET-based buffer

amplifier. The buffer amplifier provides sufficient gain of the LO

signal to drive the resistive switch. Additionally, feedback

circuitry provides the necessary bias to the FET buffer amplifier

and RF/IF ports to achieve optimum modulation efficiency for

common cellular frequencies. The GC node is the “gate-contact”

of the RF/IF port resistive switch. The GC node enables external

control of the bias level of the switching FET, allowing the user to

override the internal bias generation circuitry, and allow further

optimization of the mixer’s dynamic performance at frequencies

outside of the 800 MHz to 2000 MHz band.

The mixing of RF and LO signals is achieved by switching the

channel conductance from the RF/IF port to ground at the rate

of the LO. The RF signal is passed through an external band-

pass network to help reject image bands and reduce the

broadband noise presented to the mixer. The band-limited RF

signal is presented to the time-varying load of the RF/IF port,

which causes the envelope of the RF signal to be amplitude

modulated at the rate of the LO. A filter network applied to the

IF port is necessary to reject the RF signal and pass the wanted

mixing product. In a down-conversion application, the IF filter

network is designed to pass the difference frequency and

present an open circuit to the incident RF frequency. Similarly,

for an up-conversion application, the filter is designed to pass

the sum frequency and reject the incident RF. As a result, the

frequency response of the mixer is determined by the response

characteristics of the external RF/IF filter networks.

IMPLEMENTATION PROCEDURE

The ADL5350 is a simple single-ended mixer that relies on off-

chip circuitry to achieve effective RF dynamic performance.

The following steps should be followed to achieve optimum

performance (see Figure 58 for component designations):

RF/IF

GND2

LOIN

GND1

RF/IF

GC

VPOS

L4

C4

C2

L2

C6

C1

LO

C3

L3

L1

RF

IF

GND1

ADL5350

12

3

4

87

6

5

Figure 58. Reference Schematic

1.

Tune LO buffer supply inductor for lowest supply current.

To start this procedure, it is necessary to provide an initial

guess. Table 8 can be used as a starting point. It is not necessary

to terminate or populate the RF and IF port networks to

complete this first step. The RFIF pins can be left open while

tuning the LO buffer networks.

Table 8. Recommended LO Bias Inductor

Desired LO Frequency

380 MHz

68 nH

750 MHz

24 nH

1000 MHz

18 nH

1750 MHz

3.8 nH

2000 MHz

2.1 nH

intended frequency of operation.

To test the supply current consumption, power up the device

and apply the desired LO signal. Next, attempt to increase and

decrease the LO frequency. If the current consumption

increases as the LO frequency is decreased, then increase the

value of L4. If the current consumption decreases as the LO

frequency also decreases, then decrease the value of L4. After

determining the optimum inductor value, the current

consumption should be minimized at the desired LO frequency.