ADL5350ACPZ-R7 datasheet(17/24 Pages) AD

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ADL5350

Rev. 0 | Page 17 of 24

FUNCTIONAL DESCRIPTION

CIRCUIT DESCRIPTION

The ADL5350 is a GaAs pHEMT, 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.

GND1

GND2

LOIN

INPUT

VPOS

INPUT

OR OUTPUT

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 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 upconversion

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

RF/IF

VPOS

GND1

ADL5350

Figure 58. Reference Schematic

1. Table 7 shows the recommended LO bias inductor

values for a variety of LO frequencies. To ensure efficient

commutation of the mixer, the bias inductor needs to

be properly set. For other frequencies within the range

shown, the values can be interpolated. For frequencies

outside this range, see the Applications Information section.

Table 7. Recommended LO Bias Inductor

Desired LO Frequency (MHz)

Recommended LO Bias

Inductor, L41 (nH)

380

750

1000

1750

3.8

2000

2.1

1 The bias inductor should have a self-resonant frequency greater than

the intended frequency of operation.

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