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AD6459

GSM 3 V Receiver IF Subsystem

Analog Devices 

AD6459

PRODUCT OVERVIEW

The AD6459 provides most of the active circuitry required to

realize a complete low power, single-conversion superhetero-

dyne receiver, or the latter part of a double-conversion receiver,

at input frequencies up to 500 MHz, with an IF from 5 MHz to

50 MHz. The internal I/Q demodulators, and their associated

phase-locked loop, support a wide variety of modulation modes,

including n-PSK, n-QAM and GMSK. A single positive supply

voltage of 3 V is required (2.7 V minimum, 5.5 V maximum) at

a typical supply current of 8 mA at midgain. In the following

discussion, VPOS will be used to denote the power supply voltage,

which will be normally assumed to be 3 V.

Figure 20 shows the main sections of the AD6459. It consists of

a variable-gain UHF mixer and a linear two-stage IF strip,

which together provide a calibrated voltage-controlled gain range

of more than 76 dB, followed by dual quadrature demodulators.

These are driven by inphase and quadrature clocks that are

generated by a Phase-Locked Loop (PLL), which is locked to a

corrected external reference. A CMOS-compatible power-down

interface completes the AD6459.

Mixer

The UHF mixer is an improved Gilbert-cell design and can

operate from low frequencies (it is internally dc-coupled) up to

an RF input of 500 MHz. The dynamic range at the input of the

mixer is determined, at the upper end, by the maximum input

signal level of ± 90 mV (–11 dBm in 50 Ω between RFHI and

RFLO) up to which the mixer remains essentially linear, and at

the lower end, by the noise level. It is customary to define the

linearity of a mixer in terms of its 1 dB gain-compression point

and third-order intercept, which for the AD6459 are –11 dBm

and 0 dBm, respectively, in a 50 Ω system.

The mixer’s RF input port is differential; that is, pin RFLO is

functionally identical to RFHI, and these nodes are internally

biased. The RF port can be modeled as a parallel RC circuit as

shown in Figure 19.

RFHI

RFLO

CSH

RSH

Figure 19. Mixer Port Modeled as a Parallel RC Network

The local oscillator (LO) input is internally biased at VP–0.8 V

and must be ac coupled. The LO interface includes a preampli-

fier that minimizes the drive requirements, thus simplifying the

oscillator design and reducing LO leakage from the RF port.

The LO requires a single-sided drive of ± 50 mV, or –16 dBm

in a 50 Ω system. For operation above 300 MHz, noise figure

can be improved by increasing the LO level.

RFHI 6

RFLO 5

VPS1 20

VPS2 18

PRUP 3

LOIP

4

MXOP

9

10

MXOM

LC

BANDPASS

FILTER

IFIP

11

+

12

–

IFIM

BIAS

CIRCUIT

AGC VOLTAGE

AD6459

2

COM1

7

COM2

0°

PLL

50°

GAIN TO

COMPENSATION

4.7kΩ

17 IRXP

16 IRXN

4.7kΩ

1 FREF

19 FLTR

4.7kΩ

15 QRXP

14 QRXN

4.7kΩ

13 GAIN

8 GREF

Figure 20. Functional Block Diagram

–8–

REV. 0

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