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AD6459ARS

GSM 3 V Receiver IF Subsystem

Analog Devices 

The output of the mixer is differential. The nominal conversion

gain is specified for operation into a 19.5 MHz LC IF bandpass

filter as shown in Figure 21 and Table I.

The conversion gain is measured between the mixer input and

the input of this filter and varies between –5 dB and +15 dB.

MXOP

C1

C2

IFIP

L1

MXOM

C1

IFIM

Figure 21. Suggested IF Filter Inserted Between the

Mixer’s Output Port and the Amplifier’s Input Port

Table I. Filter Component Values for Selected Frequencies

Frequency

C1

L1

C2

13 MHz

19.5 MHz

26 MHz

40 MHz

27 pF

27 pF

22 pF

22 pF

0.82 µH

0.56 µH

0.39 µH

0.12 µH

180 pF

110 pF

82 pF

100 pF

The maximum permissible signal level between MXOP and

MXOM is determined by the maximum gain control voltage.

The mixer output port, having pull-up resistors of 250 Ω to

VPOS, is shown in Figure 22.

250Ω

MXOP

VPOS

250Ω

MXOM

Figure 22. Mixer Output Port

IF Amplifier

Most of the gain in the AD6459 is provided by the IF amplifier

strip, which comprises two stages. Both are fully differential and

each has a gain span of 26 dB for the AGC voltage range of 0.2

V to 2.25 V. Thus, in conjunction with the variable gain of the

mixer, the total gain span is 76 dB. The overall IF gain varies

from –13 dB to 45 dB for the nominal AGC voltage of 0.2 V to

2.25 V. Maximum gain is at VGAIN = 0.2 V.

The IF input is differential, at IFIP and IFIM. Figure 23 shows

a simplified schematic of the IF interface modeled as parallel

RC network.

The IF’s small-signal bandwidth is approximately 50 MHz from

IFIP and IFIM through the demodulator.

IFHI

IFLO

CSH

RSH

Figure 23. IF Amplifier Port Modeled as a Parallel RC

Network

AD6459

Gain Scaling

The AD6459’s overall gain, expressed in decibels, is linear with

respect to the AGC voltage VGAIN at pin GAIN. The gain of all

sections is maximum when VGAIN is 0.2 V and falls off as the

bias is increased to VGAIN = 2.25 V. The gain is independent

of the power supply voltage. The gain of all stages changes

simultaneously. The AD6459’s gain scaling is also tempera-

ture compensated.

Note that GAIN pin of the AD6459 is an input driven by an

external low impedance voltage source, normally a DAC, under

the control of the radio’s digital processor.

The gain-control scaling is directly proportional to the reference

voltage applied to the pin GREF and is independent of the

power supply voltage. When this input is set to the nominal

value of 1.2 V, the scale is nominally 27 mV/dB (37 dB/V).

Under these conditions, 76 dB of gain range (mixer plus IF)

corresponds to a control voltage of 0.2 V ≤ VGAIN Ϲ 2.25 V.

The final centering of this 2.05 V range depends on the inser-

tion losses of the IF filters used.

Pin GREF can be tied to an external voltage reference (VREF)

provided, for example, by an AD1580 (1.21 V) voltage reference.

When using the Analog Devices AD7013 (IS54, TETRA, and

satellite receiver applications) and AD7015 or AD6421 (GSM,

DCS1800, PCS1900) baseband converters, the external refer-

ence may also be provided by the reference output of the

baseband converters. The interface between the AD6459 and

the AD6421 baseband converter is shown in Figure 24. The

AD7015 baseband converter provides a VR of 1.23 V. An auxil-

iary DAC in the AD7015 can be used to generate the AGC

voltage. Since it uses the same reference voltage, the numerical

input to this DAC provides an accurate RSSI value in digital

form, no longer requiring the reference voltage to have high

absolute accuracy.

AD6459

IRXP

IRXN

100pF

100pF

AD6421

IRXP

IRXP

QRXP

QRXP

100pF

100pF

IRXP

IRXN

GREF

GAIN

1nF

0.1µF

160Ω

BREFOUT

BREFCAP

AGC DAC

FREF

VCTCXO

AFC DAC

Figure 24. Interfacing the AD6459 to the AD6421

Baseband Converter

REV. 0

–9–

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