ADF4602BCPZ datasheet(20/36 Pages) AD

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ADF4602

Rev. 0 | Page 20 of 36

The receive baseband outputs have a programmable common

mode voltage of 1.2 V or 1.4 V, selectable via the vcmsel bit in

Gain Control

Gain control is distributed throughout the receive signal chain

as shown in Figure 39. The RF front end contains 30 dB of control

range: 18 dB in the LNA and 12 dB in the mixer transconductance

stage. The two baseband active filter stages each provide 18 dB

of gain control range in 6 dB steps. Filter characteristics (ripple

and group delay) are best conserved if the active filter stages

have equal gain. This results in a total of 36 dB gain control in

4× 12 dB steps for the filter stage. The variable gain amplifier

(VGA) implements 24 dB of gain controllable in 1 dB steps. The

base gain of the mixer is 18 dB, and the base gain of the VGA is

−6 dB. This gives a total of 102 dB gain with 90 dB of gain

control range.

The base gain of the mixer stage is 18 dB in WCDMA mode

and 27 dB in GSM mode.

Table 6. Receive Gain Control in WCDMA mode

Stage

Gain Control

Control Steps

LNA

0 dB to +18 dB

3 × 6 dB steps

Mixer

+18 dB to +30 dB (WCDMA)

+27 dB to +39 dB (GSM)

2 × 6 dB steps

Filter

0 dB to +36 dB

3 × 12 dB steps

VGA

−6 dB to +18 dB

24 × 1 dB steps

To simplify programming and to ensure optimum receiver

performance and dynamic range, the user simply programs the

total desired receive gain in dB via the rx_gain[6:0] bits in

automatically distributes the gain between the various blocks. To

allow some flexibility, predefined user inputs control the gain

threshold points at which the LNA and mixer gain steps occur.

Bit settings mixstep[3:0] and lnastep[3:0] control the mixer and

LNA gain threshold steps, respectively. An Excel spreadsheet

detailing the receive gain decode system is available from

Analog Devices, Inc., on request. Figure 38 shows an example

gain distribution profile.

120

110

100

–10

100

110 120

REQUESTED Rx GAIN (dB)

RF GAIN

BASEBAND GAIN

CHIP GAIN

MIXSTEP = 10

LNASTEP = 6

GAINCAL = 8

Figure 38. Gain Distribution Between RF and Baseband Blocks for Default

Setting

–5

–10

100

110 120

REQUESTED Rx GAIN (dB)

LNA GAIN

MIXER GAIN

FILTER GAIN

VGA GAIN

MIXSTEP = 10

LNASTEP = 6

GAINCAL = 8

Figure 39. More Detailed Gain Distribution Profile

In addition, a gain calibration setting in Register 15 (gaincal[4:0]) is

used to account for losses in the RF front end.

The total gain in the ADF4602 is given by

ReceiveGain = rxgain[6:0] − gaincal[4:0] + X

(2)

where X = 8 in WCDMA filter mode, and X = 17 in GSM filter

mode. Rxgain[6:0] is the receive gain programmed in Register

11. Gaincal[4:0] is the gain calibration setting in Register 15,

and is calculated using the following formula:

gaincal[4:0] = 8 − front_end_losses

(3)

where front_end_losses is the loss in the receive path due to

duplexers/switches. This is useful for referencing the

programmed gain to the antenna and accounting for any losses

in the path.

For example, if the total receive front-end loss is 2 dB, the user

should program gaincal[4:0] to 6 dB. If the user then requestes

80 dB of gain by programming rxgain[6:0] to 80 dB, the

ADF4602 uses Equation 4 to give

ReceiveGain = 80 − 6 + 8 = 82 dB

(4)

82 dB is the receive gain used internally by the ADF4602.

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