ADF7020BCP datasheet(16/40 Pages) AD

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ADF7020

Preliminary Technical Data

Rev. PrH | Page 16 of 40

RECEIVER SECTION

RF FRONT END

The ADF7020 is based on a fully integrated, low IF receiver

architecture. The low IF architecture facilitates a very low

external component count and does not suffer from power-line-

induced interference problems.

Figure 14 shows the structure of the receiver front end. The

many programming options allow users to trade off sensitivity,

linearity, and current consumption against each other in the

way best suitable for their applications. To achieve a high level

of resilience against spurious reception, the LNA features a

differential input. Switch SW2 shorts the LNA input when

transmit mode is selected (R0_DB27 = 0). This feature

facilitates the design of a combined LNA/PA matching network,

avoiding the need for an external Rx/Tx switch. See the LNA/PA

Matching section for details on the design of the matching

network.

SW2

LNA

RFIN

RFINB

Tx/Rx SELECT

[R0_DB27]

LNA MODE

[R6_DB15]

LNA CURRENT

[R6_DB(16:17)]

MIXER LINEARITY

[R6_DB18]

I (TO FILTER)

Q (TO FILTER)

LNA GAIN

[R9_DB(20:21)]

LNA/MIXER ENABLE

[R8_DB6]

Figure 14. ADF7020 RF Front End

The LNA is followed by a quadrature downconversion mixer,

which converts the RF signal to the IF frequency of 200 kHz. It

is important to consider that the output frequency of the

synthesizer must be programmed to a value 200 kHz below the

center frequency of the received channel.

The LNA has two basic operating modes: high gain/low noise

mode and low gain/low power mode. To switch between these

two modes, use the LNA_mode bit, R6_DB15. The mixer is also

configurable between a low current and an enhanced linearity

mode using the mixer_linearity bit, R6_DB18.

Based on the specific sensitivity and linearity requirements of

the application, it is recommended to adjust control bits

LNA_mode (R6_DB15) and mixer_linearity (R6_DB18) as

outlined in Table 5.

The gain of the LNA is configured by the LNA_gain field,

R9_DB(20:21), and can be set by either the user or the AGC

logic.

IF Filter Settings/Calibration

Out-of-band interference is rejected by means of a fourth-order

Butterworth polyphase IF filter centered around a frequency of

200 kHz. The bandwidth of the IF filter can be programmed

between 100 kHz and 200 kHz by means of Control Bits

R1_DB(22:23), and should be chosen as a compromise between

interference rejection, attenuation of the desired signal, and the

AFC pull-in range.

To compensate for manufacturing tolerances, the IF filter

should be calibrated once after power-up. The IF filter

calibration logic requires that the IF filter divider in Bits

R6_DB(20:28) be set dependent on the crystal frequency. Once

initiated by setting Bit R6_DB19, the calibration is performed

automatically without any user intervention. The calibration

time is 200 µs, during which the ADF7020 should not be

accessed. It is important not to initiate the calibration cycle

before the crystal oscillator has fully settled. If the AGC loop is

disabled, the gain of IF filter can be set to three levels using the

filter_gain field, R9_DB(20:21). The filter gain is adjusted

automatically, if the AGC loop is enabled.

The signal in the image channel of the low IF mixer, located at a

frequency of 400 kHz below the desired channel, is rejected due

to the image rejection of the polyphase filter. The image

rejection performance of the IF filter is subject to manufactur-

ing tolerances, and, to some extent, temperature drift. To

improve the image rejection, a calibration procedure can be

performed as outlined in the Image Rejection Calibration

section.

Table 5. LNA/Mixer Modes

Receiver Mode

LNA Mode

(R6_DB15)

LNA Gain Value

R9_DB(21:20)

Mixer Linearity

(R6_DB18)

Sensitivity

(DR = 9.6 kbps,

fDEV = 10 kHz)

Rx Current

Consumption

(mA)

Input IP3 (dBm)

High Sensitivity Mode

(default)

−110.5

−35

RxMode2

−104

−15.9

Low Current Mode

−91

−3.2

Enhanced Linearity Mode

−101

6.8

RxMode5

TBD

−8.25

RxMode6

TBD

−28.8

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