ADF7021 datasheet(38/64 Pages) AD

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ADF7021

Data Sheet

Rev. B | Page 38 of 64

APPLICATIONS INFORMATION

IF FILTER BANDWIDTH CALIBRATION

The IF filter should be calibrated on every power-up in receive

mode to correct for errors in the bandwidth and filter center

frequency due to process variations. The automatic calibration

requires no external intervention once it is initiated by a write

to Register 5. Depending on numerous factors, such as IF filter

bandwidth, received signal bandwidth, and temperature

variation, the user must determine whether to carry out a

coarse calibration or a fine calibration. For information on

calibration setup, refer to the IF Filter section.

The performance of both calibration methods is outlined in

Table 21.

Table 21. IF Filter Calibration Specifications

Filter Calibration

Method

Center Frequency

Accuracy1

Calibration

Time (Typ)

Coarse Cal

100 kHz ± 2.5 kHz

200 µs

Fine Cal

100 kHz ± 0.5 kHz

5.2 ms

After calibration.

When to Use Coarse Calibration

It is recommended to perform a coarse calibration on every

receive mode power-up. This calibration typically takes 200 µs.

The FILTER_CAL_COMPLETE signal from MUXOUT can be

used to monitor the filter calibration duration or to signal the

end of calibration. The ADF7021 should not be accessed during

calibration.

When to Use a Fine Calibration

In cases where the receive signal bandwidth is very close to the

bandwidth of the IF filter, it is recommended to perform a fine

filter calibration every time the unit powers up in receive mode.

A fine calibration should be performed if

OBW + Coarse Calibration Variation > IF_FILTER_BW

where:

OBW is the 99% occupied bandwidth of the transmit signal.

Coarse Calibration Variation is 2.5 kHz.

IF_FILTER_BW is set by R4_DB[30:31].

The FILTER_CAL_COMPLETE signal from MUXOUT (set by

R0_DB[29:31]) can be used to monitor the filter calibration

duration or to signal the end of calibration. A coarse filter

calibration is automatically performed prior to a fine filter

calibration.

When to Use Single Fine Calibration

In applications where the receiver powers up numerous times in

a short period, it is only necessary to perform a one-time fine

calibration on the initial receiver power-up.

After the initial coarse calibration and fine calibration, the result of

the fine calibration can be read back through the serial interface

using the FILTER_CAL_READBACK result (refer to the Filter

Bandwidth Calibration Readback section). On subsequent

power-ups in receive mode, the filter is manually adjusted using

the previous fine filter calibration result. This manual adjust is

performed using the IF_FILTER_ADJUST bits (R5_DB[14:19]).

This method should only be used if the successive power-ups in

receive mode are over a short duration, during which time there

is little variation in temperature (>15°C).

IF Filter Variation with Temperature

When calibrated, the filter center frequency can vary with

changes in temperature. If the ADF7021 is used in an application

where it remains in receive mode for a considerable length of

time, the user must consider this variation of filter center

frequency with temperature. This variation is typically 0.7 kHz

per 10°C, which means that if a coarse filter calibration and fine

filter calibration are performed at 25°C, the initial maximum

error is ±0.5 kHz, and the maximum possible change in the

filter center frequency over temperature (−40°C and +85°C) is

±4.5 kHz. This gives a total error of ±5 kHz.

If the receive signal occupied bandwidth is considerably less

than the IF filter bandwidth, the variation of filter center

frequency over the operating temperature range may not be

an issue. Alternatively, if the IF filter bandwidth is not wide

enough to tolerate the variation with temperature, a periodic

filter calibration can be performed, or alternatively, the on-chip

temperature sensor can be used to determine when a filter cali-

bration is necessary by monitoring for changes in temperature.

LNA/PA MATCHING

The ADF7021 exhibits optimum performance in terms of

sensitivity, transmit power, and current consumption, only if its

RF input and output ports are properly matched to the antenna

impedance. For cost sensitive applications, the ADF7021 is

equipped with an internal Rx/Tx switch that facilitates the use

of a simple, combined passive PA/LNA matching network.

Alternatively, an external Rx/Tx switch such as the ADG919 can

be used, which yields a slightly improved receiver sensitivity

and lower transmitter power consumption.

Internal Rx/Tx Switch

Figure 49 shows the ADF7021 in a configuration where

the internal Rx/Tx switch is used with a combined LNA/PA

matching network. This is the configuration used on the EVAL-

ADF7021DBX evaluation board. For most applications, the slight

performance degradation of 1 dB to 2 dB caused by the internal

Rx/Tx switch is acceptable, allowing the user to take advantage

of the cost saving potential of this solution. The design of the

combined matching network must compensate for the reactance

presented by the networks in the Tx and the Rx paths, taking

the state of the Rx/Tx switch into consideration.

ADF7021 Datasheet(PDF) 38 Page - Analog Devices

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