EVAL-ADF7021DBZ5 Datasheet, PDF (30/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
RECEIVER SECTION
RF FRONT END
The ADF7021 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 powerline-
induced interference problems.
If the AGC loop is disabled, the gain of the IF filter can be set to one
of three levels by using the FILTER_GAIN bits (R9_DB[22:23]).
The filter gain is adjusted automatically if the AGC loop is
enabled.
IF Filter Bandwidth and Center Frequency Calibration
Figure 44 shows the structure of the receiver front end. The
many programming options allow users to trade off sensitivity,
linearity, and current consumption to best suit their application.
To achieve a high level of resilience against spurious reception,
the low noise amplifier (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.
RFIN
Tx/Rx SELECT
(R0_DB27)
RFINB
SW2 LNA
LNA MODE
(R9_DB25)
LNA CURRENT
(R9_DB[26:27])
LNA GAIN
(R9_DB[20:21])
LNA/MIXER ENABLE
(R8_DB6)
I (TO FILTER)
LO
Q (TO FILTER)
MIXER LINEARITY
(R9_DB28)
To compensate for manufacturing tolerances, the IF filter should be
calibrated after power-up to ensure that the bandwidth and
center frequency are correct. Coarse and fine calibration
schemes are provided to offer a choice between fast calibration
(coarse calibration) and high filter centering accuracy (fine
calibration). Coarse calibration is enabled by setting R5_DB4
high. Fine calibration is enabled by setting R6_DB4 high.
For details on when it is necessary to perform a filter
calibration, and in what applications to use either a coarse
calibration or fine calibration, refer to the IF Filter Bandwidth
Calibration section.
It is necessary to do a coarse calibration before doing a fine
calibration. If the IF_FINE_CAL bit (R6_DB4) has already been
configured high, it is possible to do a fine calibration by writing
only to Register 5. Once initiated by writing to the part, the cali-
bration is performed automatically without any user intervention.
Calibration time is 200 μs for coarse calibration and a few
milliseconds for fine calibration, during which time the ADF7021
should not be accessed. The IF filter calibration logic requires
that the IF_FILTER_DIVIDER bits (R5_DB[5:13]) be set such that
Figure 44. RF Front End
The LNA is followed by a quadrature downconversion mixer,
which converts the RF signal to the IF frequency of 100 kHz.
An important consideration is that the output frequency of the
synthesizer must be programmed to a value 100 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 (R9_DB25). The mixer is also configurable
between a low current and an enhanced linearity mode using
the MIXER_LINEARITY bit (R9_DB28).
Based on the specific sensitivity and linearity requirements of
the application, it is recommended to adjust the LNA_MODE
bit and MIXER_LINEARITY bit as outlined in Table 14.
XTAL [Hz]
 50 kHz
IF _ FILTER _ DIVIDER
IF Filter Fine Calibration Overview
The fine calibration uses two internally generated tones at
certain offsets around the IF filter. The two tones are attenuated
by the IF filter, and the level of this attenuation is measured
using the RSSI. The filter center frequency is adjusted to allow
equal attenuation of both tones. The attenuation of the two test
tones is then remeasured. This continues for a maximum of
10 RSSI measurements, at which stage the calibration algorithm
sets the IF filter center frequency to within 0.5 kHz of 100 kHz.
The frequency of these tones is set by the following bits:
 IF_CAL_LOWER_TONE_DIVIDER (R6_DB[5:12])
The gain of the LNA is configured by the LNA_GAIN bits
(R9_DB[20:21]) and can be set by either the user or the
automatic gain control (AGC) logic.
IF FILTER
IF Filter Settings
Out-of-band interference is rejected by means of a fifth-order
Butterworth polyphase IF filter centered on a frequency of
100 kHz. The bandwidth of the IF filter can be programmed to
12.5 kHz, 18.75 kHz, or 25 kHz by R4_DB[30:31] and should be
chosen as a compromise between interference rejection and
attenuation of the desired signal.
 IF_CAL_UPPER_TONE_DIVIDER (R6_DB[13:20])
It is recommended to place the lower and upper tones as close
as possible to 65.8 kHz and 131.5 kHz, respectively, as outlined
in the following equations:
XTAL
 65.8 kHz
IF _ CAL _ LOWER _ TONE _ DIVIDE  2
XTAL
 131.5 kHz
IF _ CAL _UPPER _ TONE _ DIVIDE  2
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