EVAL-ADF7021DB2 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-ADF7021DB2' PDF : 44 Pages View PDF
ADF7021
Preliminary Technical Data
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 power-
line-induced interference problems.
Figure 19 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 suited for their applications. 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)]
I (TO FILTER)
LO
Q (TO FILTER)
MIXER LINEARITY
[R9_DB28]
LNA GAIN
[R9_DB(20:21)]
LNA/MIXER ENABLE
[R8_DB6]
Figure 19. ADF7021 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 control bits LNA
mode (R9_DB25) and mixer linearity (R9_DB28) as outlined in
Table 8.
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
automatic gain control (AGC) logic.
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
between 12.5 kHz and 25 kHz by Control Bits R4_DB[30:31], and
should be chosen as a compromise between interference
rejection, attenuation of the desired signal, and the AFC pull-in
range.
Coarse/Fine Filter Calibration
To compensate for manufacturing tolerances, the IF filter
should be calibrated once after power-up. Coarse and fine
calibration schemes are provided in order to offer a choice
between fast calibration with the coarse scheme and high filter
centering accuracy with fine calibration. Coarse calibration is
enabled by setting Bit R5_DB4 high. Fine calibration is enabled
by setting Bit R6_DB4 high. It is necessary to do a coarse cali-
bration 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 calibration is performed automatically
without any user intervention. Calibration time is 200 μs for coarse
calibration and a few milliseconds for fine calibration, during
which the ADF7021 should not be accessed. The IF filter
calibration logic requires that the IF filter divider in Bits
R5_DB[5:13] be set depending on the crystal frequency. 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[22:23]. The filter gain is adjusted automatically if the
AGC loop is enabled.
Table 8. LNA/Mixer Modes
Receiver Mode
High Sensitivity Mode (default)
Rx Mode2
Low Current Mode
Enhanced Linearity Mode
Rx Mode5
Rx Mode6
LNA Mode
(R9_DB25)
0
1
1
1
1
0
LNA
Gain Value
(R9_DB[20:21])
30
10
3
3
10
30
Mixer
Linearity
(R9_DB28)
0
0
0
1
1
1
Sensitivity
(DR = 9.6 kbps,
FDEV = 4.8 kHz)
−117
TBD
TBD
TBD
TBD
TBD
Rx Current
Consumption
(mA)
22
20
19
19
20
21
Input IP3
(dBm)
TBD
TBD
TBD
TBD
TBD
TBD
Rev. PrI | Page 18 of 44
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