EVAL-ADF7021DBZ5 Datasheet, PDF (38/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
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
Coarse Cal
100 kHz ± 2.5 kHz
Calibration
Time (Typ)
200 µs
Fine Cal
100 kHz ± 0.5 kHz
5.2 ms
1 _{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.
Rev. B | Page 38 of 64