Lna/Pa Matching - Analog Devices ADF7021-V Manual

When to Use Fine Calibration
In cases where the receive signal bandwidth is very close to the
bandwidth of the IF filter, it is recommended that a fine filter
calibration be performed every time that 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 Register 4, Bits[DB31:DB30].
The FILTER_CAL_COMPLETE signal from MUXOUT (set by
Register 0, Bits[DB31:DB29]) 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 necessary to perform fine calibration only
once, on the initial power-up in receive mode.
After the initial coarse calibration and fine calibration, the result
of the fine calibration can be read back through the serial inter-
face using the FILTER_CAL_READBACK result (see 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 adjustment
is performed using the IF_FILTER_ADJUST bits (Register 5,
Bits[DB19:DB14]).
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-V 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 1 kHz per 20°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 to +85°C) is ±3.25 kHz. This gives a
total error of ±3.75 kHz.
If the receive signal occupied bandwidth is considerably
narrower than the IF filter bandwidth, the variation of filter
center frequency over the operating temperature range may
not be an issue. However, 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-V 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-V is
equipped with an internal Tx/Rx switch that facilitates the use
of a simple, combined passive LNA/PA matching network.
Alternatively, an external Tx/Rx switch such as the
be used, which yields a slightly improved receiver sensitivity
and lower transmitter power consumption.
Internal Tx/Rx Switch
Figure 49 shows the ADF7021-V in a configuration where
the internal Tx/Rx switch is used with a combined LNA/PA
matching network. This is the configuration used on the
EVAL-ADF7021-VDBxZ evaluation board. For most applica-
tions, the slight performance degradation of 1 dB to 2 dB
caused by the internal Tx/Rx 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 Tx/Rx switch into
consideration.
ANTENNA
OPTIONAL
BPF OR LPF
Figure 49. ADF7021-V with Internal Tx/Rx Switch
The procedure typically requires several iterations until an accept-
able compromise is reached. The successful implementation of
a combined LNA/PA matching network for the ADF7021-V is
critically dependent on the availability of an accurate electrical
model for the PCB. In this context, the use of a suitable CAD
package is strongly recommended. To avoid this effort, a small
form-factor reference design for the ADF7021-V is provided,
including matching and harmonic filter components. The design is
on a 4-layer PCB. Gerber files are available at www.analog.com.
Rev. 0 | Page 37 of 60
ADF7021-V
ADG919
V
BAT
ADF7021-V
L1
C1
RFOUT
Z _PA
OPT
Z _RFIN
IN
C
A
RFIN
L LNA
A
RFIN
Z _RFIN
IN
C
B
can
PA