Applications Information; If Filter Bandwidth Calibration - Analog Devices ADF7021-V Manual

ADF7021-V

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 when it is initiated by a write
to Register 5. Depending on numerous factors, such as IF filter
bandwidth, received signal bandwidth, and temperature varia-
tion, the user must determine whether to carry out a coarse
calibration or a fine calibration.
The performance of both calibration methods is shown in
Table 21.
Table 21. IF Filter Calibration Specifications
Filter Calibration Center Frequency
Method Accuracy
Coarse Calibration 100 kHz ± 2.5 kHz
Fine Calibration 100 kHz ± 0.6 kHz
1
After calibration.
Calibration Setup
IF filter calibration is initiated by writing to Register 5 and
setting the IF_CAL_COARSE bit (Bit DB4). This initiates a
coarse filter calibration. If the IF_FINE_CAL bit (Register 6,
Bit DB4) has already been set high, the coarse calibration is
followed by a fine calibration; otherwise, the calibration ends.
When initiated by writing to the part, calibration is performed
automatically without user intervention. The calibration time is
200 μs for coarse calibration and 8.2 ms for fine calibration, during
which time the ADF7021-V should not be accessed. The IF
filter calibration logic requires that the IF_FILTER_DIVIDER
bits (Register 5, Bits[DB13:DB5]) be set such that
XTAL (Hz)
IF _ FILTER _ DIVIDER
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 process continues for a maximum of
10 RSSI measurements, at which point the calibration algorithm
sets the IF filter center frequency to within 0.6 kHz of 100 kHz.
The frequency of these tones is set in Register 6 by the
IF_CAL_LOWER_TONE_DIVIDE bits (Bits[DB12:DB5]) and
the IF_CAL_UPPER_TONE_DIVIDE bits (Bits[DB20:DB13]),
as shown in the following equations.
Calibration
1
Time (Typ)
200 μs
8.2 ms
=
50 kHz
Lower Tone Frequency (kHz) =
IF_CAL_LOW
Upper Tone Frequency (kHz) =
IF_CAL_UPP
It is recommended that the lower tone and the upper tone be set
as shown in Table 22.
Table 22. IF Filter Fine Calibration Tone Frequencies
IF Filter
Bandwidth (kHz)
9
13.5
18.5
Because the filter attenuation is slightly asymmetrical, it is
necessary to have a small offset in the filter center frequency to
provide near equal rejection at the upper and lower adjacent
channels. The calibration tones listed in Table 22 provide this
small positive offset in the IF filter center frequency.
In some applications, an offset may not be required, and the
user may wish to center the IF filter at 100 kHz exactly. In this
case, the user can alter the tone frequencies from those given in
Table 22 to adjust the fine calibration result.
The calibration algorithm adjusts the filter center frequency and
measures the RSSI 10 times during the calibration. The time for
an adjustment plus RSSI measurement is given by
IF Tone Calibratio
It is recommended that the IF tone calibration time be at least
800 μs. The total time for the IF filter fine calibration is given by
IF Filter Fine Calibration Time = IF Tone Calibration Time × 10
When to Use Coarse Calibration
It is recommended that a coarse calibration be performed on
every power-up in receive mode. This calibration typically takes
200 μs. The FILTER_CAL_COMPLETE signal from MUXOUT
(set using Bits[DB31:DB29] in Register 0) can be used to monitor
the filter calibration duration or to signal the end of calibration.
The ADF7021-V should not be accessed during calibration.
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XTAL
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ER_TONE_DI VIDE 2
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ER_TONE_DI VIDE 2
Lower Tone Upper Tone
Frequency (kHz) Frequency (kHz)
78.1 116.3
79.4 116.3
78.1 119
IF_CAL_DWE
=
n Time
SEQ CLK
LL_TIME