AD8346_ データシートの表示（PDF） - Analog Devices
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AD8346_ Datasheet PDF : 8 Pages
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Application Note
Once the LO leakage and quadrature error have been calibrated,
all that remains is for the calibration coefficients to be stored in
nonvolatile RAM so that they are available when the equipment
is turned on in the field. To recap, the four calibration
coefficients are
• I channel offset voltage
• Q channel offset voltage
• I channel vs. Q channel gain imbalance
• Quadrature phase imbalance
FREQUENCY VARIATIONS
Calibrating at multiple frequencies within a band adds time to
the factory calibration, requires more nonvolatile memory for
the larger look up table, and is more cumbersome during field
operation as calibration coefficients have to be swapped out as
the frequency changes.
Now, consider what happens to the quality of calibration as the
frequency changes. In Figure 7, sideband suppression and LO
leakage have been nulled to −60 dBc and −74 dBm, respectively,
at 1900 MHz. Figure 7 also shows how the uncompensated
sideband suppression and LO leakage vary with frequency (the
flatter green and red traces a the top of the plot). Next, adjust
the frequency over a range of ±30 MHz (the typical width of a
cellular telephony band) without recalibration. The LO leakage
quickly loses its null and at some frequencies is only around
8 dB better than the uncompensated value. In the case of the
sideband suppression, the difference between the compensated
and uncompensated values becomes as low as around 1 dB.
Figure 7 suggests that factory calibration be performed at
multiple frequencies within a band to maintain nulled
performance across the band.
–30
UNADJUSTED SIDEBAND SUPPRESSION (dBc)
–35
UNADJUSTED LO LEAKAGE (dBm)
–40
–45
–50
–55
–60
–65
–70
–75
–80
1870
POST (MIDBAND)
NULLING SIDEBAND
SUPPRESSION (dBc)
POST (MIDBAND) NULLING LO LEAKAGE (dBm)
1880
1890
1900
1910
OUTPUT FREQUENCY (MHz)
1920
1930
Figure 7. Variation of LO Leakage and Sideband Suppression vs. Frequency
after Nulling Midband
AN-1039
POST CALIBRATION TEMPERATURE DRIFT
Factory calibration at multiple temperatures is even more
difficult and expensive than calibration at multiple frequencies.
As a result, it is generally only practical to perform factory
adjustment of LO leakage and sideband suppression at an
ambient temperature. Thus, what happens to post-calibration
performance as temperature varies?
In Figure 8, the LO leakage and sideband suppression have
again been nulled midband. After nulling, the device is cycled
over temperature. This again has the effect of moving sideband
suppression and LO leakage off their nulled levels. However,
notice that the performance at temperature is quite flat across
frequency and it is no longer clear at which frequency the
nulling was performed. The net improvement over temperature
is approximately 15 dB compared to the unadjusted LO leakage.
CALIBRATION VS. TIME
In the set-and-forget factory calibration scheme that has just
been described, the question of long-term drift arises because
the equipment may never be recalibrated in the field. Experi-
ments have shown that it is very difficult, if not impossible, to
measure the degradation of nulled sideband suppression and
LO leakage over time. Very mild changes in environmental
conditions tend to quickly move the device off its null. This
makes it impossible to determine whether the environment and
the test equipment are altering the experiment or if genuine
device drift over time is taking place.
However, Figure 8 shows that the question of drift over time
is less important. This is because the effect of temperature drift
is much more significant. Thus, in a system that experiences
reasonable temperature fluctuations, whatever drift over time
takes place is completely masked by the temperature drift.
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
1802
+25°C UNADJUSTED
+25°C NULLED
+85°C UNADJUSTED
+85°C NULLED
–40°C UNADJUSTED
–40°C NULLED
1812 1822 1832 1842 1852 1862
LO FREQUENCY (MHz)
1872
1882
Figure 8. Variation of LO Leakage vs. Frequency and Temperature After
Nulling Midband
Rev. 0 | Page 7 of 8
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