ad80164absvz Analog Devices, Inc., ad80164absvz Datasheet - Page 40

• Manufacturer Part Number: ad80164absvz
• Description: Dual, 12-/14-/16-bit,1 Gsps Digital-to-analog Converters
• Manufacturer: Analog Devices, Inc.

AD9776A/AD9778A/AD9779A
USING THE AD9776A/AD9778A/AD9779A TO
CORRECT FOR NONIDEAL PERFORMANCE OF
QUADRATURE MODULATORS ON THE IF TO RF
CONVERSION
Analog quadrature modulators make it very easy to realize
single sideband radios. However, there are several nonideal
aspects of quadrature modulator performance. Among these
analog degradations are
•
•
The AD9776A/AD9778A/AD9779A has the capability to
correct for both of these analog degradations. Understood that
these degradations drift over temperature; therefore if close to
optimal single sideband performance is desired, a scheme for
sensing these degradations over temperature and correcting for
them may necessary.
I/Q CHANNEL GAIN MATCHING
Gain matching is achieved by adjusting the values in the DAC
gain registers. For the I DAC, these values are in the I DAC
Control Register 0x05. For the Q DAC, these values are in the
Q DAC Control Register 0x07. These are 10 bit values. To
perform gain compensation, raise or lower the value of one of
these registers by a fixed step size, determine if the amplitude
of the unwanted image. If the unwanted image is increasing in
amplitude, stop the procedure and try the same adjustment on
the other DAC control register. Do this until the image rejection
can not be improved through further adjustment of these registers.
Figure 80. Typical Use of Auxiliary DACs DC Coupling to Quadrature
Gain mismatch—The gain in the real and imaginary signal
paths of the quadrature modulator may not be matched
perfectly. This leads to less than optimal image rejection as
the cancellation of the negative frequency image is less than
perfect.
LO feedthrough—The quadrature modulator has a finite
dc referred offset, as well as coupling from its LO port to
the signal inputs. These can lead to a significant spectral
spurs at the frequency of the quadrature modulator LO.
I OR Q DAC
AD9779A
25Ω TO 50Ω
Modulator with DC Shift
OPTIONAL
FILTERING
DAC1 OR
AD9779A
PASSIVE
DAC2
AUX
MODULATOR V+
QUADRATURE
25Ω TO 50Ω
QUAD MOD
I OR Q INPUTS
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It should be noted that LO feedthrough compensation is inde-
pendent of phase compensation. However, gain compensation
could affect the LO compensation because the gain compensa-
tion may change the common mode level of the signal. The dc
offset of some modulators is common mode level dependent.
Therefore it is recommended that the gain adjustment is
performed prior to LO compensation.
LO FEEDTHROUGH COMPENSATION
The LO feedthrough compensation is the most complex of all
three operations. This is due to the structure of the offset aux-
iliary DACs as shown in Figure 78. To achieve LO feedthrough
compensation in a circuit, each of four outputs of these AUX
DACs must be connected through a 50 Ω resistor to ground
and through a 250 Ω resistor to one of the four quadrature
modulator signal inputs. The purpose of these connections is
to drive a very small amount of current into the nodes at the
quadrature modulator inputs, therefore adding a slight dc bias
to one or the other of the quadrature modulator signal inputs.
This can be seen in the schematics for the AD9776A/AD9778A/
AD9779A evaluation board (see Figure 107).
To achieve LO feedthrough compensation, the user should start
with the default conditions of the AUX DAC sign registers, then
increment the magnitude of one or the other AUX DAC output
currents. While this is being done, the amplitude of the LO
feedthrough at the quadrature modulator output should be
sensed. If the LO feedthrough amplitude increases, try either
changing the sign of the AUX DAC being adjusted, or try
adjusting the output current of the other AUX DAC. It may
take practice before an effective algorithm is achieved. Using
the AD9776A/AD9778A/AD9779A evaluation board, the LO
feedthrough can typically be adjusted down to the noise floor,
although this is not stable over temperature.
RESULTS OF GAIN AND OFFSET CORRECTION
The results of gain and offset correction can be seen in Figure 81
and Figure 82. Figure 81 shows the output spectrum of the
quadrature demodulator before gain and offset correction.
Figure 82 shows the output spectrum after correction. The
LO feedthrough spur at 2.1 GHz has been suppressed to the
noise level. This result can be achieved by applying the correc-
tion, but the correction needs to be repeated after a large change
in temperature.
Note that the gain matching improved the negative frequency
image rejection, but there is still a significant image present.
The remaining image is now due to phase mismatch in the
quadrature modulator. Phase mismatch can be distinguished
from gain mismatch, by the shape of the image. Note that the
image in Figure 81 is relatively flat and the image in Figure 82
slopes down with frequency. Phase mismatch is frequency
dependent, so an image dominated by phase mismatch has
this sloping characteristic.