adl5373 Analog Devices, Inc., adl5373 Datasheet - Page 13

• Manufacturer Part Number: adl5373
• Description: 2300 Mhz To 3000 Mhz Quadrature Modulator
• Manufacturer: Analog Devices, Inc.

OPTIMIZATION
The carrier feedthrough and sideband suppression performance
of the ADL5373 can be improved by using optimization
techniques.
Carrier Feedthrough Nulling
Carrier feedthrough results from minute dc offsets that occur
between each of the differential baseband inputs. In an ideal
modulator, the quantities (V
are equal to zero, which results in no carrier feedthrough. In a real
modulator, those two quantities are nonzero and, when mixed
with the LO, they result in a finite amount of carrier feedthrough.
The ADL5373 is designed to provide a minimal amount of carrier
feedthrough. Should even lower carrier feedthrough levels be
required, minor adjustments can be made to the (V
and (V
while the Q-channel offset is varied until a minimum carrier
feedthrough level is obtained. The Q-channel offset required to
achieve this minimum is held constant, while the offset on the
I-channel is adjusted until a new minimum is reached. Through
two iterations of this process, the carrier feedthrough can be
reduced to as low as the output noise. The ability to null is
sometimes limited by the resolution of the offset adjustment.
Figure 26 shows the relationship of carrier feedthrough vs. dc
offset as null.
Note that throughout the nulling process, the dc bias for the
baseband inputs remains at 500 mV. When no offset is applied,
When an offset of +V
The same applies to the Q channel.
V
V
V
V
V
Figure 26. Carrier Feedthrough vs. DC Offset Voltage at 2500 MHz
IOPP
IOPP
IOPP
IOPN
IOPP
QOPP
–60
–64
–68
–72
–76
–80
–84
–88
–300 –240 –180 –120
= V
− V
= 500 mV + V
− V
= 500 mV − V
− V
IOPN
IOPN
IOPN
QOPN
= 500 mV, or
= V
= V
) offsets. The I-channel offset is held constant
IOS
IOS
IOS
= 0 V
is applied to the I-channel inputs,
IOS
IOS
/2, and
/2, such that
V
P
IOPP
–60
– V
N
− V
OFFSET (µV)
0
IOPN
60
) and (V
120
QOPP
180
IOPP
− V
240
− V
QOPN
IOPN
300
)
Rev. 0 | Page 13 of 20
)
It is often desirable to perform a one-time carrier null calibra-
tion. This is usually performed at a single frequency. Figure 27
shows how carrier feedthrough varies with LO frequency over a
range of ±50 MHz on either side of a null at 2500 MHz.
Sideband Suppression Optimization
Sideband suppression results from relative gain and relative
phase offsets between the I-channel and Q-channel and can
be suppressed through adjustments to those two parameters.
Figure 28 illustrates how sideband suppression is affected by
the gain and phase imbalances.
Figure 28 underlines the fact that adjusting only one parameter
improves the sideband suppression only to a point, unless the
other parameter is also adjusted. For example, if the amplitude
offset is 0.25 dB, improving the phase imbalance better than 1°
does not yield any improvement in the sideband suppression. For
optimum sideband suppression, an iterative adjustment
between phase and amplitude is required.
The sideband suppression nulling can be performed either
through adjusting the gain for each channel or through the
modification of the phase and gain of the digital data coming
from the digital signal processor.
Figure 27. Carrier Feedthrough vs. Frequency After Nulling at 2500 MHz
Figure 28. Sideband Suppression vs. Quadrature Phase Error for
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
2450 2460 2470 2480 2490 2500 2510 2520 2530 2540 2550
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0.01
2.5dB
1.25dB
0.5dB
0.25dB
0.125dB
0.05dB
0.025dB
0.0125dB
0dB
Various Quadrature Amplitude Offsets
0.1
LO FREQUENCY (MHz)
PHASE ERROR (Degrees)
1
10
ADL5373
100