ADF4112 AD [Analog Devices], ADF4112 Datasheet - Page 12

• Manufacturer Part Number: ADF4112
• Description: 300 MHz to 1000 MHz Quadrature Modulator
• Manufacturer: AD [Analog Devices]

ADL5370
OPTIMIZATION
The carrier feedthrough and sideband suppression performance
of the ADL5370 can be improved through the use of optimiza-
tion 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
equal to zero, and this 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 ADL5370 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 450 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
IOS
is applied to the I-channel inputs
IOS
/2, and
/2, such that
V
IOPP
P
–60
–V
N
− V
OFFSET (µV)
0
IOPN
60
) and (V
120
QOPP
180
IOPP
− V
240
− V
QOPN
300
IOPN
) are
Rev. 0 | Page 12 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 450 MHz.
Sideband Suppression Optimization
Sideband suppression results from relative gain and relative
phase offsets between the I and Q channels 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 28. Sideband Suppression vs. Quadrature Phase Error for Various
Figure 27. Carrier Feedthrough vs. Frequency After Nulling at 450 MHz
–25
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0.01
400
2.5dB
1.25dB
0.5dB
0.25dB
0.125dB
0.05dB
0.025dB
0.0125dB
0dB
410
420
Quadrature Amplitude Offsets
0.1
430
PHASE ERROR (Degrees)
LO FREQUENCY (MHz)
440
450
1
460
470
10
480
490
500
100