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ADL5370 Datasheet(PDF) 12 Page - Analog Devices

Part No. ADL5370
Description  300 MHz to 1000 MHz Quadrature Modulator
Download  20 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

ADL5370 Datasheet(HTML) 12 Page - Analog Devices

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ADL5370
Rev. 0 | Page 12 of 20
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 (VIOPP − VIOPN) and (VQOPP − VQOPN) are
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 (VIOPP − VIOPN)
and (VQOPP − VQOPN) offsets. The I-channel offset is held constant
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.
–60
–88
–84
–80
–76
–72
–68
–64
–300 –240 –180 –120 –60
0
60
120
180
240
300
VP –VN OFFSET (µV)
Figure 26. Carrier Feedthrough vs. DC Offset Voltage at 450 MHz
Note that throughout the nulling process, the dc bias for the
baseband inputs remains at 500 mV. When no offset is applied
VIOPP = VIOPN = 500 mV, or
VIOPP − VIOPN = VIOS = 0 V
When an offset of +VIOS is applied to the I-channel inputs
VIOPP = 500 mV + VIOS/2, and
VIOPN = 500 mV − VIOS/2, such that
VIOPP − VIOPN = VIOS
The same applies to the Q channel.
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.
–25
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
400
410
420
430
440
450
460
470
480
490
500
LO FREQUENCY (MHz)
Figure 27. Carrier Feedthrough vs. Frequency After Nulling 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.
0dB
0.0125dB
0.025dB
0.05dB
0.125dB
0.25dB
0.5dB
1.25dB
2.5dB
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
0.01
0.1
1
10
100
PHASE ERROR (Degrees)
Figure 28. Sideband Suppression vs. Quadrature Phase Error for Various
Quadrature Amplitude Offsets
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.


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