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ADL5375-05ACPZ-WP Datasheet(PDF) 22 Page - Analog Devices

Part No. ADL5375-05ACPZ-WP
Description  400 MHz to 6 GHz Broadband Quadrature Modulator
Download  36 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADL5375-05ACPZ-WP Datasheet(HTML) 22 Page - Analog Devices

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ADL5375
Data Sheet
Rev. C | Page 22 of 36
APPLICATIONS INFORMATION
CARRIER FEEDTHROUGH NULLING
LO leakage results from minute dc offsets that occur on the
differential baseband inputs. In an IQ modulator, non-zero
differential offsets mix with the LO and result in LO leakage to
the RF output. In addition to this effect, some of the signal
power at the LO input couples directly to the RF output (this
may be a result of bond-wire to bond-wire coupling or coupling
through the silicon substrate). The net LO leakage at the RF
output is the vector combination of the signals that appear at
the output as a result of these two effects.
The device’s nominal carrier feedthrough can be nulled by
adding small external differential offset voltages on the I and Q
inputs.
Nulling the carrier feedthrough is a multistep process. Initially,
with the I-channel offset held constant (at 0 mV), the Q-
channel offset is varied until a minimum LO leakage level is
obtained. This Q-channel offset voltage is then held constant,
while the offset on the I-channel is adjusted until a new
minimum is reached. Through two iterations of this process,
the LO leakage can be reduced to an arbitrarily low level. This
level is only limited by the available offset voltage steps and by
the modulator’s noise floor. Figure 55 illustrates the typical
relationship between LO leakage and dc offset at 1900 MHz. In
this case, differential offset voltages of approximately +0.5 mV
and −0.5 mV on the I and Q inputs, respectively, result in the
lowest carrier feedthrough. It is important to note that the
required offset nulling voltage changes in polarity and
magnitude from device to device and overtemperature and
frequency. To ensure that all devices in a mass production
environment can be adequately nulled, an offset adjustment
range of approximately ±10 mV should be provided.
I AND Q OFFSET VOLTAGE (µV)
–92
–87
–82
–77
–72
–67
–62
–57
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
1.0
Q OFFSET SWEEP
I OFFSET SWEEP
Figure 55. Example of Typical Carrier Feedthrough vs. DC Offset Voltage
It is important to note that the carrier feedthrough is not
affected by the dc bias levels (also called the common-mode
level) on the I and Q inputs. A differential offset voltage must
be applied, so after nulling, the average voltage on the IP and
IN inputs can be slightly different. Using Figure 55 as an
example, after LO leakage nulling, the average dc level on IP
and IN can be 500.25 mV and 499.75 mV.
The same applies to the Q-channel. For the ADL5375-15, the
same theory applies except that
VIBBP = VIBBN = 1500 mV.
It is often desirable to perform a one-time carrier null. This is
usually performed at a given frequency. After this factory
calibration, the IQ modulator operates over a frequency range
on each side of the calibration frequency. The nulled LO leakage
level degrades somewhat because the LO frequency is moved
away from the calibration frequency. Despite this degradation,
the overall LO leakage across a frequency band can be expected
to be better than when no nulling is performed. This assumes
an operating frequency band that is in the 30 MHz to 60 MHz
range.
LO leakage nulling is discussed further in AN-1039, Correcting
Imperfections in IQ Modulators to Improve RF Signal Fidelity.
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 56 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 56. Sideband Suppression vs. Quadrature Phase Error for
Various Quadrature Amplitude Offsets
Figure 56 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 by 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 baseband signal processor.


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