ADL5375ACPZ-R7 datasheet(22/36 Pages) AD

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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

0.2

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

–10

–20

–30

–40

–50

–60

–70

–80

–90

0.01

0.1

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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