ADL5375-15ACPZ-R71 datasheet(22/32 Pages) AD

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ADL5375

Rev. 0 | Page 22 of 32

OPTIMIZATION

The carrier feedthrough and sideband suppression performance of

the ADL5375 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 (VIBBP − VIBBN) and (VQBBP − VQBBN)

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, result in a finite amount of carrier feedthrough. The

ADL5375 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 (VIBBP − VIBBN)

and (VQBBP − V QBBN) 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 55 illustrates the typical relationship between carrier

feedthrough and dc offset around the null.

–60

–88

–84

–80

–76

–72

–68

–64

–300 –240 –180 –120 –60

120

180

240

300

VP – VN OFFSET (µV)

Figure 55. Example of Typical Carrier Feedthrough vs. DC Offset Voltage

Using the ADL5375-05 version as an example, note that

throughout the nulling process, the dc bias for the base-

band inputs remains at 500 mV. When no offset is applied,

VIBBP = VIBBN = 500 mV, or

VIBBP − VIBBN = VIOS = 0 V

When an offset of +VIOS is applied to the I-channel inputs,

VIBBP = 500 mV + VIOS/2, and

VIBBN = 500 mV − VIOS/2, such that

VIBBP − VIBBN = VIOS

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 calibra-

tion. This is usually performed at a given frequency and the

radio allowed to operate over a frequency range on each side

of that frequency. The nulled carrier feedthrough level degrades

somewhat as the LO frequency is moved away from the

frequency at which the null was performed. This variation is

very small across a 30 MHz or 60 MHz cellular band, however.

This small variation is due to the effects of LO-to-RF output

leakage around the package and on the board as the frequency

changes. Despite the degradation, the LO leakage can be

expected to be better than when no nulling is performed.

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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