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ADL5385

30 MHz to 2200 MHz Quadrature Modulator

Analog Devices 

ADL5385

Data Sheet

OPTIMIZATION

The carrier feedthrough and sideband suppression performance

of the ADL5385 can be improved through the use of 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 (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, result in a finite amount of carrier feedthrough. The

ADL5385 is designed to provide a minimal amount of carrier

feedthrough. If even lower carrier feedthrough levels are 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 better 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 30 shows the

relationship of carrier feedthrough vs. dc offset.

–58

–62

–66

–70

–74

–78

–82

–86

–90

–94

VP-VN OFFEST (µV)

Figure 30. 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, while

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 calibration.

This is usually performed at a single frequency. Figure 31 shows

how carrier feedthrough varies with LO frequency over a range

of ±50 MHz on either side of a null at 350 MHz.

–25

–30

–35

–40

–45

–50

–55

–60

–65

–70

–75

–80

–85

300 310 320 330 340 350 360 370 380 390 400

OUTPUT FREQUENCY (MHz)

Figure 31. Carrier Feedthrough vs. Frequency After Nulling at 350 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 32 illustrates

how sideband suppression is affected by the gain and phase

imbalances.

0

–10

2.5dB

–20 1.25dB

–30 0.5dB

0.25dB

–40 0.125dB

–50 0.05dB

0.025dB

–60 0.0125dB

–70

0dB

–80

–90

0.01

0.1

1

10

100

PHASE ERROR (Degrees)

Figure 32. Sideband Suppression vs. Quadrature Phase Error for Various

Quadrature Amplitude Offsets

Figure 32 underscores the fact that adjusting one parameter

improves the sideband suppression only to a point; the other

parameter must also be 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.

Rev. B | Page 16 of 26

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