ADL5374ACPZ-WP Просмотр технического описания (PDF) - Analog Devices

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ADL5374ACPZ-WP

3000 MHz to 4000 MHz Quadrature Modulator

Analog Devices 

OPTIMIZATION

The carrier feedthrough and sideband suppression performance of

the ADL5374 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

ADL5374 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 26 shows the relationship of carrier feedthrough vs. dc

offset as null.

–60

–64

–68

–72

–76

–80

–84

–88

–300 –240 –180 –120 –60 0

60 120 180 240 300

VP – VN OFFSET (µV)

Figure 26. Carrier Feedthrough vs. DC Offset Voltage at 3500 MHz

Note that throughout the nulling process, the dc bias for the

baseband 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.

ADL5374

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 3500 MHz.

–30

–35

–40

–45

–50

–55

–60

–65

–70

–75

–80

3450 3460 3470 3480 3490 3500 3510 3520 3530 3540 3550

LO FREQUENCY (MHz)

Figure 27. Carrier Feedthrough vs. fLO After Nulling at 3500 MHz

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 28 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 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 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 digital signal processor.

Rev. 0 | Page 13 of 20

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