ADL5385ACPZ-R2 datasheet(14/24 Pages) AD

14 / 24 page

ADL5385

Rev. 0 | Page 14 of 24

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 29 shows how carrier feedthrough varies with LO

frequency over a range of ±50 MHz on either side of a null at

350 MHz.

–25

–85

310

400

OUTPUT FREQUENCY (MHz)

–30

–35

–40

–45

–50

–55

–60

–65

–70

–75

–80

320

330

340

350

360

370

380

390

300

Figure 29. 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 30 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 30. Sideband Suppression vs. Quadrature Phase Error for Various

Quadrature Amplitude Offsets

Figure 30 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.

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