ADL5370(RevA) Просмотр технического описания (PDF) - Analog Devices

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ADL5370
(Rev.:RevA)

300 MHz to 1000 MHz Quadrature Modulator

Analog Devices 

ADL5370

OPTIMIZATION

The carrier feedthrough and sideband suppression performance

of the ADL5370 can be improved through the use of optimiza-

tion 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, they result in a finite amount of carrier feedthrough.

The ADL5370 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 (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 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 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, and

VIOPN = 500 mV − VIOS/2, such that

VIOPP − VIOPN = VIOS

The same applies to the Q channel.

Data Sheet

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 450 MHz.

–25

–30

–35

–40

–45

–50

–55

–60

–65

–70

–75

–80

–85

400 410 420 430 440 450 460 470 480 490 500

LO FREQUENCY (MHz)

Figure 27. Carrier Feedthrough vs. Frequency After Nulling at 450 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 28 illus-

trates 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 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. A | Page 12 of 20

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