AD8341_V01 datasheet(11/20 Pages) AD

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Data Sheet

AD8341

Rev. B | Page 11 of 20

I-Q ATTENUATORS AND BASEBAND AMPLIFIERS

The proprietary linear-responding attenuator structure is an active

solution with differential inputs and outputs that offer excellent

linearity, low noise, and greater immunity from mismatches than

other variable attenuator methods. The gain, in linear terms, of

the I and Q channels is proportional to its control voltage with a

scaling factor designed to be 2/V, i.e., a full-scale gain setpoint of

1.0 (−4.5 dB) for a VBBI (or a VBBQ) of 500 mV. The control voltages

can be driven differentially or single-ended. The combination of

the baseband amplifiers and attenuators allows for maximum

modulation bandwidths in excess of 200 MHz.

OUTPUT AMPLIFIER

The output amplifier accepts the sum of the attenuator outputs

and delivers a differential output signal into the external load.

The output pins must be pulled up to an external supply,

preferably through RF chokes. When the 50 Ω load is taken

differentially, an output P1dB and IP3 of 8.5 dBm and 17.5 dBm

is achieved, respectively, at 1.9 GHz. The output can be taken in

single-ended fashion, albeit at lower performance levels.

NOISE AND DISTORTION

The output noise floor and distortion levels vary with the gain

magnitude but do not vary significantly with the phase. At the

higher gain magnitude setpoints, the OIP3 and the noise floor

vary in direct proportion with the gain. At lower gain magnitude

setpoints, the noise floor levels off while the OIP3 continues to

vary with the gain.

GAIN AND PHASE ACCURACY

There are numerous ways to express the accuracy of the

AD8341. Ideally, the gain and phase must precisely follow the

setpoints. Figure 4 illustrates the gain error in dB from a best fit

line, normalized to the gain measured at the gain setpoint = 1.0,

for the different phase setpoints. Figure 6 shows the gain error

in a different form, normalized to the gain measured at phase

setpoint = 0°; the phase setpoint is swept from 0° to 360° for

different gain setpoints. Figure 8 and Figure 22 show analogous

errors for the phase error as a function of gain and phase setpoints.

The accuracy clearly depends on the region of operation within

the vector gain unit circle. Operation very close to the origin

generally results in larger errors as the relative accuracy of the I

and Q vectors degrades.

RF FREQUENCY RANGE

The frequency range on the RF input is limited by the internal

polyphase quadrature phase-splitter. The phase-splitter splits the

incoming RF input into two signals, 90° out of phase, as previously

described in the RF Quadrature Generator section. This polyphase

network has been designed to ensure robust quadrature accuracy

over standard fabrication process parameter variations for the

1.5 GHz to 2.4 GHz specified RF frequency range. Using the

AD8341 as a single-sideband modulator and measuring the

resul-ting sideband suppression is a good gauge of how well the

quadrature accuracy is maintained over RF frequency. A typical

plot of sideband suppression from 1.1 GHz to 2.7 GHz is shown

in Figure 28. The level of sideband suppression degradation

outside the 1.5 GHz to 2.4 GHz specified range is subject to

manufacturing process variations.

–15

–20

–45

–40

–35

–30

–25

0.7

2.7

2.5

2.3

2.1

1.9

1.7

1.5

1.3

0.9

1.1

FREQUENCY (GHz)

Figure 28. Sideband Suppression vs. Frequency

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