Data Sheet

AD8340

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 mis-

matches 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, that is, a full-scale

gain setpoint of 1.0 (−2 dB) for VBBI(Q) of 500 mV. The control

voltages can be driven differentially or single-endedly. 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 OP1dB of 11 dBm and OIP3 of 24 dBm are

achieved at 880 MHz. 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 magni-

tude 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 AD8340.

Ideally, the gain and phase should precisely follow the setpoints.

Figure 4 illustrates the gain error in decibels (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; the phase setpoint is swept from 0° to 360°

for different gain setpoints. Figure 8 and Figure 22 show analo-

gous 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 700 MHz to 1 GHz specified RF

frequency range. Using the AD8340 as a single-sideband

modulator and measuring the resulting sideband suppression is

a good gauge of how the quadrature accuracy is maintained

over RF frequency. A typical plot of sideband suppression from

500 MHz to 1.5 GHz is shown in Figure 28. The level of sideband

suppression degradation outside the 700 MHz to 1 GHz specified

range is subject to manufacturing process variations.

0

–5

–10

–15

–20

–25

–30

–35

500

1500

1400

1300

1200

1100

1000

900

800

600

700

FREQUENCY (MHz)

Figure 28. Sideband Suppression vs. Frequency

Rev. C | Page 11 of 20