AD9262EBZ datasheet(23/32 Pages) AD

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AD9262

Rev. A | Page 23 of 32

Cascaded Filter Responses

The cascaded filter responses for the three signal bandwidth

settings are for a 160 MSPS output data rate, as shown in Figure 55,

Figure 56, and Figure 57.

102030

40506070

–160

–140

–120

–100

–80

–60

–40

–20

FREQUENCY (MHz)

–0.08

–0.04

0.04

0.08

FREQUENCY (MHz)

Figure 55. 10 MHz Signal Bandwidth, 160 MSPS

–0.08

–0.04

0.04

0.08

FREQUENCY (MHz)

102030

40506070

–160

–140

–120

–100

–80

–60

–40

–20

FREQUENCY (MHz)

Figure 56. 5 MHz Signal Bandwidth, 160 MSPS

0.5

1.5

2.5

–0.08

–0.04

0.04

0.08

FREQUENCY (MHz)

102030

40506070

–160

–140

–120

–100

–80

–60

–40

–20

FREQUENCY (MHz)

Figure 57. 2.5 MHz Signal Bandwidth, 160 MSPS

DC AND QUADRATURE ERROR CORRECTION (QEC)

In direct conversion or other quadrature systems, mismatches

between the real (I) and imaginary (Q) signal paths cause

frequencies in the positive spectrum to image into the negative

spectrum and vice versa. From an RF point of view, this is

equivalent to information above the LO frequency interfering

with information below the LO frequency, and vice versa. These

mismatches may occur from gain and/or phase mismatches in

the analog quadrature demodulator or in any components in

the ADC signal chain itself. In a single-carrier zero-IF system

where the carrier has been placed symmetrically around dc, this

causes self-distortion of the carrier as the two sidebands fold

onto one another and degrade the EVM of the signal.

In a multicarrier communication system, this can be even more

problematic because carriers of widely different power levels

can interfere with one another. For example, a large carrier

centered at +f1 can have an image appear at –f1 that can be

larger than the desired carrier at this frequency.

The integrated quadrature error correction (QEC) algorithm of

the AD9262 attempts to measure and correct the amplitude and

phase imbalances of the I and Q signal paths to achieve higher

levels of image suppression than is achievable by analog means

alone. These errors can be corrected in an adapted manner

where the I and Q gain and quadrature phase mismatches are

constantly estimated and corrected. This allows changes in the

mismatches due to slow supply and temperature changes to be

constantly tracked.

The quadrature errors are corrected in a frequency independent

manner on the AD9262; therefore, systems with significant

mismatch in the baseband chain may have reduced image

suppression. The AD9262 QEC still corrects the systematic

imbalances.

The convergence time of the QEC algorithm is dependent on

the statistics of the input signal. For large signals and large

imbalance errors, this convergence time is typically less than

two million samples of the AD9262 output data rate.

LO Leakage (DC) Correction

In a direct conversion receiver subsystem, LO to RF leakage of

the quadrature modulator shows up as dc offsets at baseband.

These offsets are added to dc offsets in the baseband signal

paths, and both contribute to a carrier at dc. In a zero-IF receiver,

this dc energy can cause problems because it appears in band of

a desired channel. As part of the AD9262 QEC function, the dc

offset is suppressed by applying a low frequency notch filter to

form a null around dc. The 3 dB bandwidth of this notch filter

vs. the AD9262 output data rates is shown in Figure 58.

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