ADRF6516ACPZ-R7 datasheet(20/29 Pages) AD

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ADRF6516

Data Sheet

Rev. C | Page 20 of 29

Distortion must also be considered when maximizing the dynamic

range. At low and moderate signal levels, the output distortion

is constant and assumed to be adequate for the selected output

level. At some point, the input signal becomes large enough that

distortion at the input limits the system. The maximum tolerable

input signal depends on whether the input distortion becomes

unacceptably large or the minimum gain is reached.

The most challenging scenario in terms of dynamic range is the

presence of a large out-of-band blocker accompanying a weaker

in-band desired signal. In this case, the maximum input level is

dictated by the blocker and its inclination to cause distortion.

After filtering, the weak desired signal must be amplified to the

desired output level, possibly requiring maximum gain. Both

the distortion limits associated with the blocker at the input and

the SNR limits created by the weaker signal and higher gains are

present simultaneously. Furthermore, not only does the blocker

scenario degrade the dynamic range, it also reduces the range

of input signals that can be handled because a larger part of the

gain range is used to simply extract the weak desired signal

from the stronger blocker.

KEY PARAMETERS FOR QUADRATURE-BASED

RECEIVERS

The majority of digital communication receivers makes use of

quadrature signaling, in which bits of information are encoded

onto pairs of baseband signals that then modulate in-phase (I)

and quadrature (Q) sinusoidal carriers. Both the baseband and

modulated signals appear quite complex in the time domain with

dramatic peaks and valleys. In a typical receiver, the goal is to

recover the pair of quadrature baseband signals in the presence

of noise and interfering signals after quadrature demodulation.

In the process of filtering out-of-band noise and undesired inter-

ferers and restoring the levels of the desired I and Q baseband

signals, it is critical to retain their gain and phase integrity over

the bandwidth.

The ADRF6516 delivers flat in-band gain and group delay,

consistent with a six-pole Butterworth prototype filter, as

described in the Programmable Filters section. Furthermore,

careful design ensures excellent matching of these parameters

between the I and Q channels. Although absolute gain flatness

and group delay can be corrected with digital equalization,

mismatch introduces quadrature errors and intersymbol inter-

ference that degrade bit error rates in digital communication

systems.

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