ADRF6518ACPZ-R7 datasheet(25/39 Pages) AD

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Data Sheet

ADRF6518

Rev. A | Page 25 of 39

MAXIMIZING THE DYNAMIC RANGE

When used in the filter mode, the role of the ADRF6518 is to

increase the level of a variable in-band signal while minimizing

out-of-band signals. Ideally, this is achieved without degrading

the SNR of the incoming signal or introducing distortion to the

incoming signal.

The first goal is to maximize the output signal swing, which can

be defined by the ADC input range or the input signal capacity

of the next analog stage. For the complex waveforms often encoun-

tered in communication systems, the peak-to-average ratio, or

crest factor, must be considered when choosing the peak-to-peak

output. From the chosen output signal and the maximum gain

of the ADRF6518, the minimum input level can be defined.

As the input signal level increases, the VGA3 gain is reduced

from its maximum gain point to maintain the desired fixed

output level. VGA2 and VGA1 can then be adjusted as the input

signal level keeps increasing. This maintains the best NF for the

cascaded chain. The output noise, initially dominated by the

filter and VGA1 combination, follows the gain reduction,

yielding a progressively better SNR. At some point, the VGA3

and VGA2 gains drop sufficiently so that their noise becomes

dominant, resulting in a slower reduction in SNR from that

point. From the perspective of SNR alone, the maximum input

level is reached when the VGA1 reaches its minimum gain.

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. This can be kept in check

by monitoring peak detector voltage, VPK.

The most challenging scenario in terms of dynamic range is the

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

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

dictated by the blocker and its inclination to cause distortion.

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

desired output level, possibly requiring the maximum gain on

VGA2 and VGA3. In such a case, 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 simply used to extract the weak desired signal from

the stronger blocker.

KEY PARAMETERS FOR QUADRATURE-BASED

RECEIVERS

The majority of digital communication receivers make 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 unwanted inter-

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

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

the bandwidth.

In filter mode, the ADRF6518 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 interference that degrade bit error rates in digital

communication systems.

For wideband signals, filters can be bypassed and the

ADRF6518 then becomes a dual cascaded chain of three VGAs,

offering large gain range options, while maintaining gain and

group delay match between the two channels.

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