ADRF6518ACPZ-R7 datasheet(31/39 Pages) AD

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Data Sheet

ADRF6518

Rev. A | Page 31 of 39

EFFECT OF FILTER BW ON EVM

Figure 84 shows how changing the filter BW affects the EVM

for signals at several different symbol rates. The x-axis is

normalized such that it displays the baseband bandwidth of

each respective signal to the set filter corner. For example, a

filter corner of 10 MHz and a signal with a baseband bandwidth

of 5 MHz yields 2 Hz/Hz. Similarly, a filter corner of 50 MHz

and a signal with a baseband bandwidth of 25 MHz also yields

2 Hz/Hz. Baseband bandwidth is defined by the following:



)

(









Rate

Symbol

The general behavior shows that the higher the ratio of the filter

corner to baseband bandwidth is, the better the EVM. This

behavior starts to plateau at around a 2× ratio. This behavior

affects the higher bandwidth signal more so than the smaller

bandwidth signals. The primary reason for this behavior is that

the noise is not flat across frequency (there is some interaction

between the filters and all the gain stages). The noise shaping

degrades EVM as the filter corner starts to encroach well

outside of the signal bandwidth.

Figure 84. EVM vs. Filter Corner/Baseband BW Ratio over Symbol Rate,

Filter Corner = 63 MHz, QPSK, 1.5 V p-p Output Level, −20 dBm Input Power

PULL-DOWN RESISTORS FOR DISABLE FUNCTION

The ADRF6518 offers a disable function, by pulling ENBL low,

that brings the supply current to approximately 1 mA. For the

function to work correctly, a dc path to ground must be

established on the output pins (OPP1, OPM1, OPP2, OPM2) to

allow proper discharge of the postamplifier. Figure 85 shows

how to properly place the pull-down resistors. The resistor

value must be big enough so that it does not interfere with the

output impedance that the postamplifer sees (for example, 400 Ω),

but not so big that it prevents proper discharge, effectively

becoming an open circuit. For most applications, a value of

RPULLDOWN = 10 kΩ sufficiently satisfies these conditions.

INSTABILITY AT HIGH GAIN IN FILTER BYPASS

MODE

The user must be cautious while operating the ADRF6518 at the

highest of gains in filter bypass mode. Due to the high gain (up

to 66 dB) and wide bandwidth (up to 350 MHz with maximum

digital gains), the ADRF6518 is susceptible to oscillations when

it is in filter bypass mode and its gain is set above 60 dB. The

oscillation manifests itself with a broadband rise in the noise

floor and significantly degrades the SNR and EVM. Orthogonal

input to output signal paths on the printed circuit board (PCB)

helps reduce this oscillation. This can be seen in the top layer

silkscreen, shown in Figure 92. It is recommended that any

posts or headers not be placed for measurement purposes on

the signal paths, especially the output signal paths. Doing so

causes the output signal to radiate back to the input and induce

the oscillation at even lower gains than 60 dB.

Figure 85. Pull-Down Resistors

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

010

FILTER CORNER/BASEBAND BANDWIDTH (Hz/Hz)

5MSPS

10MSPS

20MSPS

50MSPS

VPSD

COMD

CLK

DATA

SDO/RST

VICM/AC

VPI

OPP1

OPM1

COM

VGN3

VOCM

COM

OPM2

OPP2

ADRF6518

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RPULLDOWN

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