ADRF6516ACPZ-R7 datasheet(23/29 Pages) AD

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Data Sheet

ADRF6516

Rev. C | Page 23 of 29

EFFECT OF OUTPUT VOLTAGE LEVELS ON EVM

Output voltage level can affect EVM greatly when the signal is

compressed. When changing the output voltage levels of the

ADRF6516, take care that the output signal is not in compres-

sion, which causes EVM degradation.

Figure 54 show EVM performance vs. RF input power for

several maximum differential I and Q output voltage levels

of 350 mV p-p up to 2.4 V p-p. For the lower maximum differ-

ential output voltage levels, the EVM is less than −45 dB over

approximately 20 dB of input power range.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–25

–20

–15

–10

–5

RF INPUT POWER (dBm)

350mV p-p MAX

700mV p-p MAX

1500mV p-p MAX

2400mV p-p MAX

Figure 54. EVM vs. RF Input Power at Several Output Maximum Differential

Voltage Levels (Filter Corner = 10 MHz, OFDS Pulled High)

For the largest tested maximum differential output voltage level

of 2.4 V p-p, the ADRF6516 begins to compress the signal. This

compression causes EVM to degrade, but it still remains below

−40 dB, albeit over a truncated input power range. At the high

end of the input power range, the signal is in full compression

and EVM is large. Given that the gain is near its minimum, the

input signal level must be lowered to bring the output signal out

of full compression and into the proper linear operating region.

EFFECT OF COFS VALUE ON EVM

When enabled, the dc offset compensation loop effectively

nulls any information below the high-pass corner set by the

COFS capacitor. However, loss of the low frequency information

of the modulated signal can degrade the EVM in some cases.

As the signal bandwidth becomes larger, the percentage of

information that is corrupted by the high-pass corner becomes

smaller. In such cases, it is important to select a COFS capacitor

that is large enough to minimize the high-pass corner frequency,

which prevents loss of information and degraded EVM.

Figure 55 shows degradation of the EVM vs. RF input power as

the COFS capacitor value becomes smaller, which increases the

high-pass corner for the dc offset compensation loop.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–35

–30

–25

–20

–15

–10

–5

RF INPUT POWER (dBm)

COFS = 1µF

COFS = 220nF

COFS = 1nF

Figure 55. EVM vs. RF Input Power at Several COFS Values (Filter Corner = 10 MHz,

256-QAM, 14 MSPS Signal with α = 0.35; Output Differential Signal Level

Held to 700 mV p-p; OFDS Pulled Low)

Figure 56 shows the effect that COFS has on several modulated

signal bandwidths. The modulated bandwidth was swept while

using 1000 pF and 1 µF values for COFS. Total gain was set to

15 dB, so the high-pass filter corner of the 1000 pF capacitor is

26.67 kHz, and the high-pass filter corner of the 1 µF capacitor

is 26.67 Hz. It is recommended that at moderate signal band-

widths, a 1 µF capacitor for COFS be used to obtain the best EVM

when using the dc offset compensation loop.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

SIGNAL BANDWIDTH CORNER (MHz)

COFS = 1µF

COFS = 1000pF

Figure 56. EVM vs. Signal Bandwidth Corner with COFS = 1 µF

and COFS = 1000 pF (Filter Corner = 10 MHz)

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