AD8368 datasheet(16/20 Pages) AD

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AD8368

Rev. A | Page 16 of 20

The choice of CDETO is a compromise of averaging time constant,

response time, and carrier leakage. If CDETO is selected to be too

small to speed up the response time, the AGC loop could start

tracking and leveling any amplitude envelope and corrupt the

constellation. Figure 38 illustrates a 16 QAM, 100 ksymbols per

second constellation with a degraded error vector magnitude

(EVM) of 5%. By increasing CDETO to 0.01 μF, the EVM is

improved to 1.1%.

Figure 39 illustrates the measured EVM performance for a

16 QAM modulation at 10 Msymbols per second using

CDETO = 1 nF.

–40

–30

–20

–10

POWER IN (dBm)

REF –4.9dBm

CF 100MHz

SR 10kHz

CONST DIAG

MEAS SIGNAL

16 QAM

–1U

–1.31289U

262.578mU/

1.31289U

Figure 39. Error Vector Magnitude Performance for 16 QAM

10 Msymbols per second

Figure 38. Degraded Error Vector Magnitude Performance for 16 QAM at

100 ksymbols per second (CDETO Too Small)

AD8368 Datasheet(PDF) 16 Page - Analog Devices