ADL5336ACPZ-R7 datasheet(22/30 Pages) AD

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ADL5336

Data Sheet

Rev. C | Page 22 of 30

EFFECT OF CAGC ON EVM

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

response time, and carrier leakage. If CAGC 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. The AGC loop bandwidth (BW) is given by the

equation

BWLOOP = 1/(2π × RAGC × CAGC)

where RAGC is the on-chip equivalent resistance of the loop.

By increasing CAGC (which decreases the loop BW), EVM can be

improved because the signal is outside of the AGC loop BW,

and therefore, the AGC no longer levels the amplitude envelope

of the signal. Figure 64 illustrates this behavior with three different

AGC capacitor values while the ADL5336 VGAs are cascaded.

There is a drastic degradation of EVM when the smaller capacitor

values are used. This example uses a 16 QAM modulated signal

at 4.5 Msym/sec using a pulse shaping filter and an alpha of

0.35. The frequency used was 140 MHz and output setpoints for

both VGAs were 250 mV rms. Both VGAs were set to maximum

gain codes of 11.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–65

–55

–45

–35

–25

–15

–5

RF INPUT POWER (dBm)

CAGC = 0.1µF

CAGC = 1000pF

CAGC = 100pF

Figure 64. EVM vs. RF Input Power over Several CAGC Values

AGC INSENSITIVITY TO MODULATION TYPE

Given that CAGC is chosen correctly for the symbol rate of the

modulated signal and carrier frequency, EVM should not degrade

much with different modulation types. The four different

modulation types, and how EVM changes with each, are shown in

Figure 65. There is an approximately 4 dB spread across the curves.

All modulated signals were set to 4.5 Msym/sec using a pulse

shaping filter and an alpha of 0.35. The frequency used was

140 MHz. CAGC = 0.1 µF and output setpoints for both VGAs were

250 mV rms. Both VGAs were set to maximum gain codes of 11.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–65

–55

–45

–35

–25

–15

–5

RF INPUT POWER (dBm)

16QAM

256QAM

QPSK

8PSK

Figure 65. EVM vs. RF Input Power Over Several Modulation Types

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