adl5336 Analog Devices, Inc., adl5336 Datasheet - Page 22

• Manufacturer Part Number: adl5336
• Description: Cascadable If Vgas With Programmable Rms Detectors
• Manufacturer: Analog Devices, Inc.

ADL5336
EFFECT OF C
The choice of C
response time, and carrier leakage. If C
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
where R
By increasing C
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
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.
BW
–10
–15
–20
–25
–30
–35
–40
–45
–50
–5
0
–65
Figure 64. EVM vs. RF Input Power over Several C
AGC
LOOP
is the on-chip equivalent resistance of the loop.
C
C
C
= 1/(2π × R
–55
AGC
AGC
AGC
AGC
AGC
AGC
= 0.1µF
= 1000pF
= 100pF
–45
is a compromise of averaging time constant,
(which decreases the loop BW), EVM can be
ON EVM
RF INPUT POWER (dBm)
–35
AGC
× C
–25
AGC
ADL5336
)
–15
AGC
–5
VGAs are cascaded.
is selected to be too
5
AGC
Values
15
25
Rev. A | Page 22 of 32
AGC INSENSITIVITY TO MODULATION TYPE
Given that C
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. C
250 mV rms. Both VGAs were set to maximum gain codes of 11.
Figure 65. EVM vs. RF Input Power Over Several Modulation Types
–10
–15
–20
–25
–30
–35
–40
–45
–50
–5
0
–65
AGC
AGC
–55
= 0.1 μF and output setpoints for both VGAs were
is chosen correctly for the symbol rate of the
–45
RF INPUT POWER (dBm)
–35
–25
–15
–5
5
16QAM
256QAM
QPSK
8PSK
15
25