ADL5380-29A-EVALZ AD [Analog Devices], ADL5380-29A-EVALZ Datasheet - Page 25

• Manufacturer Part Number: ADL5380-29A-EVALZ
• Description: 400 MHz to 6 GHz Quadrature Demodulator
• Manufacturer: AD [Analog Devices]

Figure 83 shows the zero-IF EVM performance of a 10 MHz
IEEE 802.16e WiMAX signal through the ADL5380. The
differential dc offsets on the ADL5380 are in the order of a few
millivolts. However, ac coupling the baseband outputs with 10 μF
capacitors eliminates dc offsets and enhances EVM performance.
With a 10 MHz BW signal, 10 μF ac coupling capacitors with
the 500 Ω differential load results in a high-pass corner frequency
of ~64 Hz, which absorbs an insignificant amount of modulated
signal energy from the baseband signal. By using ac coupling
capacitors at the baseband outputs, the dc offset effects, which
can limit dynamic range at low input power levels, can be
eliminated.
Figure 83. EVM, RF = 2.6 GHz, RF = 3.5 GHz, and RF = 5.8 GHz, IF = 0 Hz vs.
RF Input Power for a 16 QAM 10 MHz Bandwidth Mobile WiMAX Signal
–10
–20
–30
–40
–50
–60
0
–75
–65
ω
(AC-Coupled Baseband Outputs)
LSB
–55
ω
IF
RF INPUT POWER (dBm)
ω
–45
LO
ω
IF
–35
ω
USB
–25
5.8GHz
2.6GHz
3.5GHz
COS
SIN
–15
ω
ω
LO
LO
t
t
–5
Figure 85. Illustration of the Image Problem
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–
–
ω
ω
IF
IF
Figure 84 exhibits multiple W-CDMA low-IF EVM performance
curves over a wide RF input power range into the ADL5380. In
the case of zero-IF, the noise contribution by the vector signal
analyzer becomes predominant at lower power levels, making it
difficult to measure SNR accurately.
7.5 MHz vs. RF Input Power for a W-CDMA Signal (AC-Coupled Baseband Outputs)
LOW IF IMAGE REJECTION
The image rejection ratio is the ratio of the intermediate frequency
(IF) signal level produced by the desired input frequency to that
produced by the image frequency. The image rejection ratio is
expressed in decibels. Appropriate image rejection is critical
because the image power can be much higher than that of the
desired signal, thereby plaguing the down-conversion process.
Figure 85 illustrates the image problem. If the upper sideband
(lower sideband) is the desired band, a 90° shift to the Q channel
(I channel) cancels the image at the lower sideband (upper sideband).
Phase and gain balance between I and Q channels are critical
for high levels of image rejection.
Figure 84. EVM, RF = 1900 MHz, IF = 0 Hz, IF = 2.5 MHz, IF = 5 MHz, and IF =
0
0
–10
–15
–20
–25
–30
–35
–40
–45
–80
+
+
ω
ω
IF
IF
–70
–60
–90°
+90°
7.5MHz LOW-IF
0°
0°
–50
RF INPUT POWER (dBm)
0Hz IF
–40
2.5MHz LOW-IF
–30
0
0
–20
+
+
ω
ω
IF
IF
5MHz LOW-IF
–10
ADL5380
0
10