adl5382 Analog Devices, Inc., adl5382 Datasheet - Page 17

• Manufacturer Part Number: adl5382
• Description: 700 Mhz To 2.7 Ghz Quadrature Demodulator
• Manufacturer: Analog Devices, Inc.

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 48 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 49 shows the excellent image rejection capabilities of
the ADL5382 for low IF applications, such as W-CDMA. The
ADL5382 exhibits image rejection greater than 45 dB over a
broad frequency range.
Figure 49. Image Rejection vs. RF Frequency for a W-CDMA Signal,
70
60
50
40
30
20
10
0
700
2.5MHz LOW IF
900

1100 1300 1500 1700 1900 2100 2300 2500 2700
IF = 2.5 MHz, 5 MHz, and 7.5 MHz
LSB

7.5MHz LOW IF
IF

RF FREQUENCY (MHz)
LO

IF

5MHz LOW IF
USB
COS
SIN


LO
LO
t
t
Figure 48. Illustration of the Image Problem
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IF
IF
EXAMPLE BASEBAND INTERFACE
In most direct conversion receiver designs, it is desirable to
select a wanted carrier within a specified band. The desired
channel can be demodulated by tuning the LO to the appropriate
carrier frequency. If the desired RF band contains multiple
carriers of interest, the adjacent carriers would also be down
converted to a lower IF frequency. These adjacent carriers can
be problematic if they are large relative to the wanted carrier as
they can overdrive the baseband signal detection circuitry. As a
result, it is often necessary to insert a filter to provide sufficient
rejection of the adjacent carriers.
It is necessary to consider the overall source and load impedance
presented by the ADL5382 and ADC input to design the filter
network. The differential baseband output impedance of the
ADL5382 is 50 Ω. The ADL5382 is designed to drive a high
impedance ADC input. It may be desirable to terminate the
ADC input down to lower impedance by using a terminating
resistor, such as 500 Ω. The terminating resistor helps to better
define the input impedance at the ADC input at the cost of a
slightly reduced gain (see the Circuit Description section for
details on the emitter-follower output loading effects). The order
and type of filter network depends on the desired high frequency
rejection required, pass-band ripple, and group delay. Filter
design tables provide outlines for various filter types and orders,
illustrating the normalized inductor and capacitor values for a
1 Hz cutoff frequency and 1 Ω load. After scaling the normalized
prototype element values by the actual desired cut-off frequency
and load impedance, the series reactance elements are halved to
realize the final balanced filter network component values.
0
0
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IF
IF
–90°
+90°
0°
0°
0
0
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IF
IF
ADL5382