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ADL5523 Datasheet(PDF) 18 Page - Analog Devices
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ADL5523 Datasheet(HTML) 18 Page - Analog Devices
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Rev. C | Page 18 of 21
TUNING THE ADL5523 FOR OPTIMAL NOISE FIGURE
The ADL5523 is a monolithic low noise amplifier (LNA) in a
3 mm × 3 mm LFCSP. The evaluation board, as shipped from
the factory, gives a noise figure of 0.9 dB over a bandwidth of
several hundred megahertz. The specific frequency where optimal
noise is reached depends on the tuning.
The bandwidth of the ADL5523 is 400 MHz to 4 GHz, although
noise figure degrades above 2.5 GHz as the gain begins to roll off.
This section is based on Analog Devices, Inc., lab measurements.
Although there are plots in which the Agilent Advanced Design
System (ADS) environment is used, the data in these plots come
entirely from Analog Devices lab measurements.
Tuning of the LNA begins with S22 (output tuning). Tuning of
the LNA output is done by placing reactive components on the
bias line, referred to in the schematic in Figure 54 as VPOS.
On the LNA evaluation board, S22 tuning is achieved by either
the use of an inductor (L2) on the bias line or a shunt capacitor
(C3) on the bias line to ground. Typically, either L2 is required
or C3 but not both.
The evaluation board uses a slider on the bias line to make tuning
for S22 as easy as possible. The slider is an area of ground etch
adjacent to the bias line that is clear of solder mask. The bias
line in this area is also free of solder mask. This allows a capacitor
(C3) to be placed anywhere on the bias line to ground, which
provides easy and accurate tuning for S22.
Note that the PCB layout shows two capacitors, C3 and C4.
Typically, only one of these capacitors is needed for good
The slider is seen in the LNA PCB layout in Figure 58 as the
area near the red arrows to the right of the bias line. With a 0 Ω
resistor in place of L2, moving a 1 nF capacitor from the top to
the bottom effectively tunes S22 from 1400 MHz to 3500 MHz.
Table 8 shows the component values and placement required for
S22 tuning from 800 MHz to 3200 MHz. For lower frequencies,
higher values of L2 can be used to tune S22, and for frequencies
from 3.2 GHz to 4.0 GHz, smaller values of capacitors can be
used on the slider.
Table 8. Capacitor and Inductor Tuning and Placement for
LNA S22 Tuning
Figure 58. PCB Layout for LNA Evaluation Board (Note Slider on Bias Line
with Capacitor Placement for S22 Tuning Noted by Arrows)
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