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ADL5523

Data Sheet

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 S22

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

S22 tuning.

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

Frequency (MHz)

L2 (nH)

C3 (nF)

C3 Placement

800

3.4

Open

N/A

1400

0 Ω

1 nF

2000

0 Ω

1 nF

2400

0 Ω

1 nF

2800

0 Ω

1 nF

3200

0 Ω

1 nF

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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