ADL5521ACPZ-R7 datasheet(34/45 Pages) AD

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ADL5521

Preliminary Technical Data

Rev. PrC| Page 34 of 45

TUNING THE ADL5521/23 EVAL BOARD FOR OPTIMAL NOISE FIGURE

The ADL5521 and ADL5523 are monolithic LNAs in a 3x3mm

LFCSP package. The eval board, as shipped from the factory,

should give a noise figure of 0.9dB over a bandwidth of several

hundred MHz. The specific frequency where optimal noise is

reached depends on the tuning.

The bandwidth of the ADL5521 is 400MHz to 4GHz, although

noise figure will degrade above 2.5GHz as the gain begins to roll

off.

Note – The factory eval board has a bias resistor on the LNA of

3.3K Ω. If this bias resistor is increased to 5.2K Ω, the optimal

noise figure will drop to 0.8dB, but the tradeoff is that the OIP3

will typically drop from 35 to 33dB. The change in S parameters

will be insignificant when changing bias resistors, so this section

will only take into account measurements done with 3.3K Ω bias

resistor.

Contents of this note are based completely on lab measurements.

Although there are plots in which the Agilent ADS environment is

used, the data in these plots comes completely from 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 schematics in Figure 88 as VPOS.

On the LNA eval board, S22 tuning is achieved by either the use of

an inductor (L2) on the bias line, or a shunt cap 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 in order 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

and so provides easy, very accurate tuning for S22.

Note that the PCB layout shows two capacitors, C3 and C4.

Typically only one of these is needed for good S22 tuning.

The slider can be seen in the LNA PCB layout in Figure 117 as the

red area to the right of the bias line. With a 0 Ω jumper in place of

L2, moving a 1nF capacitor from the top to the bottom effectively

tunes S22 from 1400 MHz to 3500MHz. Table 7 shows the

component values and placement required for S22 tuning from

800MHz to 3200MHz. For lower frequencies, higher values of L2

can be used to tune S22, and for frequencies from 3.2GHx to

4.0GHz, smaller values of capacitors can be used on the slider.

The results for S22 tuned for different frequencies are shown in

Figure 118 to Figure 123.

Frequency

(MHz)

L2 (nH)

C3 (nF)

Placement

800

3.4

Open

1400

0 Ω

Open

2000

0 Ω

1nF

2400

0 Ω

1nF

2800

0 Ω

1nf

3200

0 Ω

1nf

Table 7. Capacitor and Inductor Tuning and Placement for LNA S22 Tuning

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