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ADL5523 Datasheet(PDF) 19 Page - Analog Devices
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ADL5523 Datasheet(HTML) 19 Page - Analog Devices
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Rev. C | Page 19 of 21
TUNING THE LNA INPUT FOR OPTIMAL GAIN
LNAs are generally tuned for either gain or noise optimization,
or some trade-off between the two. One figure of merit of an LNA
is how much trade-off must be made for one of these parameters to
optimize the other. With the ADL5523, an S11 of 6 dB to 8 dB
at the input to the matching network can still be achieved
typically when optimizing for noise.
For optimal gain matching, the goal is to use a matching network
that converts the input impedance of the LNA to the characteristic
impedance of the system, typically 50 Ω. Correct tuning for gain
matching results in a conjugate match. That is, the impedance of
the matching network at the LNA input, looking back toward
the generator, is always the complex conjugate of the LNA input
impedance when matched for gain.
Once S11*, the complex conjugate of S11, is known, a matching
circuit must be found that transforms the 50 Ω system impedance
into the conjugate S11 impedance. To do this, the designer starts at
the origin of the Smith Chart circle and finds components that
move the 50 Ω match to S11*.
The related impedances for gain matching are shown in Figure 59.
A Smith Chart representation of the conjugate match is shown
in Figure 60.
Figure 59. Matching LNA Input for Gain
Figure 60. Smith Chart Representation of Conjugate Match
TUNING THE LNA INPUT FOR OPTIMAL NOISE
The point in the Smith Chart at which matching for optimal
noise occurs is typically referred to as gamma optimal or Γ
Typically, it is significantly different from the gain matching
point; finding Γ
is not as obvious as the gain match. Γ
function of the semiconductor structure and characteristics of
the LNA. The fabrication facility that produces the LNA typically
has this information. Γ
can also be determined by doing
source pull testing in the lab.
Noise matching for the ADL5523 is actually very easy because
the area of the Smith Chart where the noise figure is optimal or
near optimal is not confined to a narrow area around Γ
is very advantageous because it means that component variations
play a smaller part in the board-to-board variation of noise figure.
The matching area for optimal noise for the ADL5523 is shown
in Figure 61. Note that textbooks usually define noise circles as
a conjugate match. However, for the purpose of this data sheet,
the circle is a direct match. To find the correct matching circuit,
the designer must start with the S11 of the LNA and select
components that move the S11 to within this circle.
An important aspect of the overall ADL5523 ease of tuning is
that as long as S22 is matched for a particular frequency, the
noise matching area remains very consistent in its placement for
that frequency. If S22 is matched, take the measured S11 and
move it into the red circle shown in Figure 61 for optimal noise
Figure 61. Area of Optimal Noise Matching for ADL5523
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