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ADL5521 Datasheet(PDF) 37 Page - Analog Devices |
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ADL5521 Datasheet(HTML) 37 Page - Analog Devices |
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37 / 45 page  Preliminary Technical Data ADL5521 Rev. PrC| Page 37 of 45 Tuning the LNA Input for Optimal Gain LNAs are generally tuned for either gain or noise optimization, or some tradeoff between the two. One figure of merit of an LNA is how much tradeoff must be made for one of these parameters to optimize the other. With the ADL5521 and ADL5523, S11 of 6 to 8dB at the input to the matching network can still be typically achieved 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, will always be the complex conjugate of the LNA input impedance when matched for gain. Once the conjugate of S11 is known, a matching circuit must be found which transforms the 50 Ω system impedance into the conjugate S11 impedance. To do this, the designer starts at the origin of the circle and finds components that move the 50 Ω match to S11*. The related impedances for gain matching are shown in Figure 124. A Smith Chart representation of the conjugate match is shown in Figure 125. LNA S11 S11* Matching Network 50 Ω 50 Ω Figure 124. Matching LNA Input for Gain S11 S11* Figure 125. Smith Chart Representation of Conjugate Match Tuning the LNA Input for Optimal Noise Figure The point in the Smith Chart at which matching for optimal noise occurs is typically referred to as Gamma Optimal, or ΓOPT. It’s often different than the gain matching point. Finding ΓOPT is not as obvious as the gain match. ΓOPT is a function of the semiconductor structure and characteristics of the LNA. Typically, the fabrication facility that produces the LNA will have this information. ΓOPT can also be determined by doing source pull testing in the lab. Noise matching for the ADL5521 and 23 is actually very easy, as the area of the Smith Chart where the noise figure is optimal or near optimal is not confined to a narrow area around ΓOPT. This is very advantageous as it means that component variations will play a smaller part in board to board variation of noise figure. The matching area for optimal noise for the ADL5523 and ADL5521 is shown in Figure 126. Note that textbooks usually define noise circles as a conjugate match. However, for the purpose of this note this circle is a direct match, we will do things slightly differently. In our case to find the correct matching circuit, the designer must start with the S11 of the LNA, then select components which move the S11 to within this circle. One important aspect of the overall ADL5521 and 23 ease of tuning is that as long as S22 is matched for a particular frequency, this noise matching area remains very consistent in its placement for that frequency. Said another way, if S22 is matched, we simply have to take the measured S11 and move it into the black circle for optimal noise matching. |
Similar Part No. - ADL5521_08 |
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Similar Description - ADL5521_08 |
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