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ADL5523 Datasheet(PDF) 19 Page  Analog Devices 

ADL5523 Datasheet(HTML) 19 Page  Analog Devices 
19 / 21 page Data Sheet ADL5523 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 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 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. LNA S11 S11* 50Ω 50Ω MATCHING NETWORK Figure 59. Matching LNA Input for Gain S11* S11 Figure 60. 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. Typically, it is significantly different from 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. The fabrication facility that produces the LNA typically has this information. ΓOPT 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 ΓOPT. This is very advantageous because it means that component variations play a smaller part in the boardtoboard 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 matching. 5 10 10 5 1 1 0.5 0.5 0.2 0.2 0.2 0.5 1 5 10 Figure 61. Area of Optimal Noise Matching for ADL5523 
