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In operation, the top end of the ADL5205 gain scale is varied by setting the input attenuator at minimum attenuation, then varying the gain of the internal VGA. Since the noise figure of the variable gain amp is nearly constant with its gain setting, this gives a nearly constant noise floor vs. gain for the top 11 dB of gain range. Once the VGA is set to its lowest gain, further gain reduction can only be achieved by increasing the attenuator value, which gives a corresponding 1:1 degradation in noise floor with gain for the rest of the gain curve.  

PTM Published on: 2016-10-05