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AD8347 High Noise Figure?

Question asked by brihei on Apr 21, 2011
Latest reply on Apr 25, 2011 by Qui

Figure 19 of the AD8347 datasheet shows the noise figure versus the VGIN voltage. At the high end VGIN is 1.0 V, corresponding to an attenuation in the mixer stage of about 22 dB. Figure 19 shows the noise figure at VGIN=1.0V to be about 62 dB. Why is that? If the gain is set at -22 dB, I would expect the noise figure to be +22 dB. Why does the mixer add more noise than a normal attenuator? Sorry if this is a stupid question.