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Gain, Noise Figure, Output Intercept Point

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duffman268

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I have a cascaded system and I'm supposed to find the gain, noise figure, and the output intercept point.

I believe the net gain of the system is 3 dB. ( - 1 dB + 12 dB - 2 dB - 6 dB = 3 dB) I'm assuming that those values are the gain levels of the BP filter.

I found the value of OIP3 to be 5.806 dBm, and the NF = 4.69 dB. My concern is that I've made some assumptions. For instance, the IIP3 of the two BP filters are infinite. With the noise figure, I assumed that for the BP filters, the noise figures (NF) are 1 dB, and 2 dB, which gives a noise factor (F) of 1.26, and 1.59, respectively. This is all assuming that those are the noise figures (NF).

If anyone wants to see the math on this, I can upload my methods.
 

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  • NF, G, OIP3.jpg
    NF, G, OIP3.jpg
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On IP3, what would be the Output IP3 of the mixer if it had no gain or loss in front of it? What would the mixer output IP3 be if you put 9 db gain in front of it with infinitely high IP3 for the 9 db gain? (i.e. mixer is solely responsible for IP3 generation). Does your calcuated OIP3 make sense against these questions?

In reality, cascade IP3 calculations are valid only when one block's IP3 dominates. In reality, there is almost alway some phase shifting of the earlier's stages IP3 products that cause summation of previously created, and amplified, IP3 components to shift out of direct phase match with IP3 components created in successive stage(s). This causes high side IP3 components to be different in amplitude then low side IP3 components. And this phase shifting is usually unpreditable.
 
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