ThomasLee
Newbie level 5
Hello everybody!
I don´t know if this has been discussed before, but I hope you can help me.
I have a general problem with system calculation.
Think of a system with these blocks: LNA with 2dB Noise Figure and 15dB available gain, a mixer with 12 dB Noise Figure (SSB) and e.g. 5 dB gain.
These blocks shall be used for a direct conversion receiver. Here my assumptions, correct me if I am wrong:
For a direct conversion receiver the DSB noise figure is applicable because there is no image. So I subtract 3 dB from 12 dB and I get 9 dB DSB NF for the mixer. With the LNA gain and NF I can calculate the cascaded NF to 2.56 dB.
But if I don´t subtract the 3dB first (like for SSB), I use 12 dB for the cascaded NF calculation, then I get 3.13 dB total NF.
So far so good. Then I say take this whole block, regard it as a low noise mixer and use it for direct conversion, so I can subtract 3dB and I´d get a system with 0.13 dB noise figure!!! What am I doing wrong?
Thank you!
I don´t know if this has been discussed before, but I hope you can help me.
I have a general problem with system calculation.
Think of a system with these blocks: LNA with 2dB Noise Figure and 15dB available gain, a mixer with 12 dB Noise Figure (SSB) and e.g. 5 dB gain.
These blocks shall be used for a direct conversion receiver. Here my assumptions, correct me if I am wrong:
For a direct conversion receiver the DSB noise figure is applicable because there is no image. So I subtract 3 dB from 12 dB and I get 9 dB DSB NF for the mixer. With the LNA gain and NF I can calculate the cascaded NF to 2.56 dB.
But if I don´t subtract the 3dB first (like for SSB), I use 12 dB for the cascaded NF calculation, then I get 3.13 dB total NF.
So far so good. Then I say take this whole block, regard it as a low noise mixer and use it for direct conversion, so I can subtract 3dB and I´d get a system with 0.13 dB noise figure!!! What am I doing wrong?
Thank you!