I\u2019ve been receiving a few questions from the readers. (Yes, I actually have readers\u2014other than you.)<\/p>\n
I misread the email; the initial version of the I\/Q noise figure discussion was completely wrong.<\/p>\n
<\/p>\n
What’s the difference of noise figure between quadrature [downconverter] and single downconverter? In the quadrature downconverter, you have I and Q path, if required Nf=10dB for this quadrature downconverter, what’s the Nf for the path of I or Q? Can I see the Nf for RF to I path is 7dB, the same thing for Q?<\/p>\n
— K. C.<\/p><\/blockquote>\n
Unfortunately, the answer is no. The important thing to remember when computing things in dB (and summing them) is whether they add in-phase (constructively) or not. Since the I and Q signals are completely orthogonal, if a 10 dB NF is required for the quadrature downconverter (I + jQ), then 10 dB is required for the each of the I and Q paths.<\/p>\n
If, however, we had two paths that summed constructively<\/em>, and we needed a 10 dB overall noise figure, then yes, each path would only need to deliver 10 dB (per path). Unfortunately, I and Q don\u2019t do this constructive summation.<\/p>\n
Loop gain in Cadence ADE<\/h2>\n
Can u please elaborate on loop gain\/Stablity simulation using Cadence ADE?<\/p>\n
— H. M.<\/p><\/blockquote>\n
Unfortunately, the answer is no. Or sort of. Sure, I can elaborate, but I can\u2019t do a very good step-by-step job because I don\u2019t have access to Cadence ADE. So, I\u2019m running on my own memory, which (as my friends know) is very dangerous.<\/p>\n
However, here\u2019s a rough step-by-step:<\/p>\n
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- Break the loop by inserting an Iprobe element. (Usually, I choose an Iprobe). You usually want to break the loop in a point that\u2019s high-impedance. Typically, I choose the highest impedance output node (transconductor) looking into a compensation capacitor. So, the Iprobe would measure current going from the transconductor into its compensation capacitor.<\/li>\n
- Start ADE (Tools \u2013> Analog Environment). If it\u2019s not already set in the ADE window, select Setup (?) \u2013> Simulator\/Directory\/Host and select spectre.<\/li>\n
- Select Setup \u2013> Analysis and select stb<\/strong> (stability). You\u2019ll need to select start and stop frequencies. There\u2019s also a button to select the loop gain element (Iprobe) that you\u2019re using to break the loop. (You can have multiple Iprobes in a single schematic, for each loop you want to analyze.)<\/li>\n
- After you run it, you can choose Outputs \u2013> Plot \u2013> Loop Gain (?) or something like that to see amplitude and phase vs frequency. You can also choose the loopgain result from the results browser and do a direct phaseMargin()<\/span> measurement on it, which will pick the phase margin. (Something like phaseMargin(-getData(\u2018loopgain\u2019 ?result \u2018stb\u2019))<\/span> \u2013 but I\u2019m typing from memory.)<\/li>\n
- Keep in mind that Cadence returns the exact loop gain, which is (or should be) 180 degrees at dc. However, the phaseMargin()<\/span> function expects the loop gain to be 0 degrees at dc and then go to 180 (unstable) at some point later. So, you have to invert the result when using phaseMargin()<\/span>.<\/li>\n<\/ol>\n