zinmel
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Posts: 2
Pittsburgh, PA
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I think if the circuit is highly integrated RFIC chip, then s-parameter won't be that important as conventional microwave circuit design. Therefore, for a LNA to drive mixer on-chip I didn't even look at the S21 because of load termination is only a capacitive load. Actually, I believe the information of voltage gain should be more useful than S11. Regarding to the issue pointed out by "RFICGUIDE", I think he is right, if the signal in the chip needs to be drawn out, then s-parameter is getting more and more important. Actually, I used to be a RF board designer. I used s-parameter a lot, especially for designing the matching circuit of differential I/O SAW filter. The SAW filter has the frequency dependent termination because it is in the position after mixer and before IF amplifier. Both of them are the signals from and into the chip, therefore, package will make its source and load termination very frequency dependent. Due to isolation between SAW's input and output is very bad in the passband, without the helps of circuit optimization it will be very difficult to design its input and output matching circuit. But like I just said, for a highly integrated RFIC, signal in the chip is less preferable to be drawn out of chip, although I don't know so far, there is any circuit can replace IF high-Q SAW filter such as the application of GSM, probably Gm-C filter. That is the only puzzel in my head?? probably someone else can answer this question. If there are some counter circuit on-chip to replace the SAW filter or some high-Q devices, then the signal in chip won't be necessary to come out anymore. If that is the case, I believe the s-parameter will only be necessary like the point connecting the antenna to chip or filter to chip. And also if that is the case, then as Ken put at the very beggining, the usefullness of s-parameter will become less and less. ??? -Zinmel
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