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Design >> RF Design >> Two questions about S11 of interconnect
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Message started by Kyle on Apr 26^th, 2014, 7:59pm

Title: Two questions about S11 of interconnect
Post by Kyle on Apr 26^th, 2014, 7:59pm

Hi, everyone,
I have two questions about the s11 of interconnect,
1. what‘s the physical meaning of the largest point of S11 ? Could I call it as the resonate point?
2. In the wide band, s11 seems to be approximate periodic, why ?

Title: Re: Two questions about S11 of interconnect
Post by aaron_do on Apr 26^th, 2014, 10:27pm

Hi,

I assume this is a two-port simulation with a t-line as the DUT. Your t-line is not 50-ohm. On the smith chart, your plot will trace a circle around some centre-point close to the real axis. This point is the characteristic impedance of your line. Because it is a circle and at DC S11 must be zero (perfect through), the S11 will periodically intersect zero with a period depending on the electrical length of the line. At this point, the imaginary impedance is zero, so you could possibly call it a resonant point, but the point where S11 has its smallest magnitude in dB would also be a resonant point.

If you were to change your port impedances to equal this characteristic impedance, then your S11 would be zero across all frequencies (-infinite dB, assuming lossless).

regards,
Aaron

Edit:

Sorry i think i answered too quickly. The center of the circle is not the characterisitic impedance. The characteristic impedance should be the geometric mean of the minimum and maximum resistance......

Title: Re: Two questions about S11 of interconnect
Post by Kyle on Apr 27^th, 2014, 12:52am

Hi, Aaron
Thank you for your reply, it is really helpful,
i am still confused about the following part,

the S11 will periodically intersect zero with a period depending on the electrical length of the line.

i think the wavelength of T-line depends on both frequency and geometry, and it will change with frequency with fixed geometry. so we can just identify the wavelength or electrical length of the line at a single frequency point. is that right？

Title: Re: Two questions about S11 of interconnect
Post by aaron_do on Apr 27^th, 2014, 5:54pm

Hi,

sorry I probably should have said length, not electrical length.

regards,
Aaron