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Simulators >> Circuit Simulators >> Weird Z11
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Message started by ywguo on Oct 22^nd, 2013, 2:58am

Title: Weird Z11
Post by ywguo on Oct 22^nd, 2013, 2:58am

Hi Guys,

I got weird Z11 parameter with SP analysis of spectre simulator. That is a 50 Ω microstrip. So I simulated with a n2port and read the S-parameter file.

I got weird Z11 if I put two port cells at both ports. But Z11 looks good if the port cell at the second port is replaced with a 50 Ω resistor.

The schematics and the Z11 vs. frequency are shown below.

I read Kundert's The Designer's Guide to Spice & Spectre. For SP analysis, only the AC magnitude at one port is set to non-zero at any time. So I think both configuration should give correct result. Who can explain the weird Z11?

Best Regards,
Yawei

Title: Re: Weird Z11
Post by ywguo on Oct 22^nd, 2013, 3:02am

That is Z11 vs. frequency if I simulate with two port cells.

Title: Re: Weird Z11
Post by ywguo on Oct 22^nd, 2013, 3:07am

The schematic of one port cell on the first port and one 50 Ω resistor on the second port.

Title: Re: Weird Z11
Post by ywguo on Oct 22^nd, 2013, 3:12am

That is Z11 vs. frequency of one 50 Ω resistor on the second port.

Title: Re: Weird Z11
Post by ywguo on Oct 22^nd, 2013, 3:21am

Furthermore, for the schematic two_port_cell, Z11 looks good if only one port is selected for SP analysis.

Code:

NPORT0 (PORT0_PLUS 0 NPORT0_t2 0) nport thermalnoise=yes \
file="~/test.s2p"
PORT3 (NPORT0_t2 0) port r=50 type=sine
PORT0 (PORT0_PLUS 0) port r=50 type=sine
...
sp sp ports=[PORT0] start=1 stop=2G dec=10 annotate=status

But Z11 is weird if both port cells are selected for SP analysis.

Code:

Thanks for any comments.
Yawei

Title: Re: Weird Z11
Post by raja.cedt on Oct 22^nd, 2013, 9:51am

Dear ywguo,

You can't find input impedance with Z(1,1) because you have to find input impedance with 50ohm loading, where as real(Z(1,1)) with open circuit in the second port. I normally Load 2nd port with 50ohm(or what) and find the impedance by ac source at port 1. or there is a built in function in cadence called Zm.

Hope this helps...