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Simulators >> Circuit Simulators >> Clarification needed using "port" source in Virtuoso
https://designers-guide.org/forum/YaBB.pl?num=1357004205

Message started by Sharath Raju on Dec 31^st, 2012, 5:36pm

Title: Clarification needed using "port" source in Virtuoso
Post by Sharath Raju on Dec 31^st, 2012, 5:36pm

Hi,

I understand that the internal resistance of the "port" source is 50 ohm by default. If the peak voltage amplitude is set to 1 V and the termination is set to 50 ohm, shouldn't the peak voltage across the 50 ohm load be 0.5 V due to a voltage division between the 50 ohm impedance of the source and the load ?

To do a sanity check, I replaced the port source with a "vsin" source in series with an external 50 ohm resistance. The source voltage divides in half as expected.

Attached is the schematic and simulation result. Note that /L and /Lprime are the load voltage waveforms with the port and vsin sources respectively.

Thanks,
Sharath

Title: Re: Clarification needed using "port" source in Virtuoso
Post by Ken Kundert on Dec 31^st, 2012, 6:29pm

The Spectre port source is modeled after a signal generator. In a signal generator, you specify the level you want with the implied assumption that the generator is properly terminated. So if you specify 1V, you get 1V but only if the source is terminated in its reference impedance. If you do not terminate it (you leave it open), you get 2V.

-Ken

Title: Re: Clarification needed using "port" source in Virtuoso
Post by Sharath Raju on Dec 31^st, 2012, 6:37pm

Thanks Ken..changed the load to 100 Mega ohm and quickly saw that the voltage jumped to 2V amplitude.