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pancho_hideboo

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Re: ac gm simulation
Reply #15 - Mar 17^th, 2009, 8:54pm

Do you have basic knowledge of RF circuit �theory although "RF" is included in your name "vivarf" ?
Maybe you are "unvivarf".

You should learn small signal analysis with a starter of the following post.
http://www.designers-guide.org/Forum/YaBB.pl?num=1236321645

vivarf wrote on Mar 17^th, 2009, 8:36pm:

The original definition: Y21=Id/Vin

Here Id and Vin are both small signal value in AC Analysis.

vivarf wrote on Mar 17^th, 2009, 8:36pm:

The result you defined is taken out current: Y21=i("/M2/D" ?result "ac") � (single-ended)

This is because I set Vin as 1.0V in AC analysis.

You might think calculation of g2 and g3 is irregular since it uses both DC analysis result and AC analysis result.
But it is matched to an actual measurements using actual instruments.

S-parameters are measured by Vector Network Analyzer(SP-Analysis) with sweeping DC bias points feeded via Bias-Tee.
Y21 is evaluated from S-parameters. g2 and g3 are calculated by derivation of g1=real(Y21) regarding DC bias points difference.

I always say to people in this Forum, "You have to learn measurements using actual instruments". Not "EDA Tool Play".

vivarf wrote on Mar 17^th, 2009, 8:36pm:

From the original definition, I thought Y21 = deriv(i("/M2/D" ?result "ac"))

No. You are still misunderstanding the original definition.
You can't understand gm in DC Analysis and Y21 in AC analysis.
First of all, you can't understand small signal analysis such as AC, XF, SP Analysis at all.
Learn Y,Z,S-Prameters of linear system.

Maybe you are student. So discuss such too basic issues with friends.

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« Last Edit: Mar 18^th, 2009, 6:20am by pancho_hideboo »

Kita━━━━━━(ﾟ∀ﾟ)━━━━━━ !!!!!
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vivarf

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Re: ac gm simulation
Reply #16 - Mar 17^th, 2009, 11:22pm

Quote:

This is because I set Vin as 1.0V in AC analysis.

Now I got it (at least in this situation).

Quote:

No. You are still misunderstanding the original definition.
You can't understand gm in DC Analysis and Y21 in AC analysis. First of all, you can't understand small signal analysis such as AC, XF, SP Analysis at all.
Learn Y,Z,S-Prameters of linear system.

Maybe you are student. So discuss such too basic issues with friends.

You were right, I should have learnt those more carefully.

Thank you pancho.

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Andrew Beckett

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Re: ac gm simulation
Reply #17 - Mar 20^th, 2009, 2:19pm

wave wrote on Mar 17^th, 2009, 7:36pm:

Ken Kundert wrote on Mar 17^th, 2009, 9:36am:

In Spectre there is nothing wrong with using a zero ohm resistor to measure current, just like it is fine to use a zero volt voltage source. Using the iprobe is actually preferred because it is a very simple and light weight component.

-Ken

Ken - When I use tiny R's, I find abstol, reltol type of issues.
Usually atleast 100 mOhms is a good compromise.

Actually Ken talked about a 0 ohm resistor, not a tiny value resistor. Tiny value resistors can indeed cause convergence difficulties, but 0 ohm should be fine.

Hopefully my response is a little more polite than some of the responses in this thread ;)

Regards,

Andrew.

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pancho_hideboo

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Re: ac gm simulation
Reply #18 - Mar 22^nd, 2009, 8:52pm

Andrew Beckett wrote on Mar 20^th, 2009, 2:19pm:

Hopefully my response is a little more polite than some of the responses in this thread �;)

I'm always very honest. Cheesy

Attached figure is comparison using two tones HB-QPSS of Cadence Spectre.
Apparently resistor of 0ohm cosumes much memory and it's speed is slow compared to vsource of vdc=0V.

I always feel that simulation speed is slow and memory consumption is large if I use iterative instances which include resistor of 0ohm instead of vsource of vdc=0V.

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Kita━━━━━━(ﾟ∀ﾟ)━━━━━━ !!!!!
http://www7.plala.or.jp/ungeromeppa/flash/kita.html
http://www.youtube.com/watch?v=mjIxGh55bMM&feature=related

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Andrew Beckett

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Re: ac gm simulation
Reply #19 - Mar 22^nd, 2009, 10:53pm

I didn't say that a zero-ohm resistor was completely free; you'd be better off with an iprobe (even that isn't completely free if you have many of them). If you've only got a single resistor/vsource/iprobe it's not going to make significant difference - my point was rather that using a small value resistor can cause convergence difficulties, whereas 0ohms should be OK.

I wasn't doubting the honesty of your reply (honest!)

Regards,

Andrew.

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Ken Kundert

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Re: ac gm simulation
Reply #20 - Mar 23^rd, 2009, 11:29pm

I have to differ with both of you. First, on the zero ohm resistors, from a simulation perspective, they are structurally identical to 0-volt voltage sources. In fact, they tend to have a smaller memory footprint, and so if anything they should be faster. Unless there is a bug in the simulator, which is hard to imagine because they are so simple and heavily used, it is very unlikely that there is a significant speed difference between the two.

Concerning your results, I would council you not to put too much stock in such small differences. I have looked at a very large number of simulation speed issues through the years and have found that they are often due to happenstance.

Second, on convergence problems with small resistors, this would only be true with small parasitic resistors. This is one reason why they are automatically deleted. However, normal resistors have a more sophisticated formulation that allows them to work with any resistance, no matter how large or how small. The only reason why you would have trouble is if the thresh parameter was set incorrectly, which again occurs only very rarely, and when it does occur it is often the result of overly aggressive tightening of tolerances, particular abstol.

Finally, concerning your honesty Pancho, I cannot know what you really believe. However I have always found your opinions about Spectre and SpectreRF to be excessively negative, and frankly far outside outside the mainstream. You and I both know that you have very personal reasons to dislike Cadence, and I think you are letting your feelings about Cadence affect what you say about Spectre. In addition, I often find your manner on this forum to be overly harsh, and occasionally insulting. However, at the same time you are very helpful to the people on this forum. You are very active, and very knowledgeable, and on the whole I value your contribution. Perhaps you can try to be a little more even handed in your treatment of Spectre and a little more gentle in your treatment of the other members of the forum. After all, we are all friends here just trying to help each other out.

-Ken

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