Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <7FVr9.170534$S32.firstname.lastname@example.org> <7yWr9.170725$S32.email@example.com>
Subject: Re: Aylward, Engelhardt: Noise in a transient sim?
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Date: Fri, 18 Oct 2002 18:04:21 +0100
NNTP-Posting-Date: Fri, 18 Oct 2002 18:04:22 BST
"Mike" wrote in message
> "Kevin Aylward" wrote in message
> > I think I see what your getting at. You mean effectively having the
> > noise of a device that is usually only given in the frequency
> > given in the time domain. This would seem to be a bit tricky, and
> > probably more involved then what one really requires. i.e. I can't
> > bothered with all that effort:-)
> That is what I mean, and yes, it is what I require...
> > The deal is though, you can get a pretty good estimate by worst
> > and erroring on the high side. I have simulated, e.g. VCO's and
> > put various signals on the power supply to see the effect. Its often
> > case that it is extraneous noise that is the issue with high
> > designs, such that the normal circuit noise is not that relevant. If
> > have digital switching in analogue circuits, it difficult to predict
> > just what noise is going to be anyway.
> I can isolate my circuits from supply and digital noise well enough
> dominant contributors to my VCO jitter are the FET noise sources.
Famous last words:-)
You know, I rarely find this to be the case. You must have a pretty
digital noise free environment. I assume this is jfets, rather then
2.5GHz cmos i.c. vco's. Getting, say 80+dbs supply rejection from DC to
5Ghz is some feat in an ic.
> There are various estimation methods that yield reasonably accurate
> jitter estimates from AC simulations; Hajimiri's Impulse Sensitivity
> Function is the one I see used most often. It works well, but to
> best accuracy, you have to run separate simulations at different bias
> over a cycle to find the noise at each bias point, in addition to all
> transient simulations required to construct the sensitivity function.
> significantly easier, and usually faster, to run a transient noise
> simulation and extract the noise from that.
I do have one or two papers on this sort of stuff... Always thought it
was a bit of an overkill. In addition, none of them seem to agree with
each other. I take a more engineering approach. Estimate the maximum
noise, multiply it by pi, and see if looks ok. In real life, the
performance is always worse then what you think. I have recently spent a
bit of time in getting large rejections in VCOs and their bias circuits.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.