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Colpitts kick-start in LT Spice

Started by Unknown November 11, 2019
On 13/11/19 10:50 am, John Larkin wrote:
> On Wed, 13 Nov 2019 08:23:41 +1100, Clifford Heath > <no.spam@please.net> wrote: > >> On 12/11/19 10:51 pm, Jeroen Belleman wrote: >>> A real oscillator will start up just fine, but a simulation may or >>> may not. Either way, it would be wrong to take this as a model of >>> reality. You *must* kick-start an oscillator simulation. >> >> I have both simulated and built quite a few oscillators using LTSpice, >> allowing them to start by amplifying the noise in the models. Anything >> reasonable seems to start in LTSpice and in real life, once you get the >> gain and phase right. I've never used a kick-start in any case. > > High-Q oscillators, especially crystal oscillators, can take thousands > or millions of cycles to settle. And need a small time step to model > accurately. That can make a run take hours.
True, but the way they start and settle tells you quite a lot about the loop conditions, especially amplitude stability.
> I like to force initial conditions close to what the steady-state > values will be. It starts instantly. Then I can zoom up on what it's > doing and see if the amplitude is increasing or decreasing. >
On 11/12/2019 2:58 PM, John Larkin wrote:
> On Tue, 12 Nov 2019 14:04:56 -0600, John S <Sophi.2@invalid.org> > wrote: > >> On 11/12/2019 10:48 AM, jlarkin@highlandsniptechnology.com wrote: >> >>> You removed my time step. Set it to 1 ms and wait, very patiently, to >>> see a beautiful bizarre startup that doesn't oscillate. >> >> Why? >> >>> With an unspecified time step, LT Spice LC tank frequencies are not >>> very accurate. >> >> What kind of not accurate? Frequency, voltage, etc? Please specify. > > Frequency. > >> And how do you know about very accurate? Have you built them to verify? > > Built? I compare the LT spice frequency to the calculated value. Even > bare LCs are a bit wrong at the default settings. > >> >> I showed you that your own model self-starts in LTSpice without a >> kickstart. You said it never would, but it does. You never mentioned >> accuracy. Build one and bench test it. Report back here with your >> results. Be honest and don't change the stream in the middle of a horse. > > The issue isn't bench testing, it is the hazards of LT Spice, and the > general weirdness of numerical simulation of physical systems. > > It used to be that LT Spice wouldn't allow you to run if one end of a > capacitor was open, or if you have two caps in series. Now it does. >
If you are unhappy with LTSpice, don't use it or make your own simulator. LTSpice is free. What's wrong with you?
On Thu, 14 Nov 2019 15:46:32 -0600, John S <Sophi.2@invalid.org>
wrote:

>On 11/12/2019 2:58 PM, John Larkin wrote: >> On Tue, 12 Nov 2019 14:04:56 -0600, John S <Sophi.2@invalid.org> >> wrote: >> >>> On 11/12/2019 10:48 AM, jlarkin@highlandsniptechnology.com wrote: >>> >>>> You removed my time step. Set it to 1 ms and wait, very patiently, to >>>> see a beautiful bizarre startup that doesn't oscillate. >>> >>> Why? >>> >>>> With an unspecified time step, LT Spice LC tank frequencies are not >>>> very accurate. >>> >>> What kind of not accurate? Frequency, voltage, etc? Please specify. >> >> Frequency. >> >>> And how do you know about very accurate? Have you built them to verify? >> >> Built? I compare the LT spice frequency to the calculated value. Even >> bare LCs are a bit wrong at the default settings. >> >>> >>> I showed you that your own model self-starts in LTSpice without a >>> kickstart. You said it never would, but it does. You never mentioned >>> accuracy. Build one and bench test it. Report back here with your >>> results. Be honest and don't change the stream in the middle of a horse. >> >> The issue isn't bench testing, it is the hazards of LT Spice, and the >> general weirdness of numerical simulation of physical systems. >> >> It used to be that LT Spice wouldn't allow you to run if one end of a >> capacitor was open, or if you have two caps in series. Now it does. >> > >If you are unhappy with LTSpice, don't use it or make your own >simulator. LTSpice is free. What's wrong with you?
I'm not unhapy with LT. I love it. But any numerical simulation of a complex physical system will have weirdnesses and numerical instabilities and has to be driven carefully. I have written my own circuit and control system simulators. But LT Spice is better. Well, one PLL could probably only be simulated with custom code. You are more interested in being obnoxious than in designing electronics. What's wrong with you? -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Nov 13, 2019, Steve Wilson wrote
(in article<XnsAB06794AFB969idtokenpost@144.76.35.198>):

> klaus.kragelund@gmail.com wrote: > > > On Wednesday, 13 November 2019 13:52:07 UTC+1, Steve Wilson wrote: > > > Steve Wilson <no@spam.com> wrote: > > > > > > > John Larkin<jlarkin@highland_atwork_technology.com> wrote: > > [...] > > > > > > > You start the oscillator and let it run for an appropriate length of > > > > > > time, > > > > > > > > That's the problem. The appropriate screen time may be hours. Maybe > > > > > days with a good time step. > > > > > > > I have never seen more than a few minutes. Your time step is way too > > > > small. > > > > > > Vig, April 2012, Page 81, shows the maximum Q of a crystal: > > > > > > The maximum Q of a resonator can be expressed as: > > > > > > Qmax = 1/(2*pi*F*t) > > > > > > For 10 MHz, > > > Qmax = 1/(2*pi*1e7*1e-14) = 1,591,549 > > > > > > where F is the frequency in Hz, and t is an empirically determined > > > "motional time constant" in seconds, which varies with the angles of > > > cut and the mode of vibration. For example, t = 1 x 10 -14 s for the > > > AT-cut's c-mode (Qmax= 3.2 million at 5 MHz), t = 9.9 x 10 -15 s for > > > the SC-cut's c-mode, and t = 4.9 x 10 -15 s for the BT-cut's b-mode. > > > > > > https://www.vcamerica.com/pub/media/documents/vig3.pdf
It can be difficult to decode Vig&#2013266100;s slides without his talk. Look around and find Vig&#2013266100;s full slide deck in MS PowerPoint. On the Notes pages (not available in the pdf version it appears), you will find background information and literature references. This usually solves the Hih? problem. Joe Gwinn
On 11/14/2019 5:28 PM, John Larkin wrote:
> On Thu, 14 Nov 2019 15:46:32 -0600, John S <Sophi.2@invalid.org> > wrote: > >> On 11/12/2019 2:58 PM, John Larkin wrote: >>> On Tue, 12 Nov 2019 14:04:56 -0600, John S <Sophi.2@invalid.org> >>> wrote: >>> >>>> On 11/12/2019 10:48 AM, jlarkin@highlandsniptechnology.com wrote: >>>> >>>>> You removed my time step. Set it to 1 ms and wait, very patiently, to >>>>> see a beautiful bizarre startup that doesn't oscillate. >>>> >>>> Why? >>>> >>>>> With an unspecified time step, LT Spice LC tank frequencies are not >>>>> very accurate. >>>> >>>> What kind of not accurate? Frequency, voltage, etc? Please specify. >>> >>> Frequency. >>> >>>> And how do you know about very accurate? Have you built them to verify? >>> >>> Built? I compare the LT spice frequency to the calculated value. Even >>> bare LCs are a bit wrong at the default settings. >>> >>>> >>>> I showed you that your own model self-starts in LTSpice without a >>>> kickstart. You said it never would, but it does. You never mentioned >>>> accuracy. Build one and bench test it. Report back here with your >>>> results. Be honest and don't change the stream in the middle of a horse. >>> >>> The issue isn't bench testing, it is the hazards of LT Spice, and the >>> general weirdness of numerical simulation of physical systems. >>> >>> It used to be that LT Spice wouldn't allow you to run if one end of a >>> capacitor was open, or if you have two caps in series. Now it does. >>> >> >> If you are unhappy with LTSpice, don't use it or make your own >> simulator. LTSpice is free. What's wrong with you? > > I'm not unhapy with LT. I love it. But any numerical simulation of a > complex physical system will have weirdnesses and numerical > instabilities and has to be driven carefully. > > I have written my own circuit and control system simulators. But LT > Spice is better. Well, one PLL could probably only be simulated with > custom code. > > You are more interested in being obnoxious than in designing > electronics. What's wrong with you? >
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