Analog circuit analysis discussion  – Colpitts oscillator  - Page 1
Forum section: electronics / projects

author: AnneRanch
author_rank: Contributor
timestamp: September 17, 2019, 02:48:45 pm
content: I am looking to discuss circuit details of Colpitts oscillator.I have found several circuit versions and having a difficult time to model them using Ltspice .My objective is to fully understand individual components functions.I am not looking for troubleshooting , design help , tutorials, u-tube or other references etc.I would like to engage in a meaningful discussion with someone who “been there, done that” on component level.For starters“the circuit configuration should have minimal gain , just to self-start the oscillation”If I select 2N2222 as an active device – how to I make sure it has such “minimal gain “ ?I would appreciate forum members to carefully read my post and reply only when they have a meaningful contribution to post.Cheers

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2019, 03:08:33 pm
content: Since you mention a specific active device, it would greatly aid the discussion if you posted the particular Colpitts implementaton you are talking about.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 17, 2019, 03:09:29 pm
content: Quote from: AnneRanch on September 17, 2019, 02:48:45 pmTry setting theminimummaximum time step for the transient analysis in LTSpice to something on par with the period of oscillation if your oscillator isn't oscillating.Quote from: AnneRanch on September 17, 2019, 02:48:45 pmThat's a bit spicy dontchathink?EDIT - oops, mistakenly wrote "minimum time step" instead of maximum.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2019, 03:58:48 pm
content: Quote from: MagicSmoker on September 17, 2019, 03:09:29 pmSkip initial operating point solution or start DC supply voltages at 0V can also help.QuoteThey've probably been trolled in the past. Just ignore posts which don't meaningfully contribute to the discussion or report them to the moderator, if they're that bad.

author: AnneRanch
author_rank: Contributor
timestamp: September 17, 2019, 04:13:21 pm
content: WhopsI did reply then it asked me to log in AFTERWARS.Not sure why.So hopefully here is the current LTspice file.The model is NOT tuned properly.

author: AnneRanch
author_rank: Contributor
timestamp: September 17, 2019, 04:14:39 pm
content: Sorry, posted WRONG file.

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2019, 04:35:31 pm
content: Quote from: Zero999 on September 17, 2019, 03:58:48 pmYes, introducing a small perturbation somewhere is usually enough to get things going. For instance, adding the initial condition:Code:.ic I(L1) = 1m

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 17, 2019, 05:04:28 pm
content: Circuit oscillates at about 14.7MHz for me just by setting the maximum timestep to 10ns. /thread

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2019, 05:30:37 pm
content: Quote from: MagicSmoker on September 17, 2019, 05:04:28 pmIndeed, the circuit oscillates for me with just the original.trans .5mdirective. It does take about 300ns to get going, though, which suggests that there is just enough perturbation caused by round-off error to start it up.With an initial condition like.ic I(L1) = 1moscillation starts immediately.FWIW, I'm using LTSpice XVII(x64), the September 11 2019 14:02:52 build.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 17, 2019, 05:40:49 pm
content: Quote from: ledtester on September 17, 2019, 05:30:37 pmI didn't even try it as-is; I just took the OP's claim it wouldn't simulate at face value. The .ic directive is a good one, too.

author: AnneRanch
author_rank: Contributor
timestamp: September 17, 2019, 05:54:03 pm
content: OK, gentlemenapparently my "spicy" initial comment did not take.Please read my posts.I have no issues with the circuit oscillating.However I cannot keep up with "adjust this and that" posts here, do my stuff and "honey-dooes".I have modified , annotated the TI circuit to match the book and changed some values.Attached.Perhaps I need to post SPECIFIC questions to get the discussion going in direction I was initially after.From the TI book -they determine the ratio of C2 / C3 experimentally as .47from that I get the real combined value of C2/C3 - standard Colpitts calculation for oscillator frequencyThen there is a lengthy calculation involving C1They use the initial C2C3 /C2+C3 AKA standard frequency value....My question is - where did the C1 calculation come from and WHAT is the function of C1?Cheers

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2019, 10:08:44 pm
content: Quote from: AnneRanch on September 17, 2019, 05:54:03 pmWe are reading your posts.QuoteThat's always a good thing to do on an internet forum.QuoteWhat TI book???QuoteI don't see a C1 designation on anything you've posted in this thread.

author: AnneRanch
author_rank: Contributor
timestamp: September 18, 2019, 12:32:31 am
content: My question is - where did the C1 calculation come from and WHAT is the function of C1?Per TIC1 is AC bypass of R2 C1 = 300 pf - value chosen to avoid self resonant frequency near 90 MHz

author: duak
author_rank: Super Contributor
timestamp: September 18, 2019, 03:53:03 am
content: Curiouser & curiouser... I'm also having trouble with the reference designators, the Spice model & your questions. Is the schematic posted mayhaps not what is desired?Also, the schematic looks oddly like a regenerative receiver. ie., antenna on the left, headphone on the right and a control that will vary emitter current.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 18, 2019, 11:23:44 am
content: The OP doesn't deserve help, I've concluded. Not only because the questions asked are nonsense (re: the non-existent C1), but mainly because of this gem of a thread:https://www.eevblog.com/forum/repair/464-delayed-sweep-switch-worn-out/Keep demanding we help you for free and see where that gets you.