Going Equation Postal - Page 1
Forum section: electronics / general

author: PerPC
author_rank: Contributor
timestamp: April 28, 2012, 07:38:21 pm
content: GRRRRRRRRRRRRRRRRRRRRRRRRRR......i absoutely HATE being impoverished. HOW do I LIVE in the good ole U.S of A and I cant get a descent education!!!!?SMH >.< . Well, Well -- i suppose the grass is always greener on the other side of the fence. Over here trying to cram some tech stuff in my head absolutely driving myself mad. And these equations...smh. Yet, once again, it could be worse; I mean i could be living in an age without the works of the internet. Like that one episode when dave was talking about how he had to read index after index...lmao!!! i would probably be able to afford one index and that would be it. mini-smurf tech Mike

author: IanB
author_rank: Super Contributor
timestamp: April 28, 2012, 07:51:31 pm
content: I don't quite get what you are ranting about?Are you referring to the cost of being a student and the fees universities charge? If so, then sure, you have a definite cause to rant.However, there are many free or low cost educational resources these days. The major limitation is the lack of a formal certificate to show.

author: PerPC
author_rank: Contributor
timestamp: April 28, 2012, 08:08:14 pm
content: Well telling me there are free and affordable resources isnt helping much Ian. How about solving the problem and providing some resources. Oh im so gonna love it here. I said it in my lil catch phrase "aspiring" tech and from that response i see im where i need to be.

author: IanB
author_rank: Super Contributor
timestamp: April 28, 2012, 08:15:24 pm
content: We can't give you resources unless you give some indication of what subject matter you are looking for and at what level.Are you simply letting off steam or are you asking for help?I've got to say your opening post read like you were letting off steam. When people are letting off steam the appropriate reaction is to say "Yeah, too right, roger that, you go boy!"If you want some help, you might like to try a different approach, like actually asking a question.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 28, 2012, 08:39:17 pm
content: Quote from: PerPC on April 28, 2012, 08:08:14 pmTry asking a question. Also, while every site on the internet (including this one) gets its share of abrasive personalities, we are happy with the ones we have and don't really need people who describe themselves as an "aspiring douche." It is also easier to tolerate an acerbic expert than a beginner. Being right and useful covers a multitude of faults.Anyway, if you want traditional college course lectures for free, check outhttp://academicearth.org/. They have a huge (but not comprehensive) range of topics. For a more thorough covering of engineering MIT open courseware is another great set of resources. Wikipedia and mathworld both have nearly complete educations in mathematics from basic algebra to advanced graduate level topics. There are lots of domain specific tutorial sites. You will find them the natural way. If you google about a given topic often enough you will notice the same sites popping up. You learn to trust them.A final word of warning. Don't mistake academic earth or mathworld as an equivalent to a more traditional education. The information is all there, but it takes a lot more discipline and motivation to learn by yourself with nobody to correct you if you veer off course. It is the easiest thing in the world to read something, think you understand it, and keep going. Then some point in the future you hit a brick wall because you think you understand something but you don't really have it down, or you have a misunderstanding. One important job of an instructor is to try to force you to prove you learned it right the first time. Failing that, a good instructor will recognize that you have a brick wall and help you go back to find the hole that needs patching.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 28, 2012, 09:06:52 pm
content: I see a lot of disdain for equations and theory. I think what a lot of folks don't realize is that the mathematics is absolutely essential for understanding what is happening in circuits, electromagnetics, etc. The math won't give you exact model of a circuit, but it will tell you exactly what needs to be tweaked and by how much to get the circuit to behave correctly.Here is an practical example. I designed and built a simple common base oscillator using a 2n2222 and some lumped components. My circuit resonated about 5 MHz lower than intended. Instead of dismissing the theory as nonsense, I calculated the parisitic capacitance of the transistor by substituting my component values and the measured resonant frequency into f = 1 / 2 pi sqrt {LC}, from which I computed the exact value of inductance needed to compensate for the capacitance. I replaced my capacitor with the calculated value, and voila, my oscillator was dead on within 500 kHz. Fixed within a matter of minutes.A buddy of mine, less mathematically inclined, was building a similar circuit. It took him several hours to get his circuit resonating at the intended frequency because he refused to calculate the parisitics and the component values needed to compensate for them. The lesson is, the model is not wrong, but rather it was applied incorrectly.Another good example is antenna theory. See my other post to get a feel for what kind of misconceptions abound because folks don't understand the mathematical model behind radiation.

author: free_electron
author_rank: Super Contributor
timestamp: April 28, 2012, 10:06:53 pm
content: Quote from: vxp036000 on April 28, 2012, 09:06:52 pmtakes out screwdriver, tweaks inductor core. done. no math needed. next !Quotewow , you must be lucky having all these values like 123.7pf and 36.7pf and other non-e12 series caps laying around.. and all in 0% tolerance none the less... perfectly spot on values with all the digits your calculator just told you you needed. i never get that ...I'm not downplaying mathematics. But you can get alone fine without too much of it. There is such a thing as instinctively grasping how a circuit works. if this goes up , that goes down , if i pull here: that will come up. right , where's that trimpot ? ...Other problem is that mathematics is only as good as the completeness of the equations ... and when you really need the mathematics ( as i said , they are important in SOME cases ) they tend to get so complex so fast that you need a piece of software to do the maths....

author: IanB
author_rank: Super Contributor
timestamp: April 28, 2012, 10:09:01 pm
content: Quote from: free_electron on April 28, 2012, 10:06:53 pmI think he replaced the capacitor value in the LC formula, not in real life...

author: vxp036000
author_rank: Regular Contributor
timestamp: April 28, 2012, 10:13:01 pm
content: Yes, I had reasonable tolerance components. At risk of stating the obvious, I chose a combination of practical capacitor values that gave the equivalent capacitance to what I calculated. No, it's not a fluke. I tweak amplifiers, oscillators, antennas, etc. by adjusting the mathematical model slightly to accomidate imperfections. It's way faster than blindly turning the knobs, so to speak.Quote from: free_electron on April 28, 2012, 10:06:53 pm

author: free_electron
author_rank: Super Contributor
timestamp: April 29, 2012, 12:25:27 am
content: Quote from: vxp036000 on April 28, 2012, 10:13:01 pmDid you compensate for lead inductance , stray capacitance, skin effect , dielectric absorpion , charge dispersion ,How did you decide between ceramic material or one of the film materials like mylar, mica , pen , pet , pes , or others? hat did you select and why? Q and D of the capacitors chosen. ESL ?What about dc bias / temperature / frequency derating ? Any pulse curents flowing in the system ? electrode current handling capability ?Did you run a Bode analysis and look at the resonance points of your stacked capacitors ?Are you going to mount them on a pcb. What substrate material , layer stack and plane structure do you use ? Did you look at the weave and prepreg thickness ? Any guard rings in use ? via plugging ? shielded cover or open air ? ( so you can calculate the stray of the board ... )Is your cap compatible with reflow ? lead free ( higher temperature ). Is it going to be top or bottom mounted ( bottom may need to go through the wave ? did you think about the orientation on the board ( no shadowing in the wave )what about cost of the capacitor , preferred vendor , stock , long term availability , continuity of supply ? since you need more than one in parallel. Boadr real estate cost of mounting, time on the pick and place. Did you analyse trade-off versus switching to an adjustable cap or tweaking the coil instead. What about swithing from a metal can 2n2222 to an sot-23 2n2222. the parasitics are lower. maybe it could have been solved that way ? And most of all : do the bean counters in procuring approve of your part selection?As a good engineer selecting a component you should have looked at ALL of the above things...Or was it for a one-off design on a bread board in dead-bug construction with re-used crap you had laying around? in that case : chuck the equations in the bin... slap on a trimcap and off you go. No need to 'over-engineer something, or run the mathematics for the sake of mathematics.Here's a little anecdote that happened yesterday. I got a board and 15 page long report that claimed there was something wrong with a design. Full of simulations with lots of theories what was wrong. The first slide had a scope capture made by the engineers 'towel' ( the lab-guy) and told me what was wrong ... the chip was not responding to the commands... i went to the lab , powered up the board , checked the address selection pins high and low state with a voltmeter ( the device is controlled over i2c ) and found a floater pin .. the little metal piece had fallen out of a jumper: it was just a bit of plastic across two pins... Swap jumper. System works fine.The engineer spent 7 hours preparing these sims and presentation and theorizing, board shipping cost and project delay.The same guy is notoroious for not wanting to go in a lab and hold a soldering iron. He has a 'towel boy' to do that. It is 'below him'. Running spice is all he does all day long. That, and making 'slideware'.Luckily he doesn't work for me. I would have had to buy new boots...

author: vxp036000
author_rank: Regular Contributor
timestamp: April 29, 2012, 12:34:50 am
content: I think you misunderstand my point. For the boards I work on, there is no reason to model any of the half a dozen things you mentioned that could lead to parasitics, because, quite frankly, your simulation won't be accurate enough. What I'm saying is that I can either start randomly selecting trim caps to fix my problem, or I grab a paper / pencil and run a five second calculation that tells me what the value of the trim cap needs to be. And yes, estimating the correct value component needed to tweak a design, is generally trivial (X = 1 / jwC or X = jwL, how hard is that?).

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 29, 2012, 12:54:32 am
content: Good discussion guys.I enjoyed and agreed with what you both have said.Good to hear you both got to the nub of the problem so quickly certainly quicker than I would have.Anyone who writes a report like that without actually finding a damaged jumper, should go and work for the government.

author: free_electron
author_rank: Super Contributor
timestamp: April 29, 2012, 02:05:34 am
content: Quote from: vxp036000 on April 29, 2012, 12:34:50 amNo i don't misunderstand your point. I'm just pushing back at the uselessness of it all.And i quote :Quote'Exact , dead on' give or take 500Khz. 90% fixed , now for the remaining 10%....After some maths with complex numbers , having to 'build' the calcuated value because it is non standard . and you're still off !I would have been dead on. Tune the trimcap and done. Spot-on, first time right, no calculations.Yes, maths is important. But it is even more important to know and understand when to use them. In this case they are useless since the modelling process is not practically feasible. Take out your 2n2222 and solder a different 2n2222 in and it all goes to snot. carefully build a parallel circuit of selected caps and you are STILL off !QuoteExactly. you proved that. you were still 10% off ! Your case is a typical example where the maths don't apply. Just slap a trimcap in there, tweak it, and go. there is no mathematical compensation for such things.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 29, 2012, 02:11:53 am
content: You're trim pot isn't going to do much good unless you're using a VCO :-) And what happens when you're board gets banged around? That's why I refuse to use trim pots. Most of my work is with fixed frequency oscillators. And, if you think the math is so useless, how did you know whether to pick a larger or smaller value component? And, how much larger or smaller? Yep, that's what I thought. The math.Now we can talk about how you designed your board to begin with. Where did the component values in your oscillator design come from? Thin air? What about the bias voltages? Again, it's math.

author: PerPC
author_rank: Contributor
timestamp: April 29, 2012, 03:11:35 am
content: 0_0 ooooooook. Well thanks for the resource and ill check it out. Maybe i can talk like u guys oneday. even better maybe oneday i can be like dave. i guess when it comes down to it its not what u know but your drive. Now can you please drive me some more resources that can help me in this quest? ty

author: vxp036000
author_rank: Regular Contributor
timestamp: April 29, 2012, 03:14:20 am
content: To be honest, wikipedia is one of the better resources for electronics theory. But the best resources I have, by far, and still reference today, are all my textbooks and notes from school.

author: free_electron
author_rank: Super Contributor
timestamp: April 29, 2012, 04:35:42 am
content: the problem is that textbooks are static... ten years from now they will be obsolete. the world will have movedon , new things will have been invented, new techniques developed. the old textbooks will only be useful to see how things used to be done.Electronic engineering is a life-long learning experience. books do need to be replaced every decade ...that is the strength of the internet. new ideas and technology spread like wildfire where it took years to ripple through to textbooks before.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 29, 2012, 09:04:49 am
content: Quote from: vxp036000 on April 29, 2012, 02:11:53 amWhy are you messing about with self excited oscillators?Use a Crystal,& you'll be a hell of a lot closer to frequency than 500kHz,first try!I think free_electron was using "trimpot" in a general sense,not directly related to this circuit.If you remember, he referred to adjusting the core of the inductor in his original comment.The only other trimmer that would be used would be a trimmer capacitor.Of course,everybody uses maths as a basic guide,& anybody hoping to develop an understanding of Electronics atanylevel must attain some proficiency in their use,but the real world has a way of hitting you with imponderables,which is why we have test instruments.Have you ever had the delightful job of pushing chip capacitors around paralleled UHF power amplifiers,trying to get them within 1dB,because the "Engineer" thought that every amplifier would have exactly the same characteristics?I have! and it is no fun!He also thought that 3 to 4 dB difference was close enough,anyhow,even though his underrated unbalance load resistors cooked at that level difference.I wish he had done his maths,combined with a bit of testing.

author: free_electron
author_rank: Super Contributor
timestamp: April 29, 2012, 04:01:31 pm
content: Quote from: vxp036000 on April 29, 2012, 02:11:53 amIf its that critical then you do not design oscillators like that. You make a control loop and use a physical constant as reference ( crystal or other elemental unit. Could even be a rubidium oscillator )As for picking a larger or smaller value. If you have a trimmer ( whether r l or c ) you just spin it and look at the meter. You cannot convince me you are going to to determine if you need to go up or down , look at the mechanical construction and figure out which way to turn to get your result. Anyone in his right mind pokes the screwdriver in and looks at the meter. If it goes the wrong way : change direction.And yes, some maths is needed. Calculate the bias voltage and the base l and c for your oscillator. Then build it and adjust it. No need to try to model the parasitics and go looking for complex things when there exists an easy solution.Math is not the end all of every problem. In some circumstances you just need to wing it and see what happens.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 12:04:26 am
content: AND,if you design an LC oscillator in the manner described,you have madethatoscillator work correctly.With variations in internal capacitance of individual devices,the next one may need more,or less correction.If you are making thousands of them,you maybe should look into a design where the capacitance part of "LC" is somewhat larger,so that it effectively "swamps"the variations in "parasitic capacitance".You could use a more appropriate active device,use a crystal,or as free_electron says "tweak the inductor core".The essence of good Engineering,which a surprising number of Engineers don't get,is to design circuits which will work with standard components.This enables practical manufacture.OK,there is a bit of a "fiddle" these days,in that you can work around things in software,but,if a repair is done,the repairer then needs access to the same software.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 12:12:07 am
content: If you can't tell whether you need more or less capacitance to tweak an oscillator to bring it to the right frequency, you already don't understand how oscillators work. As an employer, I will never hire a designer that cannot tell me what value component is needed where to compensate for parasitics. Someone who thinks this is not possible has not designed and built signal sources. I'm only speaking from experience.Quote from: free_electron on April 29, 2012, 04:01:31 pm

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 12:33:41 am
content: Let me know when you find a microwave transistor whose parasitic capacitance is swamped by the capacitance of your tank circuit. Having designed and built a few microwave signal sources, I will tell you that it is absolutely essential to compensate for parasitics in the active device. It is simply not feasible to choose values of capacitance significantly larger than the parasitic capacitance in the transistor.Quote from: vk6zgo on April 30, 2012, 12:04:26 am

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 01:19:01 am
content: Quote from: vxp036000 on April 30, 2012, 12:33:41 amSorry,but you have "moved the goalposts".Since when was a 2N2222 a microwave transistor?In normal LC oscillator design,"swamping" of the "parasitic" capacitance of the active device is a long established principle.Why would you build a self-excited microwave signal source, in any case?OK,in a microwaveamplifierstage,you cannot "swamp" the device capacitance,but that doesn't mean the method doesn't work at lower frequencies.Appropriate devices for microwave use are more closely specified than a 2N2222,so can be more accurately corrected by the mathematical method.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 01:22:52 am
content: Obviously my other example was not a microwave design, but rather an HF oscillator. I was giving an example of a common scenario in which modeling parasitics is required. And yes, it is common to use self-excited signal sources in the 1 to 50 GHz range.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 01:28:18 am
content: Your designer has no business designing a parallel amplifier if he thinks 3 dB difference between the two amplifiers is in anyway acceptable. At my company, he would have been sacked on the spot. And, any designer worth his salt, would be telling you what chip capacitor is needed where to correct for the imbalance. It takes a few minutes of math to figure out where the chip cap is needed, vs. wasted hour or so pushing a cap around the board. Also unacceptable in my book.And you know why the resistors cooked? The designer didn't do his math to figure out the power dissipation under an imbalanced condition. More math. All of a few minutes worth. Compare that to the cost of re-spinning a board.Quote from: vk6zgo on April 29, 2012, 09:04:49 am

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 02:14:42 am
content: Quote from: vxp036000 on April 30, 2012, 01:28:18 am

author: free_electron
author_rank: Super Contributor
timestamp: April 30, 2012, 03:41:33 am
content: QuoteBingo. seems to be common....Now, if the parasitics of an 2n2222 casue it to be off 5 Mhz you havbe other problems to deal with ... besides i can't believe the parasitics would cause it to be 5 MHz off... A 2N2222 has an Ft of what ? 200 MHz ?For the parasitics of a transsitor to kick in you have to be going harder than that. I suspect you are driving it close or over its Ft ... so you are designing it wrong to begin with. Should have taken something like a BF96x series transistor.And after lots of calculation you are still off. That is the point. Yes you can do the maths. big whoop. in the end your circuit still does not work right and cannot be released to production , cannot be replicated ( or are you going to recalculate the adjustment cap for each and every unit built ? That'll be fun. You'll have to have the people on the production floor with a whole array of caps and tweak each and every one of the boards). So all the work you did was useless.Should have designed in a trimcap or an adjustable coil to begin with.I did do a bit of work on satellite tuners ( 950MHz to 1450Mhz band ) . The LO inductor was two turns of silver coated wire. The Lo capacitor was two L-shaped pieces of metal soldered down. You would push them closer or further apart. ( i used BFG-something transistors. i think bfg17 or bfg65.. can't remember.. its 20 years ago ... ) Tweak inductor and cap with a ceramic blade , apply wax. the oscillator was controlled with two varicaps driven by a TSA5510 pll. Calibration procedure was to 'hop' the varactors continuously between min and max drive voltage tune the cap and coil. Apply hot wax , put on shield cover close pll tuning loop. These tuners were used in satellite receivers for the european astra system.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 30, 2012, 05:29:09 am
content: QuoteQuoteQuoteQuoteseems to be hypocrisy ...Why don't you ask vxp036000 what his goal posts were before you say he changed them.From what I thought he said, I feel confident that whatever exactly they were, that the goal was kicked and in 5 minutes.I think it is a mistake to jump in and criticise people without knowing considerably more about what they are trying to do.

author: free_electron
author_rank: Super Contributor
timestamp: April 30, 2012, 07:32:13 am
content: Quote from: HackedFridgeMagnet on April 30, 2012, 05:29:09 amgoal posts refers to something that happened in the beginning of this thread. oscillator with 2n2222 that was 5Mhz off... later on we switched very high frequency where it is important.Anyway. it's all a matter of perspective.vxp036000 like maths and modelling and his persepctive is 'techs can never understand something as good as an egineer since they don't have the 8 year maths laden education. we need to calculate everything.'I apply maths when appropriate. in essence: as little as possible. not because i hate maths but simply because most of the time it is useless: too many variables that you can't model in a practical way. That oscillator is a perfect examply. you can calculate and model it until you have a very long beard , you still won't have a reproducable circuit.One of the key things is to understand when maths are important and when to chuck them in the bin (not throw out the theory but the execution) . You can do a first approximation to set the operating point and after that : wing it. Maths makes sense when it saves you time. I'm faster tuning the inductor than trying to calculate a non existing capacitor value and twiddling around with that. Let alone trying to model all parasitics. For half of the stuff they are not even known. So you can have all the formulas you want. it doesn't get you any closer to a working circuit since your input is invalid.Trying to fix an oscillator frequency by calculating parts to 9 trailing digits and soldering stacks of them on top of each other them makes no sense. you will never get there. put in a trimmable part and move on. it is senseless and a waste of time even trying to model it.There is a difference between theoretical engineering and practical engineering. I subscribe to the latter and leave the former to people that never leave their desk.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 10:05:43 am
content: Quote from: vxp036000 on April 30, 2012, 01:22:52 amIn what application?--This is not a sneer,I'm honestly interested in what field these are being used.All the microwave stuff I've had anything to do with (only up to 14 GHz),used crystal locked RF sources exclusively.These were quite large units,(microwave links & similar),but I can see that the complexity needed to lock an microwave source to a lower frequency crystal may be hard to fit into very compact devices.Some of the "Hams" played around with Gunn diodes on 10GHz.They were also used in proximity alarms & stuff.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 10:29:18 am
content: Quote from: HackedFridgeMagnet on April 30, 2012, 05:29:09 amvxp036000's original example was an HF oscillator.I made the comment that it was normal practice to try to "swamp" the parasitic capacitance of the active device by making "C" in LC larger.In reply,he pointed out,quite rightly,that in a microwave circuit,that would be nearly impossible to do.In my reply,I made the point that his original example.using a 2N2222 was not a microwave circuit,& that in quoting the situation at microwave frequencies,he was"moving the goalposts".The phrase has nothing to do with how long he took to fix the original problem,it is simply a slang term describing the situation when someone shifts the basic premise of their original argument.It is quite common,we've all done it from time to time."I think it is a mistake to jump in and criticise people without knowing considerably more about what they are trying to do."I think you are pretty much "hoist with your own petard" on this own,as you lumped both free_electron's comments & mine together, & apparently didn't read back to see how we got to this point.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 30, 2012, 12:15:58 pm
content: @vk6zgo and free_electronThanks guys for going easy, I thought I was going to get badly flamed. Apologies to anyone I have offended.I have been interested in your discussion because, I must admit, my experience in oscillator design is lacking, almost non existent. I guess it's because I only ever need crystal oscillators, for real circuits and occasionally have used 555 based oscillators in breadboards for prototyping.As far as mathematics versus try it and see electronics I seem to find myself in the mathematical camp, or actually a foot in both camps which I really think any good technologist should.Just trying to think of cases where you can't really work by feel and you need to calculate.What about tuned circuit design using track impedances, I know that's moving the goalposts from oscillators but surely everybody does a calculation first, before routing the track.@vxp036000 One thing that I would be interested to know is what your 2N2222 oscillator was for and why it was valid to design it with discrete passives and associated parasitic impedances. Was it a one off?@free_electron, really appreciate your posts in this forum, both in the amount of detail and in subject matter, you seem to come from a different part of the electronic world from me, one with large RnD teams and Agilent calibrating trucks turning up at the office/factory. So I find it interesting to hear what goes on in that part of the world, so keep it up.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 09:51:24 pm
content: I suggest you re-visit my original example. The guy who was scared of the math wasted a huge amount of time testing different value components in all different places trying to get the circuit working. Since I actually understood how the circuit worked, I ran a simple calculation to find the cap value needed to correct the behavior. And it worked, a couple minute fix. For a one off board, which this was, that is perfectly acceptable.And since when were HF frequencies anywhere near the transition frequency of a 2n2222? Yes, the parisitics actually made a 10 % difference or so in frequency. I won't bore you with the details of why using a cap value much larger than the parisitic capacitance was impossible in this design.I've seen countless circuits with some sort of inexplicable behavior that "couldn't" be modeled. In every instance, a slight tweak to the model allowed for much easier troubleshooting of the circuit. The times I've seen people blindly troubleshoot without even attempting to model the circuit (and I'm talking about back of the hand calculations, not even a circuit simulator), countless hours were wasted. The folks that simply tweaked their circuit model to accomidate some non-ideality always ended up with a working circuit without spending anywhere near as much time troubleshooting.I don't doubt that randomly tweaking various components works, sometimes, but it is nowhere near as effective as modeling the circuit correctly and tweaking your design accordingly.Quote from: free_electron on April 30, 2012, 03:41:33 am

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 30, 2012, 11:07:03 pm
content: Hi vxp036000QuoteI would be interested to know details of this case, especially why the parasitics have such an effect, in particular the purpose of the circuit, the frequency, the number being built, the construction, and any interesting or relevant design decisions made. Only a few sentances would be helpful.As far as the math vs tweaking question goes, I really think it is a non argument, everybody to certain extent must do both and I guess the emphasis they put on each is just really down to personal tastes and strengths.Here's hoping I haven't killed this thread stone cold, I seem to be a bit of a thread killer.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 11:27:50 pm
content: Here is a short-rundown on why the parasitic capacitance was so significant. In my particular design, I found that the parisitic capacitance was on the order of 2.5 pF. For the doubters, I know this was close to the value of stray capacitance because the value of capacitance chosen to compensate for this amount of parasitic capacitance brought my circuit very close to the intended operating frequency. The capacitor values in the tank circuit were on the order of 5 pF. The stray capacitance of the transistor is clearly a substantial fraction of the capacitance in the tank circuit. So I added a cap in series with one of the existing ones to compensate. After the slight tweak, it worked like a charm.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2012, 11:59:28 pm
content: Quote from: vxp036000 on April 30, 2012, 11:27:50 pmJust as a matter of interest,what was the design frequency?It must have been fairly high HF,to have such a small value of capacitor in the tank circuit.Was there any reason why you couldn't have used a lower value of inductance,so the capacitance would not have been so critical?If you had given us these details in the beginning,we may not have had this discussion.