[Help] Clapp Oscillator won't start - Page 1
Forum section: electronics / rf-microwave

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 19, 2025, 08:46:55 pm
content: Can anyone tell me why my circuit isn't working. I mean surely it cannot be the parasitics right? Its supposed to work at a mere 1.5MHz (the frequency itself isn't important, I just want to make something that oscillates faster than relaxation oscillators). The simulation seem to work too. I made the inductor myself with some 0.3mm copper wire. Made the circuit on a microscope slide with copper tape strips (no particular reason, it looks cool). It didn't start up, I checked with my oscilloscope. Then just to be sure I made another circuit, but a lot smaller physically to minimise the parasitics.This one did not work either. I thought maybe it was my probing technique which was not good enough, or maybe the scope was loading the oscillator too much (I tried 10x, didn't work either) so I soldered a small helical antenna on the feedback, hoping to pick up the RF using my rtl-sdr instead. Still nothing.I haven't made any oscillators before, and they always seemed intimidating so I thought of giving them a go

author: Zero999
author_rank: Super Contributor
timestamp: September 19, 2025, 08:59:40 pm
content: The first thing I've noticed is the 180nH inductor, which is a very low value to use for 1.5MHz, giving an impedance of just 1.7 Ohms, which the transistor won't be able to drive, even with the capacitor in parallel. Choose a much higher value inductor.

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2025, 09:11:44 pm
content: In a normal Clapp oscillator, the series capacitor C3 is much smaller than the two other capacitors C4 and C5, which are usually equal to each other.Therefore, the inductor L1 is much larger than in your diagram, to resonate with the smaller C3, whose effective value is not affected much by C4 and C5.

author: mawyatt
author_rank: Super Contributor
timestamp: September 19, 2025, 09:24:37 pm
content: You need to up the impedance level as mentioned, 220nF at 1.5MHz is less than 1/2 Ω!! Ideal capacitance as showndon'tdoesn't include any device parasitics (ESR/ESL for example). Also general purpose SPICE models are usually quite limited in fidelity.There's are reason why RF is centered around 50Ω, this is a good impedance to work with at RF frequencies. Try scaling things to ~ 50Ω, the 220nF -> 2.2nF and the 180nH to ~5uH for example.Best

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 19, 2025, 09:26:09 pm
content: I really don't seem to have any idea about what I'm doing. I thought I just had to take feedback from an oscillator, put it through a 180 degree phase shifted buffer and inject the signal on the other end of the oscillator...So what you're saying is I need to make sure the impedance of LC network components is something that can be driven by the BJT amp here? So basically avoid making C too large and L too small?I don't know much about analogue stuff.

author: mawyatt
author_rank: Super Contributor
timestamp: September 19, 2025, 09:45:19 pm
content: Quote from: BadeBhaiya on September 19, 2025, 09:26:09 pmYes this goes beyond what you describe as at the "Barkhausen Criteria" which states you need a total Loop Gain of unity and phase shift (for inverting structure) of 180 degrees. Impedance levels need to be reasonable at frequency with consideration to actual components, and device models typically need to be taken as GP and not high fidelity.Edit: In addition the Loop Gain needs to be >1 to allow the oscillator to start, which will cause the gain to become "limited" as the amplitude increases so that during theintegral over entire waveform cyclethe loop gain isexactlyunity, as if it's > unity the waveform will grow and < unity will die. Thisintegral over entire waveform cycleis a unique study of oscillator behavior which is quite involved!!Best

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2025, 10:09:03 pm
content: The difference between a Clapp oscillator and a normal Colpitts oscillator is the capacitor C3 in series with the inductor.If you short out that capacitor (C3), then your drawing is a Colpitts oscillator.Changing that leg from an inductor to a series inductor/capacitor makes a huge change in the impedance level of the circuit.By increasing the two capacitors C4 and C5 to be much larger than the tiny C3, the changes in capacitance and impedance of the active device (transistor) result in much less change in frequency.There are two requirements for oscillation:1. The oscillation frequency will be that with a net 0 (or 360) degree phase angle around the circuit including the resonant network and active device.2. The magnitude of the (gain) x (feedback) at that frequency must be greater than unity, to be reduced either by the reduction in gain of the active device with increasing amplitude or by an external AGC circuit.The brute force way to enforce #2 is for the active device to be driven into saturation by the high magnitude, which by limiting the amplitude reduces the gain.The nicer way to enforce #2 is for the (non-linear) gain of the active device with large signals be reduced to that which satisfies the condition.These circuits can be easier to visualize if you use a vacuum tube as the active element: with increasing amplitude, the average voltage on the grid goes more negative, which reduces the transconductance, where the grid-cathode diode keeps the instantaneous voltage on the grid always negative by conducting with positive grid-cathode voltage, charging the grid-coupling capacitor to achieve the DC shift.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 19, 2025, 10:12:54 pm
content: Quote from: mawyatt on September 19, 2025, 09:45:19 pmCould you elaborate what you meant by "device models typically need to be taken as GP and not high fidelity"I did know it was tricky to get the oscillators started up but I never thought in terms of loop gain. Wonder how I got my simulation oscillator to ever start (I just made it inverting, and match the voltage swing).Are there any books or guides that I should read up to learn oscillator design? All online resources seem to be limited to "apply formula A, B, C and you got it", I'm not developing any intuition at all

author: mawyatt
author_rank: Super Contributor
timestamp: September 19, 2025, 10:30:19 pm
content: Quote from: BadeBhaiya on September 19, 2025, 10:12:54 pmTypical BJT SPICE models that most use are very limited in their fidelity, this is why enormous amounts of energy were put into developing improved and expanded models such as VBIC, MEXTRAM and HICUM. Any good EE text should have a chapter on Oscillators/Feedback/Stability and more details than just plugging into formulas!!Best

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2025, 10:45:09 pm
content: An excellent general reference that includes oscillators:K K Clarke & D T HessCommunication Circuits: Analysis and Design, original published 1971 by Addison-Wesley, reprint 2002 by Krieger PublishingChapter 6 covers "Sinusoidal Oscillators", and Chapters 4 and 5 cover non-linear devices in the context of sinusoidal signals.A pirated pdf edition:https://irkr.fei.tuke.sk/STS-Satelitne/comunication_circuits.pdf

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2025, 05:10:22 pm
content: Quote from: BadeBhaiya on September 19, 2025, 09:26:09 pmWhat value ESR did you set for the inductor and capacitor? If the answer is zero, then the LC circuit is undamped and will oscillate irrespective of the transistor. This is of course impossible in real life which is why the amplifier is required to counteract the losses in the LC network.

author: G0HZU
author_rank: Super Contributor
timestamp: September 21, 2025, 01:12:35 pm
content: It's definitely not a good circuit for various reasons. The LC values are a bit crazy for operation at 1.5 MHz as has already been stated. The bias point is poorly defined and will be device and temperature dependent. I also don't think that C1 and C2 are necessary. Also, C2 is only 100pF and this will affect the loop response. Was this value a typo?However, I did a quick analysis using one of my old VNA derived models of a 2N3904 and this predicts that it will oscillate if the loop gain is increased (by quite a bit) and if C1 and C2 are removed. I used a Qu of 80 for the inductor and assumed 0.05R ESR for the 220nF caps and short connections for the caps as the lead inductance will need to be very low.The loop gain can be boosted by omitting R2 but this makes the circuit even more sensitive to biasing. I assumed 15-20mA bias current. Reducing R1 to 150R will permit a higher collector current. The 180nH inductor needs to be physically quite large to get a Qu of 80 at 1.5 MHz. Either a few turns of thick wire wound as a big solenoid or a few turns wound on a suitable toroid.With the changes above the circuit is able to generate negative resistance at the 1.5 MHz resonator so it should oscillate even though it is a poor circuitQuoteWhen biased at (say) 10Vce and 10mA a genuine 2N3904 can oscillate at up to about 1 GHz if the circuit around it is optimised. This is based on my own small signal VNA model of a 2N3904 and this model should be much more accurate than a typical Spice model up at VHF and into UHF.

author: G0HZU
author_rank: Super Contributor
timestamp: September 21, 2025, 11:01:11 pm
content: Quote from: TimFox on September 19, 2025, 10:45:09 pmIf it helps, I had a quick skim through that chapter and I think it is very dated. There is some classic theory there but I would not recommend that as an introduction to oscillator theory and design unless the student was an academic type who had little intention to do any practical design work. It would be fine for someone intending to be a lecturer for example.My main gripe with RF theory books is that they nearly all appear to be written 'by' academics 'for' academics. So you get an orgy of equations on nearly every page but most of it is of limited practical use unless you want to replicate and understand the basic (and blinkered) designs in the book.The best way to learn this stuff is to be sat next to an RF engineer with access to a few basic circuits and simulations of the circuits and a few basic design rules and equations. By all means then go on to look at some theory and equations in books like the one above, but unless you are designing the same low frequency oscillator type as the ones in the book and you are doing it a just a few MHz then it will be of limited use in my opinion.

author: KE5FX
author_rank: Super Contributor
timestamp: September 22, 2025, 03:23:51 am
content: Quote from: G0HZU on September 21, 2025, 11:01:11 pmAfter a quick skim, it seems likely that if you asked Clarke and Hess how to change a light bulb, they would start with a pole-zero diagram.Wes Hayward's books would probably be more in tune with the OP's needs, orTom Lee'sbook, which has a very misleading title.

author: rf-fil
author_rank: Regular Contributor
timestamp: September 22, 2025, 05:57:16 am
content: Oscillator Design and Computer Simulation Hardcover by Randall W. Rhea is also a pretty good book for this. Lots of examples and explanation. Also, it's easier to use an RF simulator like ADS or MWO for initial oscillator design (the loop analysis). You can get things like loop gain and loaded Q from those.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 22, 2025, 08:53:59 am
content: Quote from: G0HZU on September 21, 2025, 01:12:35 pmThanks for taking the time G0HZU, every bit helps. I have moved away from trying to make the circuit work and learn designing them at all in the first place. And of course building them in real life. But that step will have to wait until I get beyond basic understanding of harmonic oscillators.Quote from: G0HZU on September 21, 2025, 11:01:11 pmYes I skimmed past a few books and I get the same feeling, a lot of these books are very academic. No worked out examples at all. I don't mind the theory, its more meat to chew on but after a point it stops translating into real world intuition built.Quote from: rf-fil on September 22, 2025, 05:57:16 amI'll give all the books listed in the thread a go.Unfortunately I do not have access to ADS or anything similar. Just LTSpice for simulations and python for number crunching. I'm sure it'll be fine either way

author: G0HZU
author_rank: Super Contributor
timestamp: September 22, 2025, 07:02:49 pm
content: QuoteAgreed. I'm a career long user of Randall W. Rhea's Eagleware Genesys SW dating back to when it was known as Circuit Busters (35 years ago) and it cost just $99 per seat. Not everyone will like Rhea's approach because it heavily relies on having access to his SW but it can be adapted to other SW platforms.I had a go at plotting the open loop gain of the original circuit using Genesys and the graph below shows just how low it is on loop gain. The zero phase point is not aligned with the gain peak either and this is mainly due to the 100pF cap C2. This is plotted using the Randall/Hock method that calculates the loop gain even with mismatched test ports. I rarely use this method.If C2 is bypassed and R2 is also bypassed, the loop gain shoots up to almost unity when the BJT is biased at 15mA. Boosting this to 20mA takes the loop gain over unity and this assumes the 180nH coil has a Qu of >80 and the ESR of the 220nF caps is about 0.05 ohm. This is shown in the other loop gain plot. This shows that the oscillator will just about start up as the loop gain is just over unity.

author: KE5FX
author_rank: Super Contributor
timestamp: September 22, 2025, 08:08:54 pm
content: Quote from: BadeBhaiya on September 22, 2025, 08:53:59 amDo not sell LLMs short as a resource, as well. They will not always do the right thing numerically (although they're slowly getting better at that) but when two leading-edge models identify similar issues and yield similar suggestions (e.g.,Gemini 2.5 ProandGPT-5 Thinking) there's a good chance that the advice will be more helpful than harmful.Here, both models fail to identify the extremely small inductor as the heart of the issue, but they do identify the problem correctly as a loading issue and give you a good place to start looking. And it's always fun to argue with a robot,as I did with GPT-5.

author: G0HZU
author_rank: Super Contributor
timestamp: September 23, 2025, 08:16:45 pm
content: I managed to find some suitable low ESR 220nF and 100nF caps and had a go at making the 1.5 MHz oscillator this evening. I built it with my proposed changes to boost the loop gain and so C1 and C2 were omitted (bypassed) and R2 22R was bypassed to boost the gain. I also changed the collector resistor to 150R and used a second PSU to provide the bias supply so I could vary it easily.I used a few turns on a T50-2 toroid from Micrometals to make the 180nH inductor. The yellow poly caps should have an ESR below about 0.05R at 1.5 MHz.The initial testing showed that it started oscillating at a collector current of just over 15mA (as predicted by the small signal model) and it really needed 20mA to give reliable starting. The scope shows about 1.57 MHz in the image below and the waveform at the base is only about 200 mVpkpk here. The voltage swing is even less at the collector and this is due to the very low impedances here at 1.5 MHz. It really is quite a poor circuit because of the low impedance of the resonator components so I don't recommend it although the harmonic distortion was very low.