Need help taming colpitts  - Page 1
Forum section: electronics / rf-microwave

author: MatCat
author_rank: Frequent Contributor
timestamp: October 02, 2023, 04:15:47 am
content: Greetings everyone,I am trying to make a VHF colpitts oscillator, for tank circuit I came up with 100nH and 23.xxxpF tank circuit, I am using 0-30pF variable caps to tune the frequency. Attached is an image of the circuit I am using, and an image from scope showing what it is doing. It is built in a breadboard for proof of concept so of course I don't expect frequency to be where I want it and that is infact true, but not important, what is important is trying to figure out why it rises like it does, then it stops oscillating. You can see it rises up to essentially the voltage rail, stops oscillating, then not shown it slowly drops back down to ground (well, AC coupled so whatever that means).the other values such as the 100uH for RFC, 10k for emitter to ground and 100nF are just rando values I pulled, so not sure if they may need to be adjusted or not or how to really properly determine them.Adjusting the tank circuit caps does infact adjust frequency, but not the rising behavior.Ok after initial post I tried a few things, first I lowered R2 the ground side of the base divider to 4.7k, made no difference, then I changed Re to 1.5k, after this change I saw some attempt at getting the oscillation going after the initial attempt as pictured below, but still ends up not oscillating after a few ms. Then I tried changing L2 to a 1 mH, this saw an improvement in the oscillation recovery attempt, but barely, then I removed L2 all together and saw a bigger improvement, but still stops oscillating after maybe 100ms or so.Another Update: I have been lowering Re, I am down to 150 ohms and now am at the situation of the 2nd scope pic where it is now indefinitely switching between no oscillation, and oscillation. Also interesting the voltage swing considering its a 5V supply!Another update: I dropped Re down to 68 ohm and got it stable, sort of! I had to dick around with the varicaps to get it stable with the 68 ohm Re, however it will run stable with a +/- 1V output, not sure if that is what I should expect or not on amplitude but its stable! See 3rd scope screenshot.

author: jonpaul
author_rank: Super Contributor
timestamp: October 02, 2023, 06:02:10 am
content: it is "squegging"https://en.wikipedia.org/wiki/SqueggingNeed to change bias networkj

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2023, 06:11:18 am
content: Quote from: MatCat on October 02, 2023, 04:15:47 amBreadboards are not recommended for radio. Parasitic capacitance and parasitic inductance of a breadboard can be considerable big for radio, then parasitic coupling is also very high, too, and breadboards make imperfect contacts with high resistance, etc.For RF, something with a ground plane is preferable. See these prototyping alternatives from W2AEW:#122: Electronic Circuit Construction Techniques: review of some prototype circuit building methodsw2aewYou must have bypass capacitors near the oscillator (in the schematic there is none), add some 10-100uF plus some 10-100nF, both in parallel with the supply, close to the oscillator, not at the other end where from the 5V is coming. Instability can be induced by the oscilloscope probe. Would be better to not touch the circuit at all. Try to clip the probe's alligator to the tip of the probe ("short circuit" the probe) to make a loop, and see if there is enough signal when you place the probe loop nearby the coil, without touching the circuit, something like this:Also, shielding the oscillator will make a big difference.#165: Why RF circuits need shielding - or how NOT to build a Theremin! (tnx 4 the title Ben!)w2aewAdd a picture of the physical circuit, to see what else might need to be changed.

author: MatCat
author_rank: Frequent Contributor
timestamp: October 02, 2023, 06:25:06 am
content: Quote from: RoGeorge on October 02, 2023, 06:11:18 amOh I understand, why I didn't expect my desired frequency on the breadboard, but I wanted to get the idea at least functional on it before I soldered up something, I am going to try the circuit on a perf board next, which shouldn't be too bad, if I can't get it where I want it on perf board I will dead bug it on copper clad.But as seen above I did get it stable, but man it doesn't want to be on this breadboard, I can look at it funny and get it to stop being stable.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2023, 06:41:57 am
content: It might work on a breadboard eventually, just that it will be unreliable. And even if it works on the breadboard, all the tuned values you found for making it stable will be valid for that particular breadboard circuit only, and not for the final PCB circuit. A perfboard should work better than a breadboard.Even better than a perfboard would be something with a ground plane. If you don't have any copper clad at hand, use a perfboard, then put the entire oscillator inside a metal can.

author: shabaz
author_rank: Frequent Contributor
timestamp: October 02, 2023, 07:04:15 am
content: At a pinch, I sometimes use copper sticky tape cut with scissors, that works nicely too (especially on copper-clad ground plane), at VHF breadboard is definitely not good, and won't closely translate to what will work on a PCB, rendering a lot of design effort redundant. Also, a jellybean 2N2222 might not work all too well. Better to use something like BFR92P.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2023, 07:29:36 am
content: Quote from: MatCat on October 02, 2023, 06:25:06 amIt is more than just the frequency, it's the entire circuit that becomes different.For example, when you plug a transistor in the breadboard, each terminal makes contact with the parallel metal plates that are inside the breadboard. That means, you now have 3 more capacitors (and inductances too) between the transistor's terminals. If you add the parasitic components to the schematic, you will get something very different from the original schematic.So, it is not only that the resonant LC frequency will be slightly different, but the schematic will become different too!All kind of parasitic L, and C, and R from the breadboard will appear as new components in your circuit. If you draw all of them on the original schematic, it will look very different from a Colpitts oscillator.The fact that the measured oscillations are at 7MHz, instead of the calculated 105MHz, indicates that the physical circuit on the breadboard differs substantially from the intended one on the paper. And the difference is not only in frequency but in the circuit topology, too, because of all the (invisible) parasitic components added by the breadboard. It's almost like a different schematic entirely than the one on the paper.

author: szoftveres
author_rank: Regular Contributor
timestamp: October 02, 2023, 12:48:06 pm
content: Change C3 to something like 22pf.A good rule of thumb to follow in order to avoiding squegging is to choose a coupling capacitance that results in an RC network corner frequency (coupling capacitor & biasing and input resistance) close to the operating frequency to the oscillator - definitely not many orders of magnitude below.As the oscillation amplitude ramps up, the b-e junction starts rectifying, and the coupling capacitor (C3) charges up. This is beneficial, for regulating the oscillation amplitude, by the virtue of de-biasing the active device, but not when the capacitor is enormously large. As the high Q LC tank becomes more unloaded due to higher input impedance from the de-biased transistor, it will nearly maintain is oscillation amplitude - this together with the high gain of the active device will result in keeping the oscillator running even though the active device is now severely de-biased. This will go on until the transistor is fully cut off.By choosing the RC corner frequency close to the operating frequency, you'll allow the capacitor to bleed off quickly enough between oscillation periods so that the active device never fully cuts off, while still regulating the amplitude.---EDIT: also L2 with 100uH seems odd - while the high inductance value would seem fine on paper, I highly doubt that the self-resonance frequency of that choke is more than 10MHz, which would make it unsuitable in this application, though it could miraculously still work.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 02, 2023, 06:11:31 pm
content: Quote from: MatCat on October 02, 2023, 04:15:47 amTry to parallel the emitter capacitor with a 10uF electrolytic.

author: markus_jlrb
author_rank: Regular Contributor
timestamp: October 02, 2023, 07:50:55 pm
content: @MatCat,see attachment.Cut the feedback and treat your circuit as an amplifier.Feed in a small input signal and measure the output signal.Try to load the output with the impedance of the input.Take a 2 channel DSO and compare both voltage atinput and output.keep amplification near 1.0+(min delta) and phasebetween in/out voltage at 360deg.(oscillation condition/requirement!)The min-delta is the parameter that influenced the startupof oscillation as well as the thermal stability of the outputsignal.Markus

author: LM21
author_rank: Regular Contributor
timestamp: October 04, 2023, 08:53:59 pm
content: You have got useful info, I think.One thing more: Study "Experimental Methods in RF Design" from Wes Hayward. It starts from the beginning, but it has an interesting text about colpits oscillators. With it and Ltspice you'll get close to a working design.There is some errata for the book, which you should also study.http://w7zoi.net/emerrata.html

author: Microbit
author_rank: Newbie
timestamp: October 14, 2023, 02:27:54 pm
content: Hello.Regardless of where you go further, the 100 uH RFC will be likely causing problems.100 uH is too high and will SRF too easy at lower frequencies, gettting instabilities.A good generic RFC value around 150 MHz would be 2.2 - 3.3 uH.Adjust if you're higher or lower.73