Colpitts oscillator working in LTSpice but not as soldered circuit - Page 1
Forum section: electronics / beginners

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 08:51:09 am
content: Hi,I am trying to make a Colpitts oscillator work, that oscillates at around 23.63 MHz in LTSpice and 26.07 MHz when using a simple calculator (such as this one:https://www.a8blog.com/en_cap3.htm).However when I build up the circuit by soldering the individual components directly together (to avoid issues due to breadboard capacitance/inductance) it will show no oscillation on the scope. Also when bringing it near to an SDR (no direct physical connection), I could pick up no signal, which worked for other oscillators so far. I tried both a 9V battery and a lab power supply.What could be the issue?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2022, 09:33:57 am
content: The scope connected to the output could effect the DC operation point. The load capacitance could also have quite some effect. So the signal ouput should be from a different position.Another point is that the circuit relies on the transistor gain to set the operation point. Real transistor gain does vary from unit to unit.For a rather high frequency circuit the circuit is also rather high impedance / low current. This makes it sensitive to parasitic capacitance.In real world one would also need to include some supply buffering/ filtering - alone for EMI resons, so that supply leads are less effective as an antenna.In many RF circuits a Pi type filter with capacitor - series resistor and another capacitor is unsed for the supply. So more than just a simple decoupling cap like with the usual digital logic.

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 10:43:20 am
content: Quote from: Kleinstein on March 13, 2022, 09:33:57 amExcellent point. Another oscillator at around 40 MHz was changing its frequency slightly when approaching my hand to the circuit.Any tips how to make this specific circuit lower impedance? I assume only R1 needs to be adjusted. How do I determine good values, depending on the targeted frequency range?Quote from: Kleinstein on March 13, 2022, 09:33:57 amC3 is meant as a coupling capacitor to the output. I thought about using a buffer, but that prevented the osciallation alltogether (in another similar circuit).Is there maybe a similarily simple Colpitts circuit, that is stable yet easy to understand and probe?

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2022, 10:55:32 am
content: Use an IC such as the: 4069, 4007, 74LVC1404, or 74LVC1GX04. The latter two are only rated to 5V, so use the LM78L05.https://www.ti.com/lit/ds/symlink/cd4069ub.pdfhttps://docs.rs-online.com/2601/0900766b800b16e3.pdfhttps://www.ti.com/lit/ds/symlink/sn74lvc1404.pdfhttps://assets.nexperia.com/documents/data-sheet/74LVC1GX04.pdfhttps://www.ti.com/lit/ds/symlink/lm78l.pdfhttps://www.onsemi.com/pub/Collateral/AND8053-D.PDF

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 11:18:04 am
content: Quote from: Kleinstein on March 13, 2022, 09:33:57 amAt least the gain doesn't seem to be the issue. I measured the transistor's actual gain which was around 500, and in the simulation tested values from 400 to 800. I'll see if a lower value R1 improves the situation.

author: mikerj
author_rank: Super Contributor
timestamp: March 13, 2022, 01:57:42 pm
content: Quote from: maelh on March 13, 2022, 11:18:04 amWhat collector current did you test at? Running at the 0.5mA or so collector current in this circuit the datasheet suggests a gain bandwidth product of around 80MHz. At 26MHz you probably don't have enough gain for this to oscillate.

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 13, 2022, 02:42:22 pm
content: Quote from: maelh on March 13, 2022, 08:51:09 amThe relation between C1 and C2 affects the feedback loop of the oscillator. Try C1=22 pF and C1=15 pF. This will increase the gain of the feedback loop from 0.1 to 0.4 and will keep the frequency around 26 MHz. I hope that helps.

author: fourfathom
author_rank: Super Contributor
timestamp: March 13, 2022, 03:03:19 pm
content: Also, take the output directly from the collector, not C3. As suggested, with the current configuration the added probe capacitance will significantly reduce the feedback. If you plan to use this circuit for anything practical, you will want to add a good buffer to the oscillator.

author: Etesla
author_rank: Regular Contributor
timestamp: March 13, 2022, 05:54:15 pm
content: You can also try putting potentiometers in for some of the biasing resistors and playing with it in real time. I have never gotten a colpitts oscillator to work in real life exactly as it does in simulation especially above like 10 Mhz. Always end up messing with the real life values until it starts working, then fidgeting with the inductor to get the frequency spot on.

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 09:04:20 pm
content: With a resistor of 1K instead of 10K, and everything else remaining the same, I got the oscillator working, even if it's not too frequency stable. I'll make more tests.Thanks all for your suggestions so far! I'll try them as well, and reply here.

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 09:14:10 pm
content: Quote from: fourfathom on March 13, 2022, 03:03:19 pmIs it because the capacitor of the probe and C3 are combined and detune (or otherwise affect the circuit -- not sure how)?If I completely remove C3 the simulation fails to oscillate.If found this article about modelling a scope probe, and see if integrating this gives me more predictable results:https://www.signalintegrityjournal.com/blogs/4-eric-bogatin-signal-integrity-journal-technical-editor/post/1433-making-lemonade-with-the-10x-scope-probe

author: maelh
author_rank: Regular Contributor
timestamp: March 13, 2022, 09:27:12 pm
content: Quote from: bsfeechannel on March 13, 2022, 02:42:22 pmHow did you compute the loop gain? It seems to be quite complex.

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 15, 2022, 03:27:54 am
content: Quote from: maelh on March 13, 2022, 09:14:10 pmThis is because Q1 will saturate.Try to increase C3 a bit, somewhere between 10 and 100p. It helps with the feedback loop gain.Quote from: maelh on March 13, 2022, 09:27:12 pmC1 and C2 form a voltage divider and its transfer function is given by H = XC2/(XC1+XC2).XC1 = 1/(2πfC1) and XC2 = 1/(2πfC2), so H = C1/C1+C2.So, if C1 = 15p and C2 = 22p, then H= 0.4.

author: maelh
author_rank: Regular Contributor
timestamp: March 23, 2022, 09:22:38 am
content: Thanks a lot for the additional details.

author: maelh
author_rank: Regular Contributor
timestamp: March 28, 2022, 07:57:06 pm
content: Quote from: mikerj on March 13, 2022, 01:57:42 pmTo trace your steps, I looked up the datasheet of the exact transistor I am using:https://www.box73.de/file_dl/bauelemente/BC547.pdfAccording to "Figure 6. Current–Gain – Bandwidth Product", you are right for 0.5 mA and Vce=10V you would expect a GBWP of about 80 MHz.Note to self: another point I want to look at later is impedance matching, especially with buffers:https://www.electronics-tutorials.ws/amplifier/input-impedance-of-an-amplifier.html

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2022, 10:30:29 pm
content: Note that figure 6 only tells you the current gain. I don't think you can use the figure 6 (CGBWP) data to predict the upper limits for oscillation as these are two different things.If it helps I have some old VNA measured s2p data for a BC547B here and I have data at 0.7mA Ic. This s2p data from the VNA shows the BC547B can oscillate at much higher frequencies than the 90MHz indicated in figure 6 of the datasheet at 0.7mA Ic. It looks like it can oscillate at >200MHz at 0.7mA Ic given the right oscillator circuit. With your circuit topology the s2p data from my VNA predicts oscillation is possible up to about 150MHz at 0.7mA Ic if you change the circuit values accordingly.Your circuit appears to have a very poorly defined DC operating point as it has a grounded emitter and a 1Meg bias resistor at the base. This means the collector current will be poorly defined from device to device and across temperature.