5..50MHz How Are the Question-Marked Elements Calculated in This Oscillator? - Page 1
Forum section: electronics / beginners

author: Smile2016
author_rank: Regular Contributor
timestamp: September 11, 2024, 06:55:35 am
content: I’d like to understand how certain elements of this oscillator are calculated, mostly those marked with a question mark. Curent values are for a 10MHz xtal. It’s an impressive oscillator with a very clean harmonic output before the comparator and extremely low phase noise. Any RF experts who can provide some insight?

author: PGPG
author_rank: Frequent Contributor
timestamp: September 11, 2024, 11:53:24 am
content: I'm not RF expert.I don't know how it works. As there is no feedback from collector than circuit between base and emitter have to ensure generation.L1,C5 is 10MHz filter (need not to be exactly 10MHz, as such filter is not very precision).C8 value is probably not critical.Short look at TLV3601 datasheet not shows as it have internal polarization of (-) input. As there are no absolutely 0 current inputs in such connection input will have to 'base on' one of ESD protection input diodes. It is at least 'not typical use'.I think L3+C2 and L2+C11 together are the main source of generation, but I'm not sure. May be the phase shift as a function of frequency in this circuit (may be together with quartz) is the key to understand this generator.C13 certainly works together with D1+C14 and C12. But why L4 with them all gives much higher frequency than 10M I don't know.

author: Smile2016
author_rank: Regular Contributor
timestamp: September 11, 2024, 08:46:49 pm
content: Quote from: PGPG on September 11, 2024, 11:53:24 amThank you, I was almost certain that L1+C5 determines the oscillation frequency in C-mode and L2+C11 and L3 are some kind of "traps" to avoid B-mode and other overtone modes, avoiding xtal to lock to wrong mode of oscillation.

author: Smile2016
author_rank: Regular Contributor
timestamp: September 11, 2024, 08:57:11 pm
content: here are my xtals.... the 10MHz one looks "clean" enough compared to the 40MHz one ....

author: shabaz
author_rank: Frequent Contributor
timestamp: September 11, 2024, 09:16:44 pm
content: Ordinarily one of several designs are commonly encountered, depending on the frequency range. There's a good book, "Oscillator Design and Computer Simulation" by Randall Rhea (sometimes on Amazon or eBay in used condition) which contains a good explanation of them, and they are as reasonable a starting point as any other. The book explains how an oscillator can be broken at a point and then what are the characteristics of that circuit which will give rise to good output, and useful, easy-to-use formulas for starting-points, such as what ballpark component values, and what ratio of component values.Beyond that, ordinarily some experimentation is needed, to tweak things for, say, reducing harmonics, or maintaining oscillation across a particular range (e.g. for LC oscillators). The simulation software is pricey though, but it would normally just try a span of values for components, stopping when the desired thresholds are reached. In the absence of that, manual tweaking is used. You might be able to use free software like QUCS, I've never tried it for such a task though, and wouldn't know how. The few times I've had to make an oscillator from scratch, I've simply taken the closest circuit (from anywhere you feel is suitable, but as mentioned, that book is a good option, and there's a lot of junk online) and then done manually what the software would be expected to do, and inevitably tweaking in front of a spectrum analyzer for hours.EDIT: Just saw there are some cheap options for it currently on Amazon. Well worth it, and it's written in clear, easy-to-follow language.

author: jwet
author_rank: Frequent Contributor
timestamp: September 12, 2024, 02:58:44 pm
content: A few more details and color to those who posted.The oscillator is a negative resistance oscillator- this is why you (nb- PGPG) don't see any obvious feedback- the changing input Z is the feedback and the gain essentially. Emitter followers with certain sources and loads can give this characteristic (capacitive loads and inductive sources). This is a common interview question for IC designers. The equivalent reflected impedance will have a negative real value under these condition, bad for a follower, good for an oscillator.In general, all the LC circuits are to enhance the main center frequency and suppress unwanted modes. Crystals usually have two closely spaced resonances usually called parallel and series resonant modes. L1 and C5 appear to be designed to enhance the lower frequency resonance. Crystals can also have subharmonic harmonic modes. L4 and C13 look to be of this type to kill gain around 3.6 Mhz. Its likely the designer derived some of these fixes empirically on the bench looking at the circuit with a spectrum analyzer. L2 and C11 (11.7 Mhz) create a HF notch, L3 and C4 (1.3 Mhz) are a low freq notch and part of the biasing- the large value of C4 also enhances the negative resistance characteristic. R4 is an interesting component not mentioned and lower's the effective crystal Q and enhances tunability. I agree with PGPG, C8 is just coupling but there should be some source of DC bias current for the comparator input, this could just be a 100k R from the positive input divider. It likely works because of ESD structure and rail clamps but is not great.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 12, 2024, 07:00:55 pm
content: Quote from: jwet on September 12, 2024, 02:58:44 pmIt was 45 years ago when last time I was analyzing such circuits. Don't remember a lot.I think that each oscillator (also with collector-base feedback) can be analyzed in such a way that you will get negative resistance at some net.Most people (me too) are used to look at circuits through the voltages at nets.Voltage is amplified from base to collector so voltage feedback (with phase shift) from collector to base makes it obvious that oscillation can take place.Voltage is not amplified from base to emitter but current is amplified. So getting some emitter current back to base can be the source of oscillation. But what is going on is harder to see at schematic. Looking at it as getting negative resistance can be simpler than searching how current gets back with right phase.It is why I have written:Quote

author: jwet
author_rank: Frequent Contributor
timestamp: September 12, 2024, 07:37:14 pm
content: You're right- most oscillator can be analyzed this way- I've seen a standard Colpitts analyze this way- don't remember where.I worked for a chip company for many years (Maxim) and we had certain pin limited IC's that couldn't spare two pins for an oscillator like a normal pierce type circuit. We had a standard, seldom used configuration that was a 1 pin oscillator- usually with a Xtal to ground, we called these negative resistance oscillators, they can "only" be analyzed using negative resistance knowledge.These are pretty esoteric circuits and techniques, really. I just thought it was an interesting aside- not trying to correct anything- everything you said was spot on. I go back a long way too, long enough to remember tunnel diodes, these are odd duck two terminal negative resistance with the right biasing. In almost all cases but a few, there are better ways to implement circuits than to resort to tunnel diodes...Take care,John

author: RFDx
author_rank: Regular Contributor
timestamp: September 13, 2024, 12:45:25 am
content: C13:DC blocking cap.L1/C5:10MHz tank circuit used as cleanup filter.C8:DC blocking cap.C2:Feedback cap. Works in conjunction with the capacitive reactance formed by the L2/L3/C11 network.L2/L3/C11:This components ensure that the input resistance at the transistor base is negative only inside a very limited frequency range, between ~9.7MHz (L2+L3+C11) and ~11.7MHz (L2+C11). This network is predominantly capacitive in this frequency range. The crystal is most likely a 10MHz 3rd overtone type.The inverting input of the comparator is floating on a breeze which should be fixed imo. Using a 8GHz transistor for a 10MHz oscillator is odd, a transistor with a medium transit frequency would technically suffice.

author: Smile2016
author_rank: Regular Contributor
timestamp: September 13, 2024, 02:54:15 am
content: Quote from: RFDx on September 13, 2024, 12:45:25 amEverything is much clearer now. L2, L3, and C11 turned out to be the key components. I forgot to take a screenshot of the 10 MHz crystal in the 3.3 MHz range, it is indeed a 3rd harmonic overtone. I’ll confirm this later. In the meantime, I’ve made some corrections and "upgrades" to the schematic. Thanks to everyone for their great help!It oscillates well, but there’s an issue at hot startup around 90°C where it won’t start. It's very sensitive to the values of L2 and C11. (C11 was incorrectly connected to GND but is corrected on the schematic now, and it should work both ways.) Frequency is slightly over 10MHz [mid V_TUNE], not sure how to pull it lower, increasing L4 helped a bit, increasing C2 will slowly kill oscillations . The BFP183W was the first reel of unused transistors I found in my junk pile. 😊

author: jwet
author_rank: Frequent Contributor
timestamp: September 13, 2024, 02:59:36 am
content: Thank us with the thanks button- my lame motivation

author: Smile2016
author_rank: Regular Contributor
timestamp: September 13, 2024, 03:43:56 am
content: Quote from: jwet on September 12, 2024, 02:58:44 pmI fixed the comparator, heheh! Here are a few plots of my 10 MHz crystal with peaks around 3.38...3.69 MHz . As RFDX mentioned, it's a 3rd harmonic overtone. I'm still working with empirical methods and learning new things at every step! You probably meant L4 instead of R4, which did help to slightly lower the frequency. Interestingly, only an inductive component can reduce the frequency on the varactor side. V_tune is around 2.000V. Please ignore Xtal values...did not calibrated the VNA , point was showing the frequencies ...Cheers !

author: RFDx
author_rank: Regular Contributor
timestamp: September 13, 2024, 06:46:03 am
content: Quote from: Smile2016 on September 13, 2024, 02:54:15 amI assume you are dealing with a (voltage controlled) OCXO build around a SC-cut crystal running on C-mode? Have you tried raising the collector current somewhat (R2=220Ohm)? You could also try some small adjustments: C11=62pF (critical value), L3=1uH, C2=220pF or less (down to 120pF). This changes would optimize the input resistance of the transistor @ 10MHz while improving the suppression of the B-mode (typ. 10% higher in frequency).