Simple Sinusoidal Oscillators - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: July 05, 2022, 06:28:00 pm
content: We needed a low level simple floating (battery powered) sine wave oscillator with ~16KHz for testing. Grabbed a couple 2N3904 NPNs, a 470uH inductor, a 0.22uF cap and a 510 ohm resistor, and slapped together a Peltz Oscillator.https://wiki.analog.com/university/courses/electronics/comms-lab-peltz-oscHere's the output when powered by a AAA cell, and actually works down to 0.7VDC!!This about as simple a low frequency sine wave oscillator as we've encountered.Others have any simple examples of low frequency sine wave oscillators?Best,

author: Benta
author_rank: Super Contributor
timestamp: July 05, 2022, 07:05:13 pm
content: Cool!

author: mawyatt
author_rank: Super Contributor
timestamp: July 05, 2022, 07:22:58 pm
content: Even with a 0.8VDC supply the output doesn't look too bad!!Best,

author: fourfathom
author_rank: Super Contributor
timestamp: July 05, 2022, 07:44:46 pm
content: Here's a simple Twin-T oscillator, with a buffer. In simulation it just barely starts up with a 1.2V supply. You might be able to improve that by playing with the resistor values -- I just threw this together.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 05, 2022, 08:04:14 pm
content: The Peltz isn't the lowest distortion thing I've ever seen but the performance vs. simplicity is fantastic!

author: Benta
author_rank: Super Contributor
timestamp: July 05, 2022, 08:22:34 pm
content: Quote from: Conrad Hoffman on July 05, 2022, 08:04:14 pmAmen to that. I'm impressed.

author: mawyatt
author_rank: Super Contributor
timestamp: July 05, 2022, 08:54:14 pm
content: Add a single series resistor to the emitter junctions of Q1 and Q2 and you have a nice injection point for an Injection Locked Oscillator, works beautifully!! Also works inserting the resistor at the collector of Q1, and this is a nice zero volt (~ground) injection point!!Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 10:49:26 pm
content: Interestingly, the amplitude should have considerable PSRR: it's constrained within one Vbe peak. Transconductance however will not, so it should become significantly flattened (with resulting FM) when driven with I_E >> Vbe/Ro (Ro being the LC equivalent parallel resistance).A proper CCS of course addresses that. As for intentional FM, you'll get lower distortion (and, maybe phase noise too?) holding it at optimal current, and using a varactor instead.I once built an, er...broadly similarcircuit, as an RC oscillator. Similar, in that it's based on CML/ECL, but significantly more complex, as a.. I forget if I did it as a hysteresis comparator, or a 555, but it went at like 100MHz.Hmm.. this might've been the circuit. I can't remember the filename, thought I took a photo of the real thing...which is probably all kinds of poorly matched, and I don't know Q12 is doing, something vestigial perhaps.....Oh here it is,never had a screenshot of it though. Just dug it out; here's Q7 emitter (Q6 base = 2.78V, just fixed R's in the sim but obviously CV input; VCC = 4.82V):Why only 52MHz? It falls off pretty fast at higher bias. Hm, decreasing R4 seems to help. Probably needs more current then, but then the collector resistors need to drop to keep bias in place... I'm not going to screw with it right now, but eh, it's a thing, I guess.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: July 06, 2022, 02:03:46 am
content: Quote from: T3sl4co1l on July 05, 2022, 10:49:26 pmYes a CCS is better but adds complexity, the simple resistor works quite well indeed!!The earlier post about the added injection resistor is not about FM, but forInjection Lockingthe oscillator to an outside Injection signal. Where the oscillator assumes the frequency of the Injection signal over a small frequency band centered around the oscillator free running frequency. This technique can even Lock around harmonic frequencies, for example 2X or 3X. We've used this Injection Locking technique in the past for many applications, including locking to a transmitted carrier (see patent 5603111, Synchronous Tracking AM Receiver), and resonate frequency dividers operating beyond 100GHz.Edit: Injection locking is a fascinating subject, dated back to the discovery by Van der Pol when experimenting with Neon bulb relaxation oscillators. He noted that two Neon oscillators became locked (same flashing rate) when they had a common connection thru the power supply (slight coupling). Much later Adler in 1946 derived the fundamental equations describing injection locking. What we discovered and patented based upon Van der Pol and Adlers work was that the injection locking concept also produces a demodulation of the injected signal, that can be utilized to perform AM, FM or PM demodulation, and subsequently developed a single (silicon) chip Microwave receiver based upon this technique.Here's some papers on the Injection Locking subject that are available, many others are behind IEEE fees unfortunately.https://chic.caltech.edu/wp-content/uploads/2019/07/08753733.pdfhttp://rfic.eecs.berkeley.edu/ee242/pdf/Module_7_4_IL.pdfhttps://www.seas.ucla.edu/brweb/papers/Conferences/RCICC2003.pdfAnyway, fun stuff!!Best,

author: moffy
author_rank: Super Contributor
timestamp: July 06, 2022, 06:14:19 am
content: Quote from: mawyatt on July 06, 2022, 02:03:46 amIt even happens in Ring Laser Gyroscopes, where there are two counter rotating laser beams that share the same cavity and mirrors. When they created the first RLGs they noticed that for low rotation rates they would get no ouput and then above a certain level they would get the output they expected, it took them a while to work it out, because they didn't have any radio guys to explain what was happening. It was only when they stumbled across some old articles about injection locking that they realised that the counter rotating lasers were being injection locked by the back scatter from the common mirrors.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 06, 2022, 08:36:51 am
content: Quote from: mawyatt on July 06, 2022, 02:03:46 amThank you for bringing in the injection locking aspect. Fascinating subject indeed.I think "injection locking" is what leads to entanglement in physics (not necessarily electric injection locking, but a similar energy exchange happens in such a way that makes particles or macro-objects synchronize between them, which will also mean entanglement is caused by a hidden variable - the sync - which right now is considered as disproved in mainstream physics, demonstrated by the Bell theorem, which theorem and implications I disagree with because of Robert H. McEachern papers published on viXra). Anyway, for now that's offtopic rambling and I'm not a physicist, so I might be totally wrong.I don't want to derail the thread into offtopic speculations, posting here only to say the link to the part II:https://chic.caltech.edu/wp-content/uploads/2019/07/08758318-small.pdf

author: RoGeorge
author_rank: Super Contributor
timestamp: July 06, 2022, 09:04:03 am
content: Quote from: moffy on July 06, 2022, 06:14:19 amSimilar happens in chip-scale optical gyrosQuoteSource:A Chip-Scale Nanophotonic Optical Gyroscope

author: mawyatt
author_rank: Super Contributor
timestamp: July 06, 2022, 04:23:10 pm
content: Quote from: moffy on July 06, 2022, 06:14:19 amMuch of my mid-career was with Honeywell, the masters of the RLG (and ESG). Although never worked on the RLG or the ESG, but did so on the Fiber Optic Gyro and granted a couple patents (Serrodyne Phase Modulator, Transimpedance Amplifier 5339055, 5216386).Honeywell's original solution to injection locking was to dither one of the 3 mirrors forming the triangular path to unlock the HeNe laser self injection locking which created a "dead zone" in the output transfer function. The result was then integrated across the "dead zone" and created a highly linear overall transfer function. Very effective solution and enabled them to outpace the stiff competition from Sperry early on which elected to use an electro-optic solution by effectively dithering the mirror index of refraction and thus modulating the laser path length.Fond memories from back then.Best,

author: fourfathom
author_rank: Super Contributor
timestamp: July 06, 2022, 06:05:39 pm
content: Quote from: mawyatt on July 06, 2022, 04:23:10 pmThat's a nice physical analog to dithering in time that we did on a reference-clock tracking loop. The designer had used an internal clock at the same nominal frequency as the reference clock, creating a one-period dead-band. Rather than make him redesign the entire FPGA (prototype, to be converted to an ASIC), I had him just use a slightly slower unrelated clock to pre-sample the input reference. All the sampling artifacts were way outside the filter bandwidth and we got a very nice phase-detector transfer curve.

author: mawyatt
author_rank: Super Contributor
timestamp: July 06, 2022, 07:42:01 pm
content: Back to the Peltz oscillator. We changed the inductor to a wire wound around a 3mm diameter of 18 turns, this creates about 0.3uH, and changed the capacitor to 1nF film. Everything just plugged into a Proto-Board and should produce a frequency of ~9.19MHz based upon the LC product. Here's the results with a 1.5VDc supply.This is such a simple little oscillator and works so well, hope others can tinker with it. You can use just about any small signal transistor (2n3904, 2n3906, 2n2222, 2n2907), any type inductor and capacitor, just make sure the impedances are reasonable for the L and C.If we ever get access to a SiGe or InP process again, would like to roll one of these with a couple 500GHz transistorsBest,

author: Benta
author_rank: Super Contributor
timestamp: July 06, 2022, 07:58:55 pm
content: @mawyatt, I'm slowly getting blown away here. This thread has been stored for future reference.And perhaps even a "Sticky".

author: LM21
author_rank: Regular Contributor
timestamp: July 06, 2022, 08:18:34 pm
content: Is there a website for 1-2 transistor oscillators. I have seen plenty of low frequency RC oscillators, but I not in a single place.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2022, 11:00:16 pm
content: The Petz oscillator is well behaved and can work with relatively low Q resonators.The classic sine oscillator for the audio range is the Wien bridge. This can can also be build with 1 or 2 transistors, though the THD may suffer a little.

author: moffy
author_rank: Super Contributor
timestamp: July 06, 2022, 11:03:16 pm
content: That's nice, the higher frequency has about 20db lower distortion on the 3rd harmonic than the audio one. Less gain out of the transistor for the higher harmonics?P.S. I think I remember the Sperry solution, they added magnetically changeable layers to one of the mirrors. Bad solution because it increased the back scatter problem as well as the stability issues.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 07, 2022, 07:03:26 am
content: By looking at all 3 spectrum measurements, what intrigues me is the fact that in the last one, 2nd, 3rdand 4thharmonics are about the same value, -70dBc.In the first measurement there are big differences between the level of them, but with the next measurements, when the level of the fundamental is lower, all the harmonics tend to become equal.Now, as explained inQuote from: mawyatt on July 05, 2022, 06:28:00 pmthe amplitude on the LC tank is limited by the forward voltage drop on the BC junction of each transistor. One transistor for each polarity of the oscillations, which means, if the two transistors have a different VBCforward drop, then the generated sinusoid will be asymmetric in respect to X axis (in time domain), which means (in frequency domain) even order harmonics.Also, the two VBCjunctions will limit the amplitude, turning the sinus into a more square"-ish" waveform. This type of distorting will translate into odd order harmonics. I don't know what leads to equal odd and even harmonics, maybe it's just a coincidence, though it doesn't look so to me, it looks like the lower the oscillation, the more equal harmonics.First things I would like to experiment with would be:- to try well paired transistor (for identical VBC, thus symmetrical waveform, thus less even harmonics)- try to put the output signal through an exponential amplifier. The idea is to try to reverse (in time domain) the amplitude limitation/distortion introduced by the forward biased BC junctions by applying to the output signal the inverse function that produced the distortion. The hope is to get lower distortions/harmonics.About the optical gyros (I know nothing about the implementation details, only the principles), I wonder if using total reflection would have lowered the backskattering. I imagine getting very flat glass surface would be easy by simply letting the melted glass to cool. Using longer wavelength for the light, so the wavelength will be big in relation to the ruggedness of the glass surface, should help too. But I guess these were considered already.Were those gyro mirrors made out of glass, or metal?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2022, 07:32:56 am
content: Quote from: RoGeorge on July 07, 2022, 07:03:26 amThe nonlinearity is not so easy to remove later, as there is the resonator in between that keeps extra energy for a longer time.This helps in reducing the hamonics quite a bit: quite often the transistors to all the way to switch on / off and thus a kind of rectangular excitation. The more linear range is much smaller - up to around a 25 mV in amplitude as the about linear range of a long tailed pair.For the symmertry it is not only the VBE drop, but also the ohmic resistance of the inductor that produces a shift. So matching alone is no suffient. It would be more a point to trim the offset.A way to reduce the harmonics would be by adding emitter resistors (maybe diodes) and adding a kind of amplitude stabilization loop, that reduces the current from the emitter side. So drive the resonantor only as hard as really needed and not too much into the limiting action of the BE junctions.

author: moffy
author_rank: Super Contributor
timestamp: July 07, 2022, 08:39:55 am
content: Quote from: RoGeorge on July 07, 2022, 07:03:26 amGetting them flat was only part of the problem with the mirrors. For the triangular laser gyro, the most common, two of the mirrors were flat and one was curved, the curved mirror was adjusted by hand at manufacture to maximise the lasers Q. They were a polished ceramic/glass called zerodure, a zero effective expansion material, below 1ppm/degC. They were a dielectric mirror with 14 incredibly thin dielectric layers deposited, the light was bent rather than reflected. It was believed that the curved mirror was the major contributor to backscatter, but the levels were incredibly small, it was atomic levels of uneveness.

author: mawyatt
author_rank: Super Contributor
timestamp: July 07, 2022, 01:09:00 pm
content: Quote from: RoGeorge on July 07, 2022, 07:03:26 amEstimating/Explaining the harmonic content is difficult in the configuration we used since this was just assembled on a plug-in Proto-Board with long leads, so lots of distributed capacitance and inductance. Think of Q1 as a common base configuration and Q2 as a feedback emitter follower to Q1's emitter. This creates a + feedback loop, the DC bias current is set by the common emitters resistor and split between the transistors if they are matched.Interested in what you find with your experiments, fun little circuit to play around with!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: July 07, 2022, 01:14:13 pm
content: Quote from: moffy on July 07, 2022, 08:39:55 amDon't remember what type mirrors were used with the RLG, but at some time these could have been diamond turned aluminum since Honeywell had that capability in-house (we utilized this on our XM21 Remote Sensing Chemical Agent Detector based upon controversial Passive Spectral Radiometry....long story about solving the impossible if someone is interested).Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: July 07, 2022, 02:51:28 pm
content: Quote from: mawyatt on July 07, 2022, 01:14:13 pmInterested? You must be kidding, not only interested but rather eager to hear more, of course.About the Peltz osc, so far the easiest way to lower the harmonics seems to be by lowering the supply voltage, though it won't oscillate when powered with less than 0.6V, but have some other ideas to try.

author: mawyatt
author_rank: Super Contributor
timestamp: July 07, 2022, 03:06:33 pm
content: Quote from: RoGeorge on July 07, 2022, 02:51:28 pmOk should start another thread.Best

author: RoGeorge
author_rank: Super Contributor
timestamp: July 08, 2022, 06:41:00 am
content: Quote from: mawyatt on July 07, 2022, 03:06:33 pmFound it, thank you:https://www.eevblog.com/forum/projects/xm21-remote-sensing-chemical-agent-detector/Back to the Peltz osc, in my attempts the most beneficial measures against distortions were:1. - reduce the power supply close to no oscillations (can oscillate with as less as 0.6-0.7V)2. - increase the R emiter (lower the T current - still oscillates with 50-100k)3. - lower the Q of the L (add a series resistor - can oscillate with XL/R ~ 1 or lower)Any of these also reduces the amplitude, which is essential to do, or else the oscillations will be limited by the VBCforward, because BC junctions are in parallel with the LC tank.Adding base resistors as suggested in the Analog Devices lab material didn't help as much as the other 3 measures listed.What puzzles me is that I couldn't get anywhere near the -65dBc harmonics showed in your latest measurement. What I see is more like -45dBc. I will try this weekend an oscillator in the kHz range, so I could measure the distorts with a sound card.Did you used other tricks than in the ADI lab (in order to get less harmonics)?

author: mawyatt
author_rank: Super Contributor
timestamp: July 08, 2022, 12:03:06 pm
content: Quote from: RoGeorge on July 08, 2022, 06:41:00 amDidn't use any base resistors or other "tricks", just a couple 2N3904s, L, C, and a 510 resistor. Reducing the collector current should improve results as the incremental gain across the waveform period when integrated must be identically unity in steady state, so higher collector current results in higher "gain" at the waveform mid-section and thus lower compressed gain at the peaks so the average over the waveform cycle is unity. At lower collector currents the gain across the waveform cycle is more uniform and should produce less distortion.As you mention, the gain "compression" at the peaks is due to the CB junctions which acts as a self limiter, and adding series base resistance allows a larger signal swing and a softer self limiting and may slightly improve the distortion, although haven't tried yet.Anyway, have fun with this, it' really an interesting little circuit!!Edit: An important note regarding the waveform cycle unity gain requirement under steady state. An ideal oscillator eventually settles to a sinusoid with constant amplitude or:Vo = V*{Exp(alpha*t)}Sin(omega*t), where alpha is identically zero under steady state and negative under waveform decay and positive under waveform expanding.(Need to figure out how to enter proper equations like others use, MathJax or other method!!)Simulations with many oscillator cycles involve waiting for the oscillator to reach steady state, so then one can get better results, but this imposes a long simulation time frame even on a fast computer. Long ago we ran into all sorts of issues attempting to simulate very high Q oscillators, one thing we did to help with the actual simulation time was utilizing a PWL current source to ignite the oscillations with a PWL current that approximated the desired oscillator output. This caused the oscillator to sort of kick start very close to the final value frequency and amplitude, and a carefully crafted current source could save considerable simulation time. Another issue is that very fact of a simulation creates a sampling effect and subtly imposes the sampling effects upon the oscillator, one of which the sampling function has a "gain" of slightly less than unity. Under steady state the oscillator loop-gain should be identically unity, however the sampling effect reduces this below unity and can cause the oscillator output to decay. This is one reason why some oscillator simulations die out after a kick start and require the time step (sampling period) to be significantly reduced for long simulations with many waveform cycles. With a smaller period between simulation samples the sampling effect more closely approaches unity {sinc function moving towards the origin, and at origin (zero sampling period, or infinite sampling rate) is unity}.Unfortunately the longer (more) the simulation of oscillator cycles the shorter the simulation time step if one wishes to achieve very accurate sim oscillator results after many cycles. One can run the simulation with a moderate time step with many oscillator cycles, then carefully attempt to recreate the waveform with the mentioned PWL current source, and restart the simulation with the modified PWL current source and much smaller time steps. This should yield a waveform result with more waveform fidelity in fewer cycles under the smaller time steps. Much later Cadence came up with various simulation augmentations to address high Q oscillator simulations, and this improved one's ability to accurately simulate high Q oscillators. Suspect Cadence has some tutorials on such.Best,

author: moffy
author_rank: Super Contributor
timestamp: July 09, 2022, 08:42:15 am
content: Number resolution can also be an issue. In LTSpice you can turn 'off' compression in the control panel and this will improve any FFTs done on the data.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2022, 09:26:32 am
content: No idea if QucsStudio is using lossy compression when saving data, like in the LTspice defaults. Since, by default, QucsStudio only saves data for the specified signals, I suspect QucsStudio doesn't compress the results at all.Just for the fun of it, attaching an LTspice simulation for 3 distinct Re values. It was just a draft, I didn't use the behavioural source trick to start the oscillator, only used the LTspice 'uic' option instead.At first I've simulated with LTspice, but QucsStudio has a very nice feature: one can simply click on a value to add a live slider for that value, which make QucsStudio the ideal tool to quickly adjust/optimize a schematic (something like seen in this video, unrelated with the Pelts oscillator, only to show how easy is to add tuning sliders:https://youtu.be/fYs7ZoVyPmM).QucsStudio is free to use but not open source, was made for Windows but works in Linux, too, using Wine (should work under Wine for Mac also, but I didn't try).

author: gf
author_rank: Super Contributor
timestamp: July 09, 2022, 10:56:48 am
content: Quote from: RoGeorge on July 08, 2022, 06:41:00 amI wonder for which time window of the transient simulation the spectrum is calculated?At least for this usecase, only the trailing samples should be considered, where steady state is (almost) reached.

author: mawyatt
author_rank: Super Contributor
timestamp: July 09, 2022, 12:04:24 pm
content: Quote from: moffy on July 09, 2022, 08:42:15 amAlso once you get down to these levels the SPICE parameters such as reltol, abstol, chgtol, and so on can be tweaked to improve simulation fidelity.Best,

author: moffy
author_rank: Super Contributor
timestamp: July 09, 2022, 12:38:04 pm
content: Quote from: mawyatt on July 09, 2022, 12:04:24 pmThey can also make convergence a pain, tweak with care.

author: Sengcid
author_rank: Regular Contributor
timestamp: July 09, 2022, 04:36:06 pm
content: A bit off topic but jfets work nicely, albeit with a high Q tank cct. I will put more details in another thread.

author: PeiCB
author_rank: Newbie
timestamp: July 10, 2022, 04:57:47 pm
content: In Wireless Worlds three volumes of "Circuit Designs special Editions" which is available for free download here:https://worldradiohistory.com/Wireless_World_Magazine.htmthere are a number of similar designs as the Pelz-oscillator described.Unfortunately I do not remember exactly where but the volumes are so rich in information it is a joy just to browse through them all!

author: doktor pyta
author_rank: Frequent Contributor
timestamp: July 10, 2022, 06:18:56 pm
content: I did't knew the name of the inventor, but back in 2013 i've built 'the shittiest VCO ever' for educational purpose:https://www.elektroda.pl/rtvforum/topic2548956.htmlAs it turned out it has some practical uses (but with X7R capacitors).Thanks for the post mawyatt.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2022, 07:05:06 pm
content: Using X7R capacitors as voltage controlled capacitors is a strange idea. It may work in a PLL loop or similar, but probably not for a stable frequency on it's own.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 10, 2022, 07:39:24 pm
content: My "entry" is a DDS locked tube Hartley Local Oscillatorhttps://app.box.com/s/ufytpndvnjm3yljoezz96afiqy2lcqfaAfter trying some other methods, this one using the 6BE6 (Mixer tube) signal grid as injection point works best.The glass tuning scales (re- artworked by Radio Daze) on the old receiver still work as originally with the DDS deselected in the app.There are presets for the DDS in the app, also a lock can be manually entered.Spect An show that the unlocked 2nd and 3rd Harmonics are about -50 and -45 dB respectively on 20 metre bandThere are phase noise sidebands on the L.O. output, I think due to proximity of strong ft8 signals coming in to RF stageWhen the DDS is switched in, the harmonics are both reduced by about 5dB.The noise sidebands are significantly reduced by DDS but I am not sure how to measure noise sidebands on the old analog Spect An.I presently have the DDS level and negative bias from the 2n2907 set for a lock range of about +/- 3 kHzThere are other mods to upgrade the receiver especially for L.O. stability to receive digital modes.-Plate and Filamant voltage regulators-The original 6AV6 is switched out for ssb and ft8 etc and a Gilbert cell is used as demodulator-Shaft drives have ball bearings added and the dial cord is replaced with the newer dual core type.-A temp sensor is added to the variable tuning cap with a PI controller on a variable speed fan.(The chassis and tuning cap run at about 10 C above ambient due mainly to the losses in the rectifier tube.This is leveraged so the fan cools more if the temp rise, and slows down if the temp falls.

author: mawyatt
author_rank: Super Contributor
timestamp: July 11, 2022, 12:02:23 am
content: With the Peltz oscilator mentioned we changed L to 10uH (9.017uH) and C to10nF (9.523nF) which should produce ~543KHz waveform with Vee at -1.5VDC.Here's the results, not bad for a simple oscillator.Best,

author: moffy
author_rank: Super Contributor
timestamp: July 11, 2022, 01:00:48 am
content: Quote from: PeiCB on July 10, 2022, 04:57:47 pmThanks for the reference, I have downloaded the 3 volumes, used to love Wireless World, so much content.I feel a little slow in recognising how the Peltz oscillator works so I have redrawn it so the differential pair is a bit more obvious.

author: mawyatt
author_rank: Super Contributor
timestamp: July 11, 2022, 02:13:08 am
content: Quote from: evb149 on July 11, 2022, 01:33:59 amHere's a few that come to mind, although not sure if these are necessarily "advantages" over the other style sine wave oscillators.Maybe "features" is a better characteristic.a) Simplicity, 2 transistors, 1 resistor, 1 inductor and capacitor.b) Simple biasing, only one resistor.c) Symmetrical self-limiting, important for low harmonics.d) Ground reference output, low DC offset when using only negative supply.e) Low voltage operation, requires only slightly more than Vbe_on (~0.7VDC).f) Wide supply operation, essentially e) to any available voltage since the emitter resistor limits the available current and the transistors never "see" more than a CB junction drop.g) See a) !!Best,

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 11, 2022, 03:15:06 am
content: It would be useful to do a high frequency hybrid pi analysis based on Moffy's clarification of the Peltz.L1 and C1 and L1_R_equiv_parallel and 2*(R_ce_H_pi) all in parallel give the circuit Q at a frequency.That can be compared with a model of Hartley.Hartley's patent claims include ...tappings ...."at least a portion of" ...full turns.That seems to include any tap > 0% to < 100%He describes damping instead of present day QLower taps on Hartley will increase the QHigher Q in these oscillators will give better harmonic suppression , but I am not sure about effect on phase noise.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2022, 09:38:27 am
content: Quote from: mawyatt on July 08, 2022, 12:03:06 pmThis forum supports MathJax, with formulas delimited by backslash dollar at both the beginning and the end of a formula (for inline formulas), i.e. \$ V_o = Ve^{\alpha t}\sin(\omega t) \$ or backslash open-square-bracket at the beginning and backslash close-square-brackets at the end for formulas centered on a new line: \[ V_o = Ve^{\alpha t}\sin(\omega t) \] where t is time ...There is a small thread with examples of posting math on EEVblog:https://www.eevblog.com/forum/beginners/how-does-posting-math-notation-work/msg3994178/#msg3994178The drawback is that MatJax formulas are not rendered at preview, they only show as formulas after posting, which makes editing a pain. There are online MatJax editors that can live-render formulas, then copy paste from there to here. There is a trick I've found working to preview MathJax in EEVblog, formulas are rendered when pressing preview from a locally saved reply webpage, like this:- press the reply button on EEVblog, so to start editing a new reply- save the reply webpage locally (i.e. with save as from the Firefox web browser file menu)- open with Firefox the locally saved EEVblog reply page- type the matjax in the offline saved reply page- press preview to see the formula rendered- important - to further edit or to copy the formula, press "Back" page in the browser, to go back to the offline reply page, then preview again and so on. The formula is only rendered when pressing preview from a locally saved webpage.Once a formula edited one way or another, copy/paste it in the "live" reply webpage. Cumbersome and time consuming.Did some experiments with the Peltz oscillator this weekend, both in simulation and in practice (for the fun of it, mostly out of curiosity, because I was not aware about this type of oscillator before, thank you for bringing it).As everybody know, "a rainy Sunday afternoon" is best suitable to improve distortions in existing oscillator designs, and yesterday it was a rainy Sunday afternoon. So I took advantage of that, and reduced even further the distortions for the Peltz oscillator by inventing thePeltz-Wyattoscillator, if I may call it so. It's aPeltz oscillatorwith aWyatt current sourceinstead of the resistor, signal much cleaner now.

author: mawyatt
author_rank: Super Contributor
timestamp: July 11, 2022, 12:27:31 pm
content: Quote from: RoGeorge on July 11, 2022, 09:38:27 amThanks for the information on using MathJax, didn't know how this would be rendered here, now need to spend some time with this.QuoteThat's certainly a good use of the Wyatt current source, well done, and please show some of your measurement resultsAnother reference for the current source from Harrison's book, Current Sources and Voltage References: A Design Reference for Electronicshttps://books.google.com/books?id=03JmxpE39N4C&pg=PA87&lpg=PA87&dq=Wyatt+Current+Source&source=bl&ots=5zevJ8Ypzb&sig=ACfU3U3DvUbu9AotugZ_GWm2zi6zyEj6pA&hl=en&sa=X&ved=2ahUKEwicmoqB9Oj2AhWOSjABHRumDjYQ6AF6BAgYEAM#v=onepage&q=Wyatt%20Current%20Source&f=falseBest,

author: RoGeorge
author_rank: Super Contributor
timestamp: July 19, 2022, 02:23:30 pm
content: Quote from: mawyatt on July 11, 2022, 12:27:31 pmThis is the Peltz oscillator after adding the Wyatt current source. The coil is from a former black and white TV (to adjust the vertical linearity IIRC), but I've removed the adjustable ferrite core, now it has ~16.9mH and 138 ohms, no magnetics.When powered from a single 1.2V battery it measures about -29.7dB (3.39%) distortions at about 3555Hz. Spectrum and distortions measured with the freebaudline v1.08software and the onboard Creative SoundCore3D audio card sampling at 24bits/96kHz (displayed dB are relative to 1VRMS):The hill in the background noise floor for frequencies above 25kHz is internal noise from the soundcard. I don't know if the displayed THD includes the 50Hz hum and the noise, too, or only the fundamental and its harmonics.The previous measurements last week were made by eyeballing at the FFT on the screen of my Rigol DS1054z oscilloscope, so not very precise. With more precise measurement using the soundcard, I see no significant differences in distortions when the differential BJT pair of the Peltz osc is powered through a simple resistor vs a constant current source in their tail (for the same amplitude of the generated signal).I think I might have measured something wrong last week, when I've observed reduced distortions after replacing the common emiters tail resistor with the current source.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 04:32:55 pm
content: Nice 1.2V oscillatorBest

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2022, 04:44:38 pm
content: The current source instead of a simple resistor would allow to adjust the amplifier gain, e.g. in an loop to regulate the amplitude. This would be expected to also reduce the THD.It was seen before that less current for the transistors (and thus less gain) gives lower distortion.Another simple point to improve on the THD may be base resitstor, so that the clamping from the BE/BC diodes is not so hard, but more gradual.

author: moffy
author_rank: Super Contributor
timestamp: July 20, 2022, 12:05:39 am
content: We just had one of the main speakers at my church go silent, was thinking of a small battery test oscillator, I think I might try the Peltz.

author: mawyatt
author_rank: Super Contributor
timestamp: July 22, 2022, 09:34:54 pm
content: Just for fun we did an analysis to find the minimum supply voltage to start/sustain oscillations for the Peltz oscillator. We were too lazy to use the Wyatt Current Source with the oscillator as RoGeorge did, and just used a 10K Resistor to the negative Vee supply.10K Emitter Resistor was selected to allow a finer adjustment since the actual parameter being varied to start/sustain oscillations is the transistor transconductance which is Ic/Vt, where Vt is the thermal voltage kT/q, and by varying the supply voltage the transistor current is varied.The Peltz oscillator was assembled on a plug-in Proto-Board with jumpers and a pair of 2N3904 transistors with a 100uH Inductor (99.28uH with Rp of 241 ohms at 16.3KHz) and 1uF Polyester Cap (963nF with Rp of 4.68K ohms) for a resonate frequency of ~16.3KHz. Measurement show oscillation start at ~5.75V Vee supply.The analysis involves computing the supply voltage when the gain around the positive feedback loop formed by Q1 and Q2 equals unity and the solution goes transcendental which requires Netwon's Method or other Numerical Solution algorithms even when simple transistor models are assumed. Here's the solution found (later we'll try and make this nicer with MathJax when we figure out how to use such).Vee > Vt [ 2 * ( R + Sqrt( R^2 + Z * R)) / Z + ln {( Vee -Vbe)/(R * Is)} ]Edit: Trying to use MathJax, shows OK on a interactive editor (see image)!!\[V_{ee} > V_t \left( 2\cdot\left(\frac{R + \sqrt{R^2 + Z \cdot R}} {Z}\right) + \ln\left(\frac{V_{ee} - V_{be}} {R\cdot I_s}\right)\right)\]Finally got the equation to render properly with the kind help from RoGeorge!!Where Vt is the thermal voltage kT/q, Z is the total real impedance "seen" at the parallel resonate LC network and includes board loss, R is the emitter bias resistor, and Vbe is the base emitter voltage of Q1 which is Vt * Ln (Ic/Is), where Is is the bipolar saturation current. As you can see this isn't exactly a simple straight forward solution, and best left for a grad studentHowever, a simple assumption can make the solution more amendable if one assumes a fixed value for Vbe, of say 0.65 volts. With this assumption and the values shown above with a small loss to account for board contact losses which makes Z ~ 215 ohms at the oscillator resonate frequency, we calculate a minimum Vee supply voltage of ~5.62 volts and measure ~5.75 volts which is ~ 255ua collector current for each transistor.Anyway this was a fun exercise while "holding down the fort" here at home this afternoon. This Peltz oscillator is a fun little circuit and great for those folks that like to tinker around with electronics stuff!!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: July 23, 2022, 12:15:12 am
content: Trying to reproduce your formula, using this live rendering webpage:https://arachnoid.com/latex/The raw formula typed in thehttps://arachnoid.com/latex/webpage wasCode:\require{cancel} \xcancel{V_{ee} ~= V_t \cdot 2\left(R + \frac{\sqrt{R^2 + Z \cdot R}} {Z} + \ln\left(\frac{V_{ee} - V_{be}} {R+I_s}\right)\right)}then I've copy pasted it here, and added backslash square brackets at the beginning and at the end, in order to be rendered by the forum in display mode (centered in a new line). This is how the formula edited live onhttps://arachnoid.com/latex/renders here: \[ \require{cancel} \xcancel{V_{ee} ~= V_t \cdot 2\left(R + \frac{\sqrt{R^2 + Z \cdot R}} {Z} + \ln\left(\frac{V_{ee} - V_{be}} {R+I_s}\right)\right)} \] Thought, a photo of a handwritten formula on paper is much faster and less prone to typo errors.As a side note, my Peltz-Wyatt oscillates just fine starting from anything above 0.7-0.8V but the amplitude is very small. For example at 0.9V the sound card measures -100dBVrms, so about 25uVpp, and the DC level is very jumpy, so I can not use the oscilloscope in averaging mode. Not sure if the signal jumps because of shot noises or because of the breadboard contacts, I suspect it's not the breadboard. The attached spectrum is for Vcc = 0.9V, barely visible above the noise floor.LATER EDIT:-----------------Added cross-strikeout because, by mistake, I've messed the parenthesis while editing the formula. My bad, sorry.

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2022, 02:34:47 am
content: Edited the previous post earlier and if you look at the image, this is the correct equation which was captured from hereV_"ee" > V_t \left( 2 * (R + sqrt\(R^2 + Z * R))/{Z} + \ln (\left(V_"ee" - V_"be")/(R * I_s)\right)\right)http://asciimath.orghttps://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-referenceUsed the link and code ReGeorge posted and did some editing to get the equation correct using the syntax you used.\[V_{ee} > V_t \left( 2\cdot\left(\frac{R + \sqrt{R^2 + Z \cdot R}} {Z}\right) + \ln\left(\frac{V_{ee} - V_{be}} {R\cdot I_s}\right)\right)\]Thanks for the helpBest,

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2022, 09:14:51 pm
content: Quote from: RoGeorge on July 23, 2022, 12:15:12 amRegarding the signal level, yes that's small, if you can't use a larger Vee supply voltage then try using a smaller Ree which will increase the transistor transconductance gain which is linear with collector emitter current. This will cause the dynamic signal swing to go further into the forward bias CB junction during the signal +- peaks such that the integral over the waveform cycle is unity gain for the positive feedback loop. In other words, the waveform will experience more dynamic gain around the center and more gain compression at the peaks such that the cycle by cycle average is unity.Thismore dynamic gain around the center and more gain compression at the peaksgets into an area of advanced research that was going on awhile back before we retired. This was related to low noise tuned/filtered amplifier and oscillator design that was completely opposed to conventional thinking, where ultra linearity was "believed" to produce the lowest noise results. The new idea was to slam the active element(s) into gain compression at cutoff and saturation where the incremental gain is almost zero, then allow large gain during transition between these compression regions, the less time spent in the "gain" transition area the lower the average overall noise per cycle.The idea was that during the high compression regions the active element(s) produce almost no noise since they are effectively highly compressed gain-wise, and only produce noise when in gain transition between the gain compression regions. BTW this was not the usual limiting type designs for high efficiency, but utilizing this limiting "gain compression" property to achieve low overall average noise per cycle. Recall this research was going on at MIT, Berkeley and/or Stanford.Anyway, a topic for another thread that others might want to investigate (can't recall much other than having dinner/drinks some time ago with one of the key early researchers involved), but makes one consider if the Peltz Oscillator is "driven" harder by higher device bias currents if the overall output noise improves??Best,

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2022, 05:38:39 am
content: For most simple uses the noise is not that relevant. The gain compression has however also a positive effect on the amplitude stability. The petz oscillator tends to produce a rather stable amplitude. With the 1 transistor Pierce or Collpitz oscillators one can run into the problem that the amplitude gets anstable and worst cost one gets an extra AM modulation for free.There are 2 effects stabilizing the amplitude: the gain saturation of the LTP when the amplitude is larger than some 50 mV and than the clamping action of the BE / BC junctions when the amplitude exceeds some 600 mV. So the amplitude and main active mechanism can effect the distortion. Chances are the clamping action would produce more pronounced distortion.

author: mawyatt
author_rank: Super Contributor
timestamp: July 24, 2022, 01:18:30 pm
content: Agree the amplitude stability under "heavy" limiting is good.The clamping or limiting action of the transistors is in the CB junctions of each transistor, Q2 for the + waveform peak and Q1 for the - waveform peak. However it's not limiting in the conventional sense, it's causing the overall cycle by cycle gain to be unity, where the mid-waveform gain is higher than 1 and the +- peaks compress to below 1.This isn't the conventional type thinking of "gain", but more of a "gain" distributed across the entire cycle where the cycle-average must be unity. So the CB junctions actually "come into play" well before the usual 0.6 diode effect at small transistor transconductance and can be estimated as the usual Diode Equation.If you check the first post there's link to the Peltz Oscillator we provided and it shows adding series R to the base of Q1 and Q2 to increase the output amplitude. What's interesting is that adding a resistor in the signal path (that doesn't change the bias) causes a signal output increase, contrary to intuition. This is because the added resistor is affecting the cycle by cycle loop gain at the waveform +- peaks where the incremental gain must dive below 1 so the cycle average is unity, and thus the waveform is larger in amplitude.Anyway, it's fun to build one of these simple Peltz Oscillators and watch the waveform behavior under variations in bias and looking at oscillator start-up conditions (why we posed the equation for startup minimum Vee). At low bias current levels the transistor transconductance is just enough to overcome the coil and capacitor losses and the loss due to Re (so cycle gain is almost unity everywhere), as the bias increases (larger Vee), so does the transconductance (linear with current). You can witness the CB junction conduction that begin to change the cycle gain at the peaks by softly introducing a dynamic loss impedance well before the usual ~0.6 volt differential.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: July 31, 2022, 11:46:16 am
content: Made a detour through the software land and when I wanted to make a few more test I've read this thread again, only to notice I was on a wrong path. ADI page was pointing at the BC junctions in parallel with the LC tank, and how they will act as limiters, and that remark fooled me into assuming the BC forward polarized are the main mechanism of limiting/distorting the signal.Quote from: Kleinstein on July 24, 2022, 05:38:39 amNoticed the similar reply before, already familiar with the idea highlighted there, yet I've somehow failed to register or to consider that.In fact, when I've read your message about BE pair clamping some pages ago I assumed that's a typo, and that you meant the anti-parallel BC diodes clamping, sorry.Indeed the input range for good linearity in a BJT pair is way lower than the 0.6V clamping caused by the forward polarized BC junctions in this particular arrangement of the LC tank. And if it were to try to extend the input range where the response of the BJT pair is still linear (since I was seeking to lower the distortions), I should have added a resistor ineachof the emitters instead of increasing the common resistor.Talking about Fig.9 at page 10 here:https://engineering.purdue.edu/wcchew/ece255s18/ece%20255%20s18%20latex%20pdf%20files/ece255Lecture_19_Mar29_Diff_Amp_Cont.pdfHave to redo, but at least now I know how to sample at 24bits/96kHz from Python.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2022, 06:01:01 pm
content: Quote from: RoGeorge on July 31, 2022, 11:46:16 amDon't think of the BC junction as a limiter, better to think of this as a means to extract energy from the LC network on a per cycle basis and create a net energy added/lost ratio per cycle which is exactly unity. Adding separate emitter resistors for Q1 and Q2 linearizes the transconductance but at the expense of reduced loop gain (lower transconductance) which require higher device bias currents, whereas adding resistors to the base of Q1 and Q2 allow more signal swing and less energy lose per cycle to the BC junctions, but have less effect on Q1 and Q2 transconductance.For the lower harmonic distortion, then one wants to balance the transconductance linearization with marginal energy lost to the BC junctions, however this may not yield the best overall SNR for the fundamental.Another side thought you could create a similar oscillator with a pair of NMOS devices, the output voltage should be much higher but won't have the benefit of the energy loss to the bipolar BC junction unless the gate has some sort of a protection diode built-in.Anyway, fun little circuit to play around and experiment with!!Keep experimentingBest,

author: RoGeorge
author_rank: Super Contributor
timestamp: July 31, 2022, 10:39:16 pm
content: In terms of energy, what bothers in respect to low distortions is the fact that energy is extracted at faster rates in the regions around the voltage peaks on the LC, which means altering the natural LC swing strongly around the peaks, which means distortions.Another thing that bothers me is that the voltage at the non grounded end of the LC tank, V(out), is swinging above and below the GND.Because the two BC junctions are directly connected to the LC tank (in anti-parallel), this implies at any time one transistor will have its CB junction forward-biased (one transistor at each half of an oscillation), so each transistor will be saturated, alternatively, for half of the period.For this reason I'm tempted to say this circuit only looks like a differential BJT pair, but shouldn't be looked as a linear BJT pair.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2022, 11:27:49 pm
content: Yes it is a simple diff pair since the total negative emitter currents thru the emitter resistor are split between Q1 and Q2 but not exactly equally. Q1 emitter current only peaks below that of Q2 because its base never goes above ground. Plot the two emitter currents and you will see this, note how the tend to equalize as the emitter resistor gets larger along with the negative supply, better approximating an emitter current source. With an high emitter current source impedance Q1 and Q2 emitter current equalize. When the total emitter currents are just enough to sustain oscillations with a high emitter source impedance Q1 and Q2 currents very close and just a hint of flat topping at the peaks, and the output voltage waveform looks like a nice sinusoid!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: August 01, 2022, 04:34:18 pm
content: Quote from: mawyatt on July 31, 2022, 11:27:49 pmFollowed your advice, thank you, plotted that and been surprised and intrigued, so plotted all trying to understand better.Next simulation is after replacing R with a CCS.In each of the two simulations above:- First plotting panel is with Vbe and Ie for each Q1 and Q2.- Second panel is for voltage and currents through L and C.- Added a series Rsense to help detect when energy is pushed or pulled out from the LC tank, and plotting that in the third plotting panel. Magenta is power on Rsense (so proportional with energy transfer with the LC tank, and the signum of the red plot I(Rsense) indicates the direction of the energy transfer. The right hand Y axis in the 3rd panel (indicating nW should be read as uW, because Rsense is 0.001 Ohm.Plots were already too crowded to step through different R (or I). Instead, looking at the different regimes the oscillator goes through from start to stable oscillations. (assuming a period of a lower amplitude - seen here during start - will look about the same as a stable oscillation in a schematic with a bigger R or a lower I)To ease the comparison of all plots at once between the two simulations, made an animated gif and stored it outside of EEVblog because the forum turns off any animations inside pictures attached here:TL;DR There's a lot of things happening there!Need to identify and decode each functioning regime.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2022, 05:29:44 pm
content: Here's a plot showing Q1 and Q2 Emitter currents and the Output Voltage V(n002). This is with a bias current with some rounding at the +-peaks, and you can see the sinusoidal output voltage. The second plot shows with a much smaller emitter resistor (510) and Vee (0.9V), note how Q2 emitter current is much larger on the + Output Voltage swing since Q2 Base goes above Ground pulling the emitter voltage shown as V(n006) higher.Would be interesting to see which of the bias cases produces the best SNR of the fundamental, the second case suffers from the larger peak emitter current of Q2 doesn't contribute to the output signal since it flows around the LC network.Note we used a startup current source to kick start the simulation and achieve a stable output with less time in both cases.Best,

author: Seekonk
author_rank: Super Contributor
timestamp: August 01, 2022, 10:25:15 pm
content: Oscillators aren't any fun unless you put a lamp in them.I had to build a calibrator for a product that had to trip somewhere from 20-100mv AC or DC. Exactly the same RMS value for both and of course it had to be adjustable. Did I say CHEAP. It used line AC and a DC source from that. I remembered seeing something in Electronics Design possibly as far back as the 70' that used a small lamp in a bridge. Line voltage could vary from 100V to 135 and it stayed spot on. Flip the switch and feed it DC and it was the same voltage.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 02, 2022, 01:39:52 pm
content: Quote from: mawyatt on August 01, 2022, 05:29:44 pmIf that is only to check if an amplifier is less noisy when driven into saturation, any amplifier should be so, if we think of noise as an input signal that would be amplified by the DUT amplifier. The Vout vs Vin plot looks linear only at small signal. For a bigger signal the plot shape looks like an S, and its slope (slope = DUT amplification) is big near zero and with no amplification at the ends of the S shape.Considering the signal + noise input as a small noise added on top of a big signal, then the big signal will put the amplifier in saturation and will act like a "bias" on the Vout vs Vin plot. When saturated the amplification is small for everybody, including for noise. A large swing of the useful signal will keep our amplifier mostly in its saturation regions, so the input noise won't be amplified either, and thus on average the saturated amplifier will show less noise at its output.In real life, this saturation and how it affect smaller signals can be observed as "blinding" (by a strong light source, or by a strong noise, etc.), when we can not see the rest of the image details because our senses were driven into saturation by the very strong "blinding" signal.I think the noise is "lowered" in a saturated amplifier just like the details are lost when blinded.The DUT doesn't see the noise any more because the DUT is blinded by the useful signal.If the F0/noise ration is needed for something else, then LTspice might help, but I think this requires special models that includes noise.Didn't play much with it, but LTspice has ".noise" type of analysis. There are also ".four" and ".meas" SPICE directives that can write the results in the log file, and they could be used in combination with a ".step param".The idea is to run two simulations:- one for ".noise" with ".step param R1", and a ".meas" to get a RMS noise value for each .step- one for ".tran" with the same ".step param R1", and a ".four" to get the amplitude of F0Then grab the data from the .log files (either manually, or with a script if the number of steps for R1 is very big) and calculate A2(F0)/NoiseRMSfor each step.On a second thought, I don't think the ".noise" simulation will include the "blinding" caused by a large signal, just like an ".ac" type of simulation doesn't include large signal behavior.Maybe just adding a noise source somewhere in the oscillator would be easier. Then run a normal ".tran" with ".step param R1", and then postprocess the result with a FFT to compute S/N ration considering only F0, as requested. On all these, overlap the noise from ".noise" at each R1 to include the internal noise of the circuit. Probably not very accurate, but better than nothing. (Not accurate because at large signal, the gain of the amplifier changes with the instantaneous amplitude of the signal, while in .noise the gain is considered constant no matter the amplitude.)Side note, a ".noise" with ".step param R1" for the Peltz oscillator will also show the frequency response of the circuit, just like one can measure the frequency response of an RF amplifier using a noise source and a spectrum analyzer.

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2022, 09:59:26 pm
content: Nice plots!! They show the effective oscillator "Q" growing as the bias is increased and reducing the effective bandwidth, however most SPICE types are linear noise analysis, so they don't take into account the non-linear circuit nature (Cadence has a non-linear noise analysis, but this is high end very costly simulation software) and it's effect on the overall noise performance.Regarding the noise sources and device linearity. One thing that occurs is that with heavy non-linearity various sources of noise get modulated into the signal of interest, this is why close-in phase noise wrapped around the signal appears. Much of this is from 1/f^n type noise from the active device(s), power supply rails, bias networks and so on, and this can corrupt things.Because of this non-linear noise modulation effect, the reduced noise from the active device(s) in saturation and/or cutoff can be sacrificed to the modulated noise causes by the saturation/cutoff, that's why we said "Would be interesting to see which of the bias cases produces the best SNR of the fundamental"Way back in the old days many seasoned engineers believed that the lowest noise higher frequency oscillators were built with 3/5 compound semiconductors (GaAs for example), we proved we good outperform any of these noise-wise with a silicon based design using the same resonators because of the much lower 1/f^n noise the silicon device possessed.Anyway, nice work and keep it upBest,

author: RoGeorge
author_rank: Super Contributor
timestamp: August 05, 2022, 09:58:10 pm
content: About that plot, thank you, I've noticed later something intriguing. Not only that a higher current will ruin the Q, but a higher current will also shift (lowers) the resonant frequency.At first I've suspected that the frequency shift might be caused by parasitic capacitance in the transistors, but it even happens in my 3.5kHz audio oscillator, where C is 220nF, a huge value in comparison with any parasitic capacitance in a BJT.Quote from: mawyatt on August 02, 2022, 09:59:26 pmWow, very clean explanation of how (amplitude) noise can turn into phase-noise because of non-linearities, thank you!Also, had to google close-in phase noise to find if that's a dedicated term.Google doesn't know very much about that, but had the luck to bump into a couple of papers that defined the term, one of which was this:QuoteSource:Close-in phase noise in electrical oscillatorsDOI: 10.1.1.119.8656In case your bias vs SNR to fundamental question was in regard to phase noise, then I'll expect that a lower I_tail will get a lower phase noise:- first, because higher bias lowers the Q of the circuit, and a lower Q oscillator is easier to perturb than a higher Q oscillator, at high Q a resonator will want to stay in its narrow band, and oscillate there at higher amplitudes, thus harder to be perturbed by a given noise.- second, because high bias current will produce oscillations with an amplitude big enough to enter into limitation, which means higher non-linearities, so even more noise will turn into phase noise by modulation on non-linearities.So far I've only simulate (last week), and did some measurements now on that sloppy breadboard built:- the frequency goes from about 3500 Hz (low current) to 2700 Hz (max current) then it stops- the skirt of the peaks seems to widen while increasing the current, but it's hard to measure precisely with how much- another observation is the 50Hz mains AM modulation becomes more visible at high current, which might mean that the oscillator is more sensitive to noises (assuming the induced mains hum is about the same, but this assumption might be wrong)The attached spectrum is for about 1 minute of averaging, with two measurements overlapped: red trace is for (potentiometer) R1=2k4 and green for R1=5k, osc powered at 5V from 4xAAA NiMH. The 50Hz is from the breadboard wires. For less than 2k4 the osc stops.Looks like more current will produce more phase noise, and will also lower the oscillation frequency in a Peltz oscillator. I'm curious if at higher frequencies would happen the same.Did you happen to have any measurements about the phase noise?

author: mawyatt
author_rank: Super Contributor
timestamp: August 06, 2022, 01:09:43 am
content: In relation to the network "Q" and phase noise, this usually involves Leeson's Equation which shows Phase Noise follows 1/Q^2.We have not made any PN measurements.One explanation of the added 50Hz around the "carrier" is with the higher bias and increased non-linearity, the modulating function has greater "gain" and converting the lower frequency mains up to and around the "carrier" just as if it were low frequency "noise" and appearing as Phase Noise.On the lowering of frequency, this may be due to the heavy forward bias CB junctions on the +- current peaks. This condition produces a large charge storage in the junction which "looks" like a heavy capacitance occurring at the +- peaks. A through detailed non-linear analysis might shed some light and may even show a oscillation dependency on the loop transconductance.Best,

author: moffy
author_rank: Super Contributor
timestamp: August 09, 2022, 01:03:56 am
content: I built my own Peltz oscillator with 9mH/2uF and got a neat 1.6kHz sine from 1.5v with 10k Re. Then I thought the Peltz oscillator would make a simple AM modulator, so I simulated the following, makes a fairly decent AM modulator.

author: moffy
author_rank: Super Contributor
timestamp: August 10, 2022, 01:42:52 am
content: I might be the only one interested, but I have improved the Peltz oscillator AM modulator by damping the Q of the resonant LC and injecting the modulating current into the emitters of the differential pair. This gave a much greater depth of modulation, and makes it easier to modulate the amplitude.Included is:1. A picture of the simulated circuit.2. A picture of the simulated output, with the green trace the point between L2 and collector of Q1. The blue trace is the output at the junction of C2 and R2.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 13, 2023, 11:31:32 am
content: Quote from: moffy on August 10, 2022, 01:42:52 amI'm all ears.Did by any chance tested it in practice? Does it works OK?Asking because I've copied last Sunday this AM modulatorand it worked very bad.My LC is a former AM ferrite rod antenna from a pocket radio, measured Q=55. Some waveforms here, captured with the 10x oscilloscope probe connected to a second winding of 6 turns on the other end of the bar (the main L has 69 turns):https://www.eevblog.com/forum/rf-microwave/how-can-i-improve-this-cheap-am-transmitter/msg4807103/#msg4807103Next day I've tried changing the PSF, then stripped the schematic to a minimalist Peltz osc, and still doesn't look good. In simulation works much better, so I'm curious if yours worked in practice as good as in simulation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2023, 03:34:11 pm
content: Did you try fixing the base bias? The unbalanced resistance to both sides I mean, 100k extra on the left side should run it pretty low. Maybe add a 10k to the right side (cap bypassed to GND) and change the 100k to 10k.It will always and necessarily run out of amplitude at low current; stable limit cycles require some minimum transconductance. Conversely, at high amplitude, Ccb and Cbe voltage modulation will dominate the FM aspect I think. I suppose feedback could be buffered (run it through an emitter follower? or maybe better yet, JFET source follower?) to help isolate Cbe, which should be the bigger effect. Ccb can be dealt with by using smaller transistors (MPSH10?) and higher supply voltage (more specifically: lower amplitude in relation to supply).Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: April 13, 2023, 04:11:44 pm
content: Quote from: T3sl4co1l on April 13, 2023, 03:34:11 pmYes, I've tried changing the bias at first. Sorry that I've wrote "PSF", that's the Romanian abbreviation for DC bias (PSF = Punctul Static de Functionare).Tried many types of BJT's, too, they perform about the same (700kHz). Hope to make more tests this weekend. For now I'm tempted to give up that schematic, and use instead any other known working MW AM modulator usually used by hams.