Dual-frequency (simultaneous dual resonance) analog sine wave gen with 1 op-amp? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: June 16, 2019, 02:08:50 pm
content: Just speculation...I just wondered:(a) If a mechanical object could resonate at 2 frequencies (seems no problem)(b) how about a Wien-bridge type of circuit? The output would contain 2 distinct sine waves.However it would have to be "interesting" - not just 2 separate Wien-bridges with the outputs 'mixed'. I imagine it would use 2 f/b circuits, but can it be done with say 1 (single) op-amp?

author: OM222O
author_rank: Frequent Contributor
timestamp: June 16, 2019, 02:23:26 pm
content: Any object can only resonate at one specific frequency afaik. It will also resonate at other harmonics of that frequency too, but not at two different frquencies. I.e: it can oscillate at 100Hz,200Hz,300Hz, etc. But not at 100Hz, 170Hz etc.Maybe there is some specific device made to be bale to do that, but I'm not aware of such thing. Besides op amps are so cheap, it would be pointless to use one rather than 3 (combine outputs of 2 using a 3rd one) especially sine quad op amps exist for the same price of the single packag3 for the most part

author: chris_leyson
author_rank: Super Contributor
timestamp: June 16, 2019, 02:52:46 pm
content: It's an interesting concept. I can't think of a feedback network that would gererate two feedback signals at the same phase let alone the same amplitude. If your amplitude is slightly off in one direction you either have no oscillation or too much. I think to get this to work you would have to use a super linear 100% AM modulator after the oscillator and maybe you could put the modulator in the feedback path. I think a lot of engineers must have looked into generating squeaky clean two tone signals for distortion measurements and I would opt for the linear AM modulator approach.

author: soldar
author_rank: Super Contributor
timestamp: June 16, 2019, 03:08:07 pm
content: To oscillate you need loop gain >1 and feedback with 180º phase change.The amplifier will provide gain at any frequency.The feedback network will only provide the correct phase shift at a single frequency.But what happens is you have two feedback loops? Do they work separately to provide two separate frequency oscillations? Or do they combine and provide a single oscillation at a different frequency?

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 16, 2019, 03:14:38 pm
content: I don't know of any kind of oscillator that can oscillate simultaneously at two independent frequencies. Obviously if they are harmonics, yes. Most actual oscillators generate harmonics!The only weird kind of oscillators I know: so-called chaotic oscillators. Chua's circuit is an example of this:https://en.wikipedia.org/wiki/Chua%27s_circuit(Attached is one possible implementation with a single op-amp.)It certainly generates several frequencies but those are not fixed.

author: 741
author_rank: Frequent Contributor
timestamp: June 16, 2019, 03:17:35 pm
content: Re "...can only resonate at one specific frequency..."I was imagining say a "dual pendulum" (one tied to 'bob' of the other), or a 'spring + weight' linked to a 'spring + weight'.There are also some simple set-ups where you can observe resonance passing from one part to another and then back again. Something like tie two pendulums with different time constants to one freely moving but rigid support. You see one pedulum go fully resonant, then quieten, then the other starts, then back again.Maybe it depends on the definition of 'object' though. Is a guitar or a car an object for the purposes of this discussion for example?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2019, 03:23:26 pm
content: I see no principle problem with this. The amplifier can work over a wide frequency range, and a feedback network can provide 180 deg. phase shaft at multiple frequencies. E.g. Crystals can resonate not only at their fundamental, but also at high frequencies that are usually not exact harmonics.A tricky part could be stabilizing the amplitude separately for the different modes. This often includes nonlinear parts - so there would be some interaction between the modes. As there is quite some interaction the result would likely be 2 couples frequencies. Even with only weak coupling there is a tendency for synchronization - with more coupling this would limit the frequency ratios quite a bit, not to just harmonics but probably to relatively simple ratios, especially if the resonant elements are not high Q.So exactly for thinks like the 2 tone test one would definitely not use this, but two oscillators that are well isolated from each other.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2019, 04:41:08 pm
content: Mechanical systems have nearly infinite degrees of freedom -- a straight bar in free space, for example, has not only a 1st order flexural mode, but 3rd, 5th, etc. (or other harmonics depending on fixed points), and that's flex in two dimensions (flexing along the longitudinal axis). And there's stretching modes, not just longitudinally but across as well. And torsion modes. And surface acoustic modes, both compressive and shear. And...In short, mechanical systems arereally, really richin terms of wave mechanics!We have a mere taste of this in electronics, with transmission lines (single propagation mode, harmonic resonant modes), waveguides and fiber optics (same things, really) (multiple modes above the first), and free waves (which really just do whatever they do until they get to an antenna, where it goes back down to one-dimensional transmission line). And EM waves are transverse propagation mode only, with the two polarization axes.Mechanics also matter at much more accessible frequencies. The speed of light is quite fast indeed, so that we can build relatively fast circuits (10s, 100s MHz) without even having to worry about transmission line effects; whereas the speed of sound in most materials is so low that even, say, a car engine running at 100s Hz must be, well---you can't really do much of value from first principles, you have to model and test and evolve the design of the damned thing to get anywhere.Regarding frequencies, mode conversion isn't at all uncommon in mechanics. While this cannot happen in a linear material, there are many nonlinear behaviors -- flexing an oscillating beam for example, modulates the rate of the oscillation. (Consider the bowed saw instrument.) Harmonic generation and mixing is common through various nonlinear flextures, and rolling and colliding contacts, and so on.Anyway, as for multi-frequency oscillators -- given this, given that we are much more limited in the ways we can make our electrons move -- there are still ways (of course).The history goes back to I think early Western Electric DTMF sets, which used a single transistor (they were expensive back then!) and a keypad matrixed selection of LC tanks, carefully tuned for the frequencies required.The trick is to allow both oscillation modes to persist, without one dominating the other, or the combination resulting in squegging or something else like that.The problem with the normal oscillator circuit is, the amplitude of the amplifier itself is the limiting factor. Which will mix both modes making IMD, and whichever one has most gain will win out.If we take a theoretical step back and consider the fundamentals of an oscillator, we might stand to synthesize a circuit based on it. If we take a normal (lossy) tank, and connect a negative resistance across it, we get an oscillator (amplitude grows without bound). If we put a nonlinear element across the tank, so that the amplitude grows to a limit, then it will stabilize. As long as the negative resistance remains negative, we could even put more (parallel) tanks in series with it (or more series-resonant tanks in parallel, same thing), and they'll all do their own thing independently.So the trick is to find:1. A component which limits tank amplitude, and2. A negative resistance network, or amplifier, that remains reasonably linear over the total amplitude and bandwidth required.Note we need a limiter that acts without severely affecting the tank itself -- a simple clamp will affect the frequency, and the oscillator will "chirp" undesirably.The answer isn't very complicated (a great relief, as these theoretical explorations can often go very differently!). A negative resistance, over a reasonable range of amplitude and bandwidth, can be made with a single transistor, a transformer, and some assorted resistors and capacitors for biasing and setting feedback gain.The limiter can be something like a MOV, perhaps with some series resistance to soften its detuning effect. (How much resistance can be afforded? Well, not more than the negative resistance, that's for sure!) A lamp (as seen in many Wien bridge oscillators) probably can't be used, because it will respond quite slowly. Or maybe it'll work out in the end, but the startup transient will have undesirable AM on it, as the two modes fight it out initially.With an op-amp, simply use an impedance converter circuit, then connect it to a pair of LC networks, each of which has a limiter on it (which might be back-to-back LEDs with some series resistance, say).Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2019, 06:08:59 pm
content: Quote from: OM222O on June 16, 2019, 02:23:26 pmMechanical objects can resonate at many different frequencies not all related by harmonic series or any other specific formula. Simple resonators like guitar strings or tuning forks are deliberately designed so that only one low frequency mode and unavoidably some of its harmonics are easily supported, but that is not the common case. For instance, a rectangular metal plate will have bending modes at two different frequencies (depending on the relative lengths of the rectangle sides), plus twisting modes, breathing modes, shear modes, and so on.Likewise an electrical circuit can have many (quasi-)normal modes determined by its structure.The troubles with turning that into a multi-tone oscillator are mode competition and intermodulation products as T3sl4co1l described.

author: duak
author_rank: Super Contributor
timestamp: June 16, 2019, 06:42:47 pm
content: There is similar prior art with the Reflex Receiver:https://en.wikipedia.org/wiki/Reflex_receiverTo minimize cost one active device amplfied both RF and AF. There was a design for one in my Handy-Dandy 101 electronic projects breadboard that I got when I was a kid.I'd bet it'd be easier to design one if the two frequencies are far apart. If the frequencies are close together, I'd think the Intermodulation Distortion performance of the amplifier could have a big effect.Sometimes the amplifier in an oscillator will have unintended oscillations at high frequency that are not harmonically related to the intended frequency. Does anyone remember snivets or Barkhausen oscillation in vacuum state devices?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2019, 07:18:31 pm
content: Modern power MOSFETs exhibit a similar symptom when switching, if a low impedance is present on the gate terminal (effectively making a grounded-gate oscillator with device capacitances and package parasitics). Though the physical mechanism differs.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: June 16, 2019, 08:15:54 pm
content: Quote from: 741 on June 16, 2019, 03:17:35 pmwanna see a crystal oscillator running at several frequencies simultaneously ?https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuts-some-unusual-behaviour-of-crystal-oscillators/

author: soldar
author_rank: Super Contributor
timestamp: June 16, 2019, 09:03:53 pm
content: Quote from: duak on June 16, 2019, 06:42:47 pmThanks. That is very neat. I had never seen that before.

author: emece67
author_rank: Frequent Contributor
timestamp: June 16, 2019, 10:19:11 pm
content: .

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2019, 11:49:08 pm
content: Quote from: duak on June 16, 2019, 06:42:47 pmThose are effectively the same thing as emitter/source follower oscillations when driving a low impedance load. The input series inductance and base/gate/grid shunt capacitance form a Colpitts oscillator if the tank resistance is lower than the negative input resistance. Tetrodes have a negative resistance kink in their transfer function at low anode voltage due to secondary emission.The solution for either is the same; add a lossy element in series with the base/gate/grid or emitter/source/cathode which is greater than the negative resistance.I would not really count this as dual resonance since it does not involve the feedback network. It is just parasitic oscillation.I could see making a feedback network which satisfies the criteria for oscillation at more than one frequency but the gain also needs to be controlled at each frequency. That is an awful lot of work compared to just using two or more separate oscillators. I think I have seen some RF oscillator designs which used duplexers to support multiple tank circuits with one active device but it was not to operate at more than one frequency simultaneously.

author: Wolfgang
author_rank: Super Contributor
timestamp: June 17, 2019, 12:08:16 am
content: Quote from: David Hess on June 16, 2019, 11:49:08 pmIf you look for useful dual resonances there are SC cut crystal oscillators where the secondary (b Mode) resonance is tracked for exact crystal temperature measurement.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 17, 2019, 03:25:53 am
content: Quote from: duak on June 16, 2019, 06:42:47 pmA superegenerative receiver comes closest, (the "self quenching type), but the very purpose ofthe quench signal is topreventsustained RF oscillation, whilst retaining the "loss cancellation"properties of positive feedback.

author: 741
author_rank: Frequent Contributor
timestamp: June 17, 2019, 11:27:09 am
content: Part of the reason for this post was to see if an elegant solution can be found, for instance adding a handful of discrete components to a Wien bridge.Even if the solution is complicated though, it might well inspire some new circuit design 'shape'. It would be nice if both frequencies were prime, eg 11KHz and 17kHz.I appreciate the comments about [a seeming need for] separately controlling the 2 amplitudes. I speculate maybe their sum (the circuit output) can be controlled as one. However, it may be that this allows one tone to dominate or something.

author: Wolfgang
author_rank: Super Contributor
timestamp: June 17, 2019, 12:03:38 pm
content: Quote from: 741 on June 17, 2019, 11:27:09 amIt *is* possible to control the two amplitudes separately, by using a diplexer approach and separate gain control.What do you do with this circuit ?

author: 741
author_rank: Frequent Contributor
timestamp: June 17, 2019, 12:52:56 pm
content: Quote from: Wolfgang on June 17, 2019, 12:03:38 pmI do not have an application in mind - this was just an idea that interested me "for its own sake".

author: Wolfgang
author_rank: Super Contributor
timestamp: June 17, 2019, 01:54:46 pm
content: Having fun and to learn something is the best of all reasons.

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2019, 09:56:06 am
content: Yes, an oscillator can output two frequencies. Often this is unintentional, such as a crystal oscillator exciting two modes or the crystal or an op-amp being unstable and generating a much higher frequency, as well as the intended oscillator frequency.

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2019, 07:07:19 am
content: Quote from: Wolfgang on June 17, 2019, 12:08:16 amI remember those now. But didn't the other mode use a separate oscillator?

author: Wolfgang
author_rank: Super Contributor
timestamp: June 19, 2019, 07:29:42 am
content: ... normally yes. But it can also happen (unintentionally) in the same Colpitts type oscillator.

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 19, 2019, 05:29:36 pm
content: Quote from: T3sl4co1l on June 16, 2019, 04:41:08 pmThat's a great example. Here's a schematic of the Western Electric 2500 Touch-tone phone:http://www.repeater-builder.com/tech-info/dtmf/bell-dtmf.htmlOnly one transistor to generate two tones. Lots of DIACs, it looks like.

author: duak
author_rank: Super Contributor
timestamp: June 19, 2019, 06:01:48 pm
content: In the early 70's the local telco was throwing out some stuff and I recall getting a manual that had a schematic similar to the above DTMF generator. So, even in 1967 (from a schematic on the above website) it was cheaper to use one transistor, two DIACs and two custom multitapped transformers? I guess they were tooled up and the circuit worked so why mess with success?