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- Опубликовано: 15 фев 2024
- This circuit fun video presents the Twin-Tee op amp oscillator - a fun circuit that can make low distortion (low THD) sine waves with simple resistors and capacitors, no inductors. It is a circuit you don't see written up too much.
Here is a link to the Texas Instruments article shown in the video:
www.ti.com/lit/pdf/slyt164
More appnotes on sine wave oscillators...
www.ti.com/lit/SLOA060
www.ti.com/lit/an/snoa665c/sn...
My show notes:
www.qsl.net/w2aew/youtube/twin...
Related FFT and THD videos on my channel:
• #65: Basics of using F...
• #305: Measuring Total ...
• #202: Basics of using ... 
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Here's an abstract for the transcript, focusing on the electrical engineering aspects of the video: (generated with gemini ultra)
*Abstract*
This video explores the design and implementation of a low-distortion sine wave oscillator using op-amps, resistors, and capacitors. The focus is on the twin-T oscillator, a design noted for its simplicity and lack of inductors. The video begins with a discussion of oscillator theory and then compares the twin-T oscillator to other common types of RC op-amp oscillators (relaxation, phase shift, Wien bridge). A twin-T notch filter is constructed and tested with a Bode plot to demonstrate its frequency response. Finally, the twin-T network is incorporated into an op-amp oscillator circuit. The oscillator output's low distortion is demonstrated through visual observation, FFT analysis, and a THD measurement.
*Keywords:* oscillator, op-amp, sine wave, twin-T, distortion, RC filter, feedback
Very eloquent! Pinned comment to the top!
I'm a retired electronic engineer, but I still learn something new from most of Alan's videos. He's a superb teacher. He prepares everything well in advance so that there's no fumbling about and he explains just the right amount of detail to make the videos useful. His pace is just right too, for me at least.
Nice scope Alan
I too, developed scope envy watching this!
You make a great teacher! Around 2001 I was taking my first EE course in college. I wanted to learn digital electronics, but couldn't wrap my head around analog concepts. I soon switched majors to computer science and didn't look back for 20 years. Then about 3 years ago I was bored during lockdown and started teaching myself electronics, mostly by watching RUclips videos and by experimenting with my new oscilloscope. At first I could barely explain the function of a resistor, but I've now made a few successful mixed signal PCBs (mostly in the digital domain, but there's some RC and LC filters and comparators in there). It's neat for me to now see this kind of video and immediately come away with a good understanding of the circuit. This is all to say, thank you and other RUclipsrs for making this kind of content!
When I worked as a product design engineer for an audiological manufacturer, they needed a variable frequency, audio oscillator whose frequency could be set by the processor. I "inherited" a design which used a switched-capacitor, bandpass filter as the feedback element with a "wide-open" op-amp. The BIG issue we had wasn't THD, but *phase* distortion that was totally hosing up audiological measurements.
As a little trivia background ... we would put two sinusoidal signals into the ear (via tiny speakers). These two signals would mix in the inner ear (a non-linear system) and a third, related signal would return to the probe (to a tiny microphone). We would measure the amplitude of the returning signal to determine the health of the entire audio chain from eardrum to inner ear and back. Pretty slick, eh?
Like what Nura does?
I got a degree in electrical engineering in 2017 with a big focus in RF.
I haven't used a lick of it since then as I ended up a draftsman in mechanical eng. Like learning another language it can fade quickly unused. But I want to get into it again and you have been a huge resource for me to refresh and learn a lot from among some other channels. Many thanks for everything.
Yep, this channel is a goldmine! I’ve learned a TON from watching his videos too.
That is basically my story. Now that the kids don’t need me that much anymore i got myself back into EE and got my amateur radio licence. And Alan is one of the best teachers around
It is always a pleasure to follow your videos and explanations. Many thanks for that
This channel, Old Hack EE, Joe Smith's channel, and the DC to Daylight series that Element14 is doing are the channels I'm leaning on to keep my brain refreshed on all the stuff I haven't really been using since graduating
I discovered this oscillator back in college while using twin-Tees to measure distortion in my homebrew amplifiers. I always needed a little positive feedback to ensure oscillation, which is different from what this fellow achieved. The good part about the positive feedback is that you can add AGC to keep the wave from hitting the rails and distorting.
Seeing others saying they wish they had these kinds of videos back in their time makes me nothing but more grateful to have access to these practical demonstrations. Please keep making videos like these.
Had to tell my wife that the honey-do list would have to wait, I have an important experiment to do! Thanks for freeing up my Saturday Alan! 73 de W3AL
Circuit Fun is my favorite segment of your channel!!! Please have more!!!
That's a nice video about sine wave oscillators. Back in the eighties in my 2nd year at university we had to design, build, and test our own sine wave oscillator. It had to be able to work in the frequency range from about 10Hz up to 100kHz. I chose to work with the Wien bridge principle, and that went well. Like you mention in your video, I used a FET as the controlling element. Most of the work went into the design of characterizing the FET, and creating a stable feedback loop, especially at the lower end of the frequency range. My design resembles a bit the design that you can find in the paper you mentioned. Distortion was below 0.5% between 100Hz and 20kHz, and it went up to just above 1% at the bottom and top end.
The limitation of tuning to a desired frequency is quite severe. Built these in the early '60s because I wanted a low THD sine wave for audio testing @ 1 kHz. Took days finding all sorts of MIL-surplus precision capacitors and resistors. The common carbon composition resistors back then were 5% and paper capacitors at 10% in the sub-uF range. I finally found some 0.1% Rs and some 2% Cs, but still too far from the desired frequency. So I spent days adding small resistors and capacitors in series and parallel to get to 1 kHz.
Also in the '60s, Popular Electronics Magazine had an article to make "Electronic Bongos" using Twin T oscillators biased just bellow oscillation and triggered by a touch pad and a differentiator. This made a damped sine wave which with some imagination sounded like small bongos.
Even today, generally available parts precision ranking is: #1 = Rs (1% common, 0.1% quite available) #2 = Cs (5% std, 2% available) and #3 = Ls (10% std., 5% available). Yes, all are available better than 0.01%, but they are often un-obtanium and un-affordium.
Keep up the great work! I was often called an OFAE, "Old Fart Analog Engineer", even in the last century, when I wasn't that old.
I to am a retired EE and enjoy your videos. Keep up the good work.
You're amazing for supplying these literature goldmines.
Ti holds a library of amazing documentation but its hard to find, awesome you put these out there.
More than greatly appreciated
Your circuit fun and basics videos are the best, very helpful to people learning electronics. Thank you.
Beautifully simple circuit with such a clean sinusoid, and that scope is pretty sick, great video. Loved seeing the bode plots and the harmonics
...a "sick" scope?!
These were used in the Roland 808 drum machine, producing arguably the most famous "beat" to ever exist
Cristal clear explained, thanks Mister
I'm always waiting for your excellent videos. Hope you can do it more frequently. I'am a rapidly aging southamerican eng. and your job is quite refreshing.
Regards.
...well, NONE of us are getting any YOUNGER...
Great circuit. Simple and elegant. Whish i had known about it before. Perfect for synchro resolver excitation applications.
BSEE 1975 retired.
great video Alan, as usual! yours is one of the best EE channels on YT! ciao from Italy! IA5IOM
Osci lator...nice one Mr. Wolke
*I like your videos. One of the key structures they demonstrate is there are single topics that can be tackled in both a scholastic and lab environment, and you don't need to spend weeks on it.* Once you have intellectually secured a structure in most the fundamentals and math for the science of engineering, circuit analysis coupled with a lab will adequately help us advance.
I was just studying Twin-T RC networks and associated the last couple of months for audio applications, so an oscillator relative is a great topic for my brain's configuration ATM. In music audio, T-notch filters are abundant in modern equipment. Even with them incorporated into common tone architecture both fixed and variable, *you still constantly see production engineers in recording studios mastering songs, and they're scooping the midrange out of the instruments to balance out the tonal structure and make it more pleasing to our ears.*
I think a lot of that need is running through so many active circuits, and the midrange one piece of gear notched out is brought back into the output bandwidth by another because such is the nature of amplification stages, *even when it's unity gain.*
oh, no. I've already recorded a video on this. Guess it will be dejavu when I release it.
I wonder if the same viewer requested it of you! I look forward to yours - we both take different approaches.
@@w2aew I just ran across it in some old book or magazine. 'hey that's interesting'
@@IMSAIGuy Cool - I look forward to it.
Im waiting video from you too!
You and Alan are very complementary, but you have more Beta. Maybe Alan has a lower frequency. 73
Hi, great video, I wish my Engeer Department were full off professionals like you. Allways well prepared presentations. PS: I love your scope.
In the 1980's I built a portable audio mixer to operate with a 16mm camera. It needed a bloop generator to put a synchronising burst of 1khz on the tape to coincide with a white frame on the film.
I used a Wien oscillator with a lamp for stabilising the frequency. Worked well.
I really enjoy these circuit descriptions, Alan. Thanks for sharing them!
Have not seen your videos pop up on my feed lately. Glad to see this one. It was very educational. Thank you.
Excellent video Alan.
Merci beaucoup, j'ai travaillé beaucoup sur les oscillateurs sinusoïdaux, je ne connaissais pas celui là
Another interesting and enjoyable video Alan. Thank you.
This is the circuit used for analogue drum synthesis for example the Roland TR808 until the early 1980s when sampling took over. Voltage triggers are fed from the sequencer, the twin T circuit can produce punchy sounding membranophone sounds like kick and snare drums, Tom Toms, bongos, congas, claves etc.
I experimented with twin tees in the early 70's. Using hybrid circuits, we built a twin tee brain wave feedback circuit, notched around 7 Hz. You could train yourself to relax and generate (7 Hz +/-) alpha waves that triggered a tone generator into earphones.
Lovely piece of work. Thanks.
Very interesting video, Alan! A pleasure to watch, with lots of information. Thanks!
A very respectable THD for a minimal complexity . Thanks for this , you've just given me an idea , to the breadboard I go.
Interesting topic , succinctly broken down . Gotta love those TI application notes too.
My response as always is an enlightened aah ! , rather than a muddled huh ? Much appreciated , though having my ideal mid-life crisis purchase MDO scope on your desk has given me a touch of envy .
Thanks in abundance , and a great day to you sir.
Wow, that’s quite elegant. Never heard of this type. Good paper, too!
It's a nice experiment, and I've done similar experiments, my conclusion is that it's very hard to beat even cheap DAC based devices. Something like the Scarlett 2 channel audio interface will output a 0.002% distortion sinewave at any frequency from 20Hz to 20kHz. I have not been able to achieve anything like that with any oscillator based circuit.
fun video, clean and informative also feel comfortable watching them!
Thanks for the video. Impressive distortion figures.
Always nice to see that theory matches reality. Btw.: nice scope
Excellent video as always. Much appreciated :)
Dear Alan, thanks a lot. As you stated in a comment below, Twin-T is not such a modern or common technic to build oscillators, and I thank you as your video helps me understand it from the designer engineer level: I have a device in my bench I need to fix and there are a 55 Hz oscillator, and I was searching to understand its principles and you clarified it.
I'm used to other types, or architectures and this one turned into a challenge/mistery.
The device measures insulating oil dissipation factor and the read value from a sample is compared to a Tan Delta known SF6 insulated capacitor, which has a very low and steady factor. In my case, device can't measure the sample tan delta, showing a negative result, and this is not consistent.
Comparing both sides of circuit helped me locate the fault, but why to build such a circuit? Device's reading error (precision) is up to 1% and THD below 0,1%.
I do thank you a lot for accepting my sugestion and producing this video.
Really cool! Thanks for showing this!
Thank you, excellent concise explanation as always.
This reminds me of using an RLC tuned circuit to obtain 50VPP for fiber optics modulators.
You always do an excellent job. Thanks so much.
Another great tip, thanks Alan! 73 - Dino KLØS
Great video lesson. My customer wanted to see a slow sinewave so, I dug into my Heath electronics course and built a twin tee for 1/9 Hz or so. My old Tek 464 with variable persistence displayed a smooth sinewave. I had to use large capacitors.
Nice circuit and presentation. Thank you.
Cool scope, nice informational video and thnx for the document.
Love your circuit fun videos! Wonder how low the THD would be with an additional filter stage at the output to attenuate the harmonics. Pretty fun circuit to play with.
Best ever channel! TNX!
So interesting and great fun to build.
In most cases, the oscillator part is easy-ish, the amplitude stabilization part dominates the distortion of those low-frequency oscillators. Any op-amp oscillator without explicit amplitude control is hitting op-amp nonlinearities to limit amplitude. If the resonator isn’t very high-Q, the distortion is pretty bad. A scope with less than 16 bits of ADC resolution isn’t the best tool to measure such distortion.
Yeah, a 50dB notch ain’t doing low distortion anything much without an amplitude control loop. A 100dB deep notch (good luck) will do the trick without an ALC. A crystal resonator will do that job admirably. But a twin-tee? Nope. It’s not like only the Wien bridge needs amplitude control. They all need it unless a very high-Q filter is used.
It is possible to have >110dB null with Twin-T, but the tempco's drift it too fast without complicated correction. Bridged-T is simpler and 75dB is easy across the audio band with cheap NE5532 (10:1 cap ratio with a dual linear pot).
SFDR of the scope input amplifiers is typically the limiting factor for distortion measurement. If you notch the fundamental (60+ dB) and only look at harmonics, 8bit scope FFT can see minus 100dB! I use a bridged-T Q=3 with a 12bit scope and have a thd floor about -130dB re 1vrms.
This is a great video! Amazing
Snazzy scope, Alan. This was a great video and I can’t wait to build it up
Excellent video. Thanks.
Awesome ! The first HP product was a bulb stabilized Wien Bridge Oscillator and Dr Hewlett did his PHD around this subject as well and I have been fascinated with it for some time now. I am currently using an OPA1656 (op-amp) and some good tight tolerance components and a CM7371 bulb also I designed a PCB, breadboards are fine but when you move to PCB its great as you know!! I have @1Khz 1v P-P a THD of 0.005% using a sound card an ARTA software to measure. Its just great fun and there is of course a bulb involved !! good old thermionics but its a shame at low power you don't get a glow ! So I have a great little low harmonic distortion sinewave generator and is powered by batteries of course and they are two 3.7v lithium cells that I strip out of 'old vapes' that people (disgustingly) just throw on the hedgerows where I live. So I have amassed quite a lot of cells I can configure how I wish for some of my projects. I have copies of those TI application notes to read now so thanks !..........cheers.
Cool little circuit design! Sometimes I see designs like this and wonder why didn’t I think of that??? 🤔 Lol
👍 Would be interesting to compare it in terms of THD with a Wien bridge oscillator that uses diodes (not a lamp).
For some more fun with this circuit, set it up so that it doesn't quite oscillate, and then when you kick it with a pulse it'll give a nice effect, like ringing a bell or similar.
I have done that and it is quite nice driving a speaker as a "bell" circuit. You can also set it up so that it clips when the signal is large and is sounds even more like a real bell. Bells have other modes that die away quickly after being struck leaving just the main tone. A bit of distortion mimics that.
Excellent video, thank you!
I just saw your video and was inspired to make an ASIC version. We got a 6MHz output and used 55x80um on Sky130 open source PDK. It will be fabricated on Tiny Tapeout 7!
Very impressive!
Simple, elegant, practical. Thanks. 73
Why is nobody commenting on your wonderfully bad sign off? "Oscillator" for "I'll see ya later"? You got a good groan out of me!
Haven't understand much since i am just learning, BUT the scope is a top notch! :) Thank you.
Thanks for another informative video. I have an old book by Ray Marston which shows a similar circuit using a 741 op-amp, so the type of op-amp doesn't appear to be critical.
Excellent!!!
Twin-Tee were used quite a lot in vintage analogue drum machines and electronic organ accompaniment percussion.
I had to look up how the 808 kick was done. As it turns out, not exactly a twin-T but rather a bridge-T oscillator which is a slightly different topology.
Oscillator 😂. Great video!
That was fun, thanks!
Great clip again,
Here is more research work:
It would be good to compare the phase noise and jitter of different oscillators.
Also, It also would be good to develop a formula for Q factor twin T filter based on R & C. That is how to get the best Q, when the product of R and C is constant so we set the frequency with that. Which one gives us the best Q when Xc>R or Xc
Have you ever discussed quadrature oscillators? This form of oscillator has the potential for various applications, including music synthesizers, signal scrambling, frequency shifters, ham and others.
...not yet...
Page 36, same TI paper.
Ron Mancini! Yeah, I know that article. :-)))
One time I've built a 500kHz oscilator on a single OpAmp (embeded in MC3361, the filter amp) by his design.
Started looking through some of my Op Amp specific books and was surprised that this was not mentioned in any of them except for "Operational Amplifiers: Theory and Servicing" by Edward Bannon. I do have probably hundreds of electronics related books and maybe this circuit was mentioned in a few of them, but was surprised at how little information there was in print. I'll have to pull out the breadboard and play around with this when I get some time.
I'll try this soon.
Nice demonstration, Alan. Not seen that one before. At least not knowingly!
U r inspiration, SIR
Just in time. Going to try this soon! I just recapped an old Sinadder and I need a good low distortion oscillator at 2000 Hz to calibrate it. The arrl book describes the wien oscillator with bulbs for a 2 two tone generator, but I don't like the idea of using bulbs in this age.
Old HP signal generators used the lightbulb trick to make a very clean sine wave. Basically that was how the company got its start.
I made regenerative receivers from Wien bridge, twin T network and phase shift network circuits. :)
That's a sweet 'scope!
Very cool ... Thank You for sharing .. Cheers :)
Thank you for the video! The notch filter is very interesting by itself. Your voice sounds a bit strained. If you are under the weather, I hope you feel better. : )
On my cheap Rigol 1054z, I was able to get an audio frequency Bode plot by using the external trigger, fed from/to? the Aux-Out on my SDG2042x. The smoothing really helps clean up the visual Moirés ( the sic visual harmonics ) on the scope.
Seeing the even harmonic distortion, I am reminded that I've been trying to learn ways to make even-harmonic distortion, to emulate tube distortion. Tube distortion is mostly even harmonics, well except for pentodes like the EL-34, which is probably not the best guitar-amp power tube, when one thinks about it. Unfortunately, adaptions of the circuit would only probably make distortion on the notch--err-peak frequency. My friend said that sawtooth waveforms tend to make even harmonics. From what I have seen, the distortion from a tube tends to be an exponential drop-off of 2, 4, 6, etc. harmonic series. When I get rich, I will look around for an old spectrum analyzer from ebay that goes down DC. I have a Hack-RF one, which are supposed to make a decent beginner spectrum analyzer, but I have to update it first. Still, that's not going to help me for audio. Hmm. Audacity?
I once read an article in IEEE Times, that stated to the effect that harmonic content aside, tubes make more distortion with a lower shelf noise (noise-floor) than is possible with solid state (non-digital) devices, such as transistors, FETs, and I think, MOSFETs.
If a guitar waveform were only simple, another wave could be mixed in with a mixer(ring modulators), but with it being pretty full, it would sound processed. Perhaps the reason why diodes and transistors don't really make tube-like distortion might be partially caused from the threashold/cut-off making a squarish crossover-like distortion. (Like seen in the LM358s, as shown in your video.)
[Ironically, I really liked your "#82: How to use a Diode as a Switch" video because beginner electronics learners aren't taught the very basics of how a diode's depletion region works. If you put a dozen electronics people in a room, and mention that, sure you can pass alternating current through a diode, it will trip-up a few people--yet, this configuration exists in transceivers. In watching, TRX Lab (great channel, too), as well as looking at the schematic for my Yaesu VX-8, I've seen quite a few dual-diodes (perhaps some PINs) in a single package, for signal chain switching. BTW, for digital, Jerry Walker has a great channel, as well.]
[Generally, I think that the tube distortion sounds pleasing because it adds a high-frequency um...sizzle, and I think that psychoacoustically, it sounds pleasing because our hearing/eardrums likely tend to naturally clip and distort with loud noises, which would hurt us a bit, BUT, if it's not actually loud enough to do that, then we are being subjected to something that something that would have hurt--but does not. It's kind of like going to a horror movie, rather than being eaten by a real monster. There's a difference, you see. : ) ]
Thank you again for making your videos. I think I have learned more from them than any other on the internet.
Very nice...
Interesting subject to get into and play with it. Thanks for the idea. This is a very straightforward and easy to understand circuit. Probably not suited for high frequencies as it gets more and more difficult to make a good noch..? It seems simple circuit at the core, but it can get very complex if you would go and research the relationship between the max damping, Q of the noch to the fact does it starts to oscillate or not, and the stability... But as always very inspiring content and indeed osci-later! 73 😀
I might try a few hundred kOhms between output and inverting input. A little negative feedback to stabilize the gain and then adjust the filter for maximum signal.
Thanks 👍
Когда то много лет назад программа EWB зависала насмерть, при попытке симуляции этого двойного т-моста
Hi, I like your calculator.
Alan abi yine harikasin
Great video as always. Is the MSO Series 2 new?
Yes, introduced last year.
Hi Alan , great video!
If I understand correctly, the feedback is provided to inverting terminal of opamp, so at any other frequency except the intended one the opamp is in negative feedback, but at the notch frequency due to reversal in phase in the filter, now it becomes positive feedback with high gain and therefore an oscillator
Is this correct?
Very good video. In need a 22khz filter circuit. I am going to experiment with putting the twin-t network in the feedback and see how it works as a filter.
Putting the twin-tee notch in the feedback path "inverts" its response, turning it input a narrow bandpass filter.
@@w2aew I need a sharp bandpass filter. I was not very clear on my comment. It will be the first active stage in a 22khz detector circuit after a voltage follower.
"Oscillator = I'll see later" - w2aew
Another great video. I love that scope but my first true lovebis your other one haha. Ive been looking for an older tek curve tracer. I have about 1400cad. Im wondering if there's a model you know off hand that goes for that price range. Im not picky about the age or if its super high end. Im building some guitar pedals and want to sort a bunch of parts i ordered. I plan to donate it later this year to a makerspace so others can use it.
You should be able to find a 577 for that price range.
Very nice! Is the primary cause of the distortion in the output because it is in a near clipping state?
Primarily, yes.
Can't say I'm a twin-T fan...due to the tuning difficulties. Years ago, I had the joy of doing production calibration of a device that used a twin-T 60 Hz notch filter. The time & frustration in adjusting three multi turn trimmers to both hit the frequency and get enough rejection was ridiculous, and doubtless subject to drift in the field. Really bad design choice.
I recently got the same scope. How you finding it? I also coughed up for the 100MHz bandwidth, source and serial, and logic analyser options. Added a bit to the cost but definitely worth it (for me)