vco - Need a circuit that control frequency of sine wave with same amplitude

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AHMED SALLAM

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i need circuit that control freq of sine wave with same amplitude when change dc input voltage (linear vco)
i research for long time to design it but dont work all time
 

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Hi,

so what does "same amplitide" mean?
* constant amplitude?
* or with increasing frequency you also want increasing amplitude?

For that latter:
* either use a "multiplier circuit"
* or use a "differentiator". May be a passive high pass filter with very low cut off freqency.

Klaus
 

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constant amplitude
 

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What is freq range desired, and Vcontrol for that range, what V range is that.

Any concern about latency from a step change in Vcontrol to freq out ?

Distortion ? Harmonic distortion needed ? Any special T and V requirements ?

Accuracy of Fout = K x Vin ?

Analog Devices make some pretty good VCOs. CD4000 logic family CD4046 has a fairly good low freq VCO in
it, although you would have to convert its square wave output to sine.

Some papers and ref material :

Internet Archive: Digital Library of Free & Borrowable Books, Movies, Music & Wayback Machine

archive.org

Internet Archive: Digital Library of Free & Borrowable Books, Movies, Music & Wayback Machine

archive.org


Regards, Dana.
 
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Single chip design, many other resources available onchip if you need them,
see right hand window for resources used/left.



If you need stuff like phase modulation use 24 Bit DDS which has that capability. You can also do stuff
like this also all onchip :

Test Bed DDS and WaveDAC burst tool

Occasionally I need to generate custom wave forms and burst a known number of cycles into a DUT. This is a single chip solution, board I typically I use is $ 10. You just drag and drop components onto schematic, wire internally with a wire wizard tool and out to pins, right click and config...
www.edaboard.com


Regards, Dana.
 

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Hi,

this is not a true VCO solution. But the same came into my mind also.
You may control the frequency by wiring the analog signal to a microcontroller that cantrols the DDC chip.

For sure you get rather good quality sine, stable frequency and stable amplitude ...

***
Analog solution:
Analog solutions all go back to the oscillation requirement for 180° phase shift and gain of 1.
But this is not simple to a achieve.

If gain is a bit smaller than 1 then the amplitude will continously decrease .. down to zero.
If gain is a bit higher than 1 then the amplitude will continously increase until it gets clipped causing some distortion.

For sure there are solutions to improve overall performance.
Thus it´s really a challnge to get clean sine, stable frequency and stable amplitude from a VCO.
Not a simple task.

Klaus
 

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this is not a true VCO solution.
Click to expand...

Huh, and I thought that if I apply a V to a pin and get a changing freq sine with stable
amplitude out on another pin it was a "true" VCO. So now I have to call it a "false" VCO
solution......

So Vin = Freq out, and if I dont tell you whats in between then can it be a "true" VCO ?

Could have fooled me. The Barkhausen condition not the only way to create oscillators.
And of course that condition is not adequate when discussing oscillators -

Barkhausen Stability Criterion and when feedback can be considered positive

My analog design book says the following: "As a signal \$ S_\epsilon \$ propagates around the loop and returns to the summer \$ \Sigma \$ as \$ S_f \$, it experiences a frequency-dependent phase
electronics.stackexchange.com

You may control the frequency by wiring the analog signal to a microcontroller that cantrols the DDC chip.
Click to expand...

My example the DelSig controls the DDS. Although in the part is a CPU totally capable
of controlling the DDS and messing up my "false" VCO.


Regards, Dana.
 

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Hi,

Wow. It´s not the first time you get triggered when I reply to a post of you. So, what´s the problem?
I even wrote that I thought about the same as you ... and even explained the difficulties of a VCO (VCO as described by Wikipedia, or AnalogDevices, or whatever result you get on an internet search for "what is a VCO"). .. and I even explained the benefit of a DDS solution.

So even if I positively validate your post ... you´re getting emotional ... and forget that this is a technical discussion. And different opinions are just normal (And non_different opinions should be allowed, too. Or don´t you agree?).

if I dont tell you whats in between then can it be a "true" VCO ?
Click to expand...
Counter question: If a diesel engine in a vehicle provides power to charge batteries .. and these batteries are used for electro-motors that move a car ... is it still a true electric car? Or a true diesel car?
[IMHO: usually it is called a "hybrid solution" .... and "hybrid" is nothing negative. It just combines "two technologies" .. usually to a solution with new features]

You mentioned Barkhausen and provided a link to a forum thread.
Did you read the thread?
One sentence says: "the original Barkhausen criterion was (and is) a necessary criterion only. That means: When a circuit oscillates it will fulfill this criterion."
So for the OP .. what´s the key-information from this link? Wrong/flase/good/bad.....?

On my "analog solutions ..." statement you argumented that "The Barkhausen condition not the only way to create oscillators". Fair enough. Can you provide an example? You underline your knowledge .. and I´m open to learn.

danadakk said:
My example the DelSig controls the DDS. Although in the part is a CPU totally capable
of controlling the DDS and messing up my "false" VCO.
Click to expand...
Re-read my sentence ... does it support your solution, or did I counter-argument?

I´m not familier with this IDE. So forgive me that I can´t see how the DelSig controls the DDS.
Indeed I don´t need to understand ... you addressed the solution to the OP, so it´s more important that the OP understands.
Don´t know what you mean with "messed up" at all.

Klaus
 

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In analog circuit design forum, I would expect a voltage controlled analog sine oscillator in the first place. No question that digital signal processing can achieve higher performance in some applications but it's not useable for all. The OP didn't yet specify any requirements.
 

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Examples here : https://en.wikipedia.org/wiki/Barkhausen_stability_criterion # 5 foot note

Your right, so much emotion in that earlier post (see the following) :


While you are at it can you tell me what a "true" VCO is, never learned that....., what
differentiates it from a non true VCO ?

I´m not familier with this IDE. So forgive me that I can´t see how the DelSig controls the DDS.
Click to expand...

The DelSig is an A/D converter, so Vin is digitized, and that value converted passed on
to DDS via code, hence V to F frequency control. Could have used DMA to do that, but got lazy.

I guess maybe I should have posted my prior post in the Mixed Signal forum, live and learn.


Regards, Dana.
 

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The trick for analog design would be to maintain constant
amplitude over varying frequency. And of course small
matters like intial accuracy, stability, distortion, drift with
environmentals, supply ripple FM-ing the output tone, etc.

If you wanted it all, something like an ADC feeding a
PLL divider code, controlling a VCO against a crystal ref
/ clock-in-a-can, throw VCO output to a counter and
sine-code ROM to a fine DAC, would put clocks well
out of band and need only a small output filter. Output
amplitude then depends mainly on DAC VDD / Vref and
downstream contributions.

Mighty busy for a breadboard, but totally normal for a
mixed signal ASIC.
 

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This digital method yields unchanging amplitude. By putting proper weighted resistors on a shift register it produces quasi-sine shape. The clock must be several times the frequency you desire. The clock can be square waves from a VCO in a 4046 IC (or whatever you choose). A smoothing capacitor removes the jaggies.

To obtain finer resolution, add more flip-flops to the sequence and revise resistor values. Notice a load can be placed across outputs 1 & 2, in order to obtain genuine AC bipolar waveforms.

This simulation is spun-off from the circuit included in Falstad's menu of included circuits (Digital Sine wave).



Link which navigates to Falstad's website and runs the above simulation on your computer:

tinyurl.com/27xbju9c
 

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danadakk said:
While you are at it can you tell me what a "true" VCO is, never learned that....., what
differentiates it from a non true VCO ?
Click to expand...
I explained it, I gave an example, I talked about the internet search and it's hits like Wikipedia, AnalogDevices... What else do you need?

danadakk said:
Examples here : https://en.wikipedia.org/wiki/Barkhausen_stability_criterion # 5 foot note
Click to expand...
Call me dumb ... but I can not find a proove for your "The Barkhausen condition not the only way to create oscillators".
I found several times that an oscillator needs to satisfy Barkhausen ... but also they say that circuits that fulfill Barkhausen do not necessarily oscillate.
I can not find a proove that there are analog oscillators that don't satisfy Barkhausen.
So what is "the other condition" to create an analog oscillator?




BradtheRad said:
This digital method yields unchanging amplitude.
Click to expand...
True. So far. I've used similar circuits for tracking mains frequency, this means for a rather narrow frequency range. Where it works great, btw.

Sad that the OP gives no information about the expected frequency range (and other specifications)
But as soon as you add an analog "smoothing filter" ... you are (very) limited in the frequency range.
The filter needs to pass the fundamental ... while suppressing the overtones. Overtones are starting at 3rd here (although 3rd amplitude well reduced compared to a square wave).
Indeed this solution comes close to a DDS ... with it's DAC style output. The benefit of the DDS is the usually much higher and fixed output sampling frequency and the much lower step size, which makes the analog reconstruction filter more useful for a wider frequency range.

In the end the OP needs to give more specifications so we can decide which solution is suitable .. or not.

Klaus
 
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Barkhausen, last time on the homework, page 15 on....attached

And I repeat :

While you are at it can you tell me what a "true" VCO is, never learned that....., what
differentiates it from a non true VCO ?
Click to expand...

Your answer is ?

I can not find a proove that there are analog oscillators that don't satisfy Barkhausen.
Click to expand...

As pointed out earlier in references Tunnel Diode oscillator comes to mind.


Knight
--- Updated ---

@OP, if you choose a digital approach you need to be concerned with wave table size
versus harmonic distortion. I made some measurements and got the following results :

Sinewave Distortion vs Table Size

Update from original post, this update is 06/15/2021 Done using DAC fed by DMA Table of various size., No post filtering applied. Spur at 12.4 Khz still trying to ID. But basic performance should still stand. Note Third Order ignores the non harmonic components. I am still trying to ID the...
www.electro-tech-online.com


Regards, Dana.
 
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danadakk said:
Barkhausen, last time on the homework, page 15 on....attached
Click to expand...
I have no clue what you refer to. I can find nothing about "homework", "attach...", "page 15". WTH are you talking about?

danadakk said:
Your answer is ?
Click to expand...
... already given.

danadakk said:
As pointed out earlier in references Tunnel Diode oscillator comes to mind.
Click to expand...
Thanks, this really is useful information. Never heard about it, learned something new.
Is it something you would recommend for the OP?

Klaus
 

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Attached, page 15. Prior post uploaded, did not display in saved post. Huh....


And then we have this, another attempt at attach. Analog "true" (whatever that means) oscillators that cannot oscillate.

Knight
 

Attachments

  • balaz2013.pdf
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Hi,

I don´t get it.

What exact information can we get from these documents
* that is not written in the thread already?
* or that is an counter-argument to what is written in the thread?

The title of the one document is "Barkhausen Criterion and Another Necessary Condition for Steady State Oscillations Existence"
Does it say Barkhausen criterion is necessary or does it say it is not necessary?
This document has no page#15

The other document, page#15 says: " A discussion of the Barkhausen Criterion which is a necessary but NOT sufficient criterion for steady
state oscillations of an electronic circuit."
Does it say Barkhausen criterion is necessary or does it say it is not necessary?

I really don´t understand what you want to tell me / us. Please tell me which information I have to focus on. .. or even better: the OP has to focus on.

Btw: I fully understand that there is a tunnel diode oscillator that does not fulfill Barkhausen.

Klaus
 

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Paper -

Barkhausen Criterion and Another Necessary Condition for Steady State Oscillations Existence
Click to expand...
Conclusion


Paper -


So post #7 (below snippet) is not sufficient (which for most of us we learned the below inaccurate criteria in school),
eg that we now know Barkhausen is not complete, not sufficient design criteria.


Analog solution:
Analog solutions all go back to the oscillation requirement for 180° phase shift and gain of 1.
But this is not simple to a achieve.
Click to expand...

Knight
 

Attachments

  • 1715937376791.png
    10.1 KB · Views: 53
Last edited:

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Hi,

In short: It´s still a mystery to me what you want to teach us.

I asked a couple of questions (delimited with a question mark). Did you answer a single one .. with a clear statement?

There is only one alsmost complete sentence of you:
danadakk said:
So post #7 (below snippet) is not sufficient (which for most of us we learned the below inaccurate criteria in school),
eg that we now know Barkhausen is not complete, not sufficient design criteria.
Click to expand...
But what do you (or should we) focus on?

I can only guess that you focus on "not sufficient" and "not complete":
If so: Nobody before said it is complete. But isn´t it still valid? (with the learned exceptions).

Nobody said "the only thing you need is a gain of 1 and a phase shift of 180° ... and nothing else."

What about some kind of amplifier... and power supply ...

And how is all this discussion related to the OP´s question?

*****
For making coffee one needs coffee beans and water. True or false?
Others may say you need a heater, you may roast the beans, for some special coffee you need a cat to eat (and s**t) the beans, sugar, milk, you need a person to do the job, ....
All true. Nobody denies that. Still "coffee beans and water" is valid for 99% (guessed) of prepared coffee.

Klaus
 

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