Variable Sine Wave Generator Design

Dean Huster · Oct 27, 2009

I guess in my book, if one wants a sine-wave (only) generator, he wants a good sine wave, i.e., low distortion. For that, I'd avoid any function generator chips and try working with a Wein bridge circuit. Distortions as low as 0.05% are easy with that. Better than a pot for frequency adjustment, use a FET-input op amp and a dual 365pF variable capacitor with the gangs in parallel. The resistors in the circuit will have to be higher, but the variable cap gives the advantage of having no noise or steps in the adjustment -- truly infinite adjustment.

Dean

Frosty_47 · Oct 27, 2009

hdc090360 said:
Frosty,

Is this the sort of simple thing you are looking for?

**broken link removed**

There is no mention of bandwidth limits, but that is a matter of careful component selection.

Yes! That is exactly it!
Thank you very much for sharing this circuit!
I will test this against my original idea of using a variable gyrator to control the frequency in colpits oscilator. Thanks again for sharing!

Frosty_47 · Oct 27, 2009

Sceadwian said:
Although I'm not a particularly good analog person I don't think something with such a low component count could possible be very linear through that wide a range. There would have to be some kind of variable capacitance to keep the distortion sane from low to high frequencies, you're talking either a varactor or ganged knobs.

Or a variable Gyrator...

Frosty_47 · Oct 27, 2009

Dean Huster said:
I guess in my book, if one wants a sine-wave (only) generator, he wants a good sine wave, i.e., low distortion. For that, I'd avoid any function generator chips and try working with a Wein bridge circuit. Distortions as low as 0.05% are easy with that. Better than a pot for frequency adjustment, use a FET-input op amp and a dual 365pF variable capacitor with the gangs in parallel. The resistors in the circuit will have to be higher, but the variable cap gives the advantage of having no noise or steps in the adjustment -- truly infinite adjustment.

Dean

Thank you for sharing my enthusiasm towards analog design. I hate IC's for their distortion and often lack of availability such as the case with MAX038.

Frosty_47 · Oct 27, 2009

MrAl said:
(With reference to the MAX038 sine synthesis chip)

Hi there Frosty,

Oh gee, ha ha, i read your post too fast i guess and didnt see that
you were not interested in that chip. I guess i could ask then:
Why are you not interested in that chip or chips like it?

Hope i didnt heat you up too much there Mr Frosty, wouldnt want
you to melt all over the sidewalk :smile:

Good luck in your search.

Because I don't whant to rely on some IC to do the job for me. Any idiot can learn to use an IC, there are few that can suplement it with analog counterpart. Further, IC's such as MAX038 are very hard to obtain and are usualy expensive. Also, discrete transistor design has its benifits such as higher gain-bandwidth product than in most IC's. This means higher frequencies can be achieved with transistor design than IC based design.

-I apologies for releasing the steam in my previous post.

Mr RB · Oct 27, 2009

BrownOut said:
The sine wave generator comprises Q5 and Q6. It amounts to a emitter coupled pair that is driven with a very linear triangle wave centered at 0V. The current of Q5 is given by:

IE(Q5)= I/(1 + e^VB(Q5)). Given sufficient base voltage, the equation approximates a sine wave to a very close tolerance. My circuit produces a slight deveation from the equation because I bias the transistors with a resistor rather than a current source (I). I've attached a graph that shows a sine wave for the interval -pi/2 to pi/2 and a plot of the given equation appropriately scaled so that the two plots superimpose. The circuit is operated back and forth between -pi/2 and pi/2 to produce the sine wave.

The circuit is still going through some refinement. I need to test the stability of the sine wave generator over the expected operating temperature range.

Q1, Q3 and C1 comprise the triangle wave generator, while Q2 and Q4 buffers and inverts it. Q7 and Q8 shift the level so that the triangle wave is centered at 0V. R30 makes a fine level adjustment. R22 and R19 sets the level of the triangle wave to produce the best sinewave from the last stage. R2/R5 are set to compensate for the 555's asymmetrical output, crude compensation, but it works. A180 degree sine may be taken from Q6.

Very nice! So if someone had access to (say) a PIC with xtal locked exact freq generation, they could improve the operation by generating a squarewave of exacly 50/50 duty cycle, and then using a current source to bias Q5 and Q6?

BrownOut · Oct 28, 2009

Mr RB said:
Very nice! So if someone had access to (say) a PIC with xtal locked exact freq generation, they could improve the operation by generating a squarewave of exacly 50/50 duty cycle, and then using a current source to bias Q5 and Q6?

Possibly, but 50/50 duty cycle can be generated from this circuit. It isn't so much that the timer doesn't generate a 50/50 cycle, rather the timer's drivers are not symmetrical. Therefore, it would totally depend on the PIC's output drivers wether that would be an improvement or not. I'd like to investigate using a symmetrical clamp on the timer's output to mitigate this issue.

Another improvment would be to use a match transistor pair for the Q5/Q6 sine wave generator.

Carltons_eng · Apr 24, 2012

Ne 555 base oscillator

BrownOut said:
Here is the one I built. It produces a square, triangle, and low-distortion sine wave. It will run from a few hertz to nearly 1MHZ. If you replace the 555 with a fast comparator circuit, you might make 2MHZ.

Please help us to make this..........

audioguru · Apr 25, 2012

Carltons_eng said:
Please help us (school kids?) to make this..........

The complete schematic is there including resistor values.
Use European BC547 transistors or American 2N3904 transistors which are nearly the same.

Then you are simply copying and soldering the circuit instead of designing it.
Is your teacher reading this?

Dean Huster · Apr 25, 2012

Frosty,
Is this the sort of simple thing you are looking for?
**broken link removed**
There is no mention of bandwidth limits, but that is a matter of careful component selection.

I can't seem to access the site indicated --- membership required or something? What's the deal?

unclejed613 · Apr 26, 2012

i kept getting a zero length file. did a search for AB112 on analog.com, first link was to the zero length file, second link was this one... https://www.electro-tech-online.com/custompdfs/2012/04/AN-112.pdf which is, once you open it, actually AB-112.

Carltons_eng · Apr 28, 2012

Thank you so much......
We have to make a Signal Generator in our Course
As there are many function generator IC's (max038, icl8038, dds ic)..... max 038 is the maximum frequency generating IC (nearly 20MHz).... So we thought to order that but unfortunately it doesnt work...... We couldnt find the fault..... the IC draws huge current overloading the supplies and voltage is droped highly.... NO any output.... As the submission date is closer we thought to make this 555 based circuit as soon as possible.......... Please be kind enough to help us...... Thank you

audioguru · Apr 28, 2012

The ICL8038 is obsolete and the MAX038 is also not made anymore. Some people think the XR2206 function generator IC is still available.

RCinFLA · Apr 28, 2012

You can beat two high frequency oscillators together in a down mixer and low pass filter the output.

Carltons_eng · Apr 28, 2012

Is it possible to make the 555 based 1 ?? i have already designed the circuit in ISIS and it works there...... now we are going to make the hardware......
npn transistors - 2n3904 pnp 2n3906 is it okay???

Ratchit · Apr 28, 2012

Carltons_eng,

Here is a amplified version of what was revealed in post #31 of this thread. The design equations are given. You will have to find opamps that can operate at the frequency you desire. Notice the frequency is controlled by one resistor. The catch is that two opamps are needed.

Ratch

unclejed613 · Apr 30, 2012

RCinFLA said:
You can beat two high frequency oscillators together in a down mixer and low pass filter the output.

i forget the model number, but there was an HP audio generator that used this method to get a VERY low distortion sine wave.

audioguru · Aug 18, 2012

Ratchit said:
Here is a amplified version of what was revealed in post #31 of this thread.

It is TOO SIMPLE because it has nothing to control the output amplitude.
The output will fade to nothing or it will keep increasing until it is a saturated square-wave.

Many years ago HP used a light bulb to control the amplitude because its resistance changes with signal voltage. Newer oscillators use a Jfet to control the output amplitude.

Ratchit · Aug 18, 2012

audioguru,

It is TOO SIMPLE because it has nothing to control the output amplitude.
The output will fade to nothing or it will keep increasing until it is a saturated square-wave.

How can you say that? R6 in series with two paralled diodes provides the nonlinear resistance needed to control the output.

Many years ago HP used a light bulb to control the amplitude because its resistance changes with signal voltage. Newer oscillators use a Jfet to control the output amplitude.

Yes, those are both tried and true ways to control the amplification. So is using diodes.

Ratch

The Electrician · Aug 18, 2012

A Bell labs researcher published a paper in 1951 showing how to build an RC oscillator covering much more than just one decade with a single turn of a potentiometer:

https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4050668

Later, around 1957, HP produced an oscillator using this principle; see the explanation:

https://www.electro-tech-online.com/custompdfs/2012/08/1957-01.pdf

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