Very low frequency sine

Thread Starter

Michal Podmanický

Joined May 11, 2019
72
Can you guide me how to generate a 20Hz sine for a reference signal?
I need THD less than about 5%.
I’ve read the phase shift oscillator is suitable for this. Is there some another analog circuitry? With LC oscillator the L is quite big. I prefer a transistors solution instead of op-amps.
 
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Thread Starter

Michal Podmanický

Joined May 11, 2019
72
Thx all, i will look at the Wien bridge, i have never work with it.

I just tried Twin-tee according W2aew on youtube.
It works well, however I don’t understand it and its functionality is a big surprise for me. I have a lot of study to do :)
 

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Ian0

Joined Aug 7, 2020
10,021
The problem with nearly all of these circuits is keeping the amplitude stable.
20Hz is rather low to use a lamp or a thermistor on the AGC as in the classic Wein circuit, because the thermal time constant of the lamp is short compared to the period of the waveform.
5% THD would probably allow a diode-clipping circuit to work, which would have no problems with amplitude stability, and give a predictable output level.
A lot depends on what you want to do with the sinewave.
THD to how many harmonics? The digital techniques give very low THD at the low harmonics, but the waveform is a series of steps that then needs filtering.
 

Audioguru again

Joined Oct 21, 2019
6,758
Few speakers produce audible 20Hz but you can see the cone vibrating. The speaker must be driven by a power amplifier that is designed to produce a frequency as low as 20Hz.
 

Thread Starter

Michal Podmanický

Joined May 11, 2019
72
It’s not an audio app. The reference signal is needed for driving a solenoid.

I measured the Twin-tee and it has the same issue about holding a stable amplitude as lan0 mention.

At some point when THD is close to zero (very nice) the amplitude starts to fall slowly to 0V. With some nonzero THD the amplitude is clipped a little but stable.

So it needs a feedback, regulator or at least some negligible zener clipper to maintain it constant. Luckily I got some ideas how to do that.
 

crutschow

Joined Mar 14, 2008
34,682
I have some stable-amplitude, phase-shift oscillators designs, but they require one op amp IC
Why don't you want to use an op amp?

Below is the Ltspice sim of an example 20Hz phase-shift oscillator using a LM324 or LM358 op amp IC op amp package that is amplitude stabilized by diode clippers, and operates from a single 5V supply:
The harmonics are more than 40dB below the 20Hz fundamental, giving a harmonic distortion of <1%.

If you want the sinewave to be centered around zero volts, you can couple the output through a series DC block capacitor into a resistive load to ground.

1713123963521.png
 
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Thread Starter

Michal Podmanický

Joined May 11, 2019
72
Thx Crutschow, it looks good.
I had similar idea about diode clipper, just with zeners on higher voltage levels. Since the amplitude changes very slowly the amount of energy clipped on diodes is almost neglected so it doesn’t affect the THD much.

For sine around Gnd I think the opamp should be supplied with +- voltage and ref should be GND.
But in that case I can’t use a polarised caps since they would work at AC.

Just one question, you have used an integrated RC in phase shift. What’s the difference/advantage compared to derivative RC?
 
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crutschow

Joined Mar 14, 2008
34,682
I had similar idea about diode clipper, just with zeners on higher voltage levels. Since the amplitude changes very slowly the amount of energy clipped on diodes is almost neglected so it doesn’t affect the THD much.
The amount of energy clipped is independent of the frequency, since at a lower frequency the time the each waveform peak is clipped is proportionally the same.
For sine around Gnd I think the opamp should be supplied with +- voltage and ref should be GND.
Yes, if you have a separate plus and minus supply then you can connect the Ref input on U1a to ground.
Also remove R1, R11, & C5, and change R8 to 499Ω.
But in that case I can’t use a polarized caps since they would work at AC.
Where do you think you need polarized caps?
The only polarized caps are the filter caps.
All the other caps should be non-polarized film (not ceramic) caps for stability.
 
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Thread Starter

Michal Podmanický

Joined May 11, 2019
72
I didn’t explain clearly, I mean amplitude change as amplitude natural decay/rise without diode clipping because of little tank overdrive, not a frequency.
The polarised caps in tank can be needed in case of transistor version where R cannot be high because of relatively low transistor input impedance (compared to op-amp).
 

crutschow

Joined Mar 14, 2008
34,682
you have used an integrated RC in phase shift. What’s the difference/advantage compared to derivative RC?
A integrator RC is a low-pass filter, which rolls off the high frequency harmonics to reduce distortion.
The three RC phase-shift stages form a 3-pole low-pass filter.
Note that the circuit Out is taken from the amplified output of the 3 LP stages.
because of little tank overdrive
There is no tank in a phase-shift oscillator.
A tank consists of an L and C, nor R and C.
 
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LvW

Joined Jun 13, 2013
1,765
Can you guide me how to generate a 20Hz sine for a reference signal?
I need THD less than about 5%.
I’ve read the phase shift oscillator is suitable for this. Is there some another analog circuitry? With LC oscillator the L is quite big. I prefer a transistors solution instead of op-amps.
Michael, there are a large number of different oscillator structures - and it is certainly a challenge to select a suitable topology for a specific application (frequency, amplitude, THD, complexity).
With this article I would like to point out that there are also oscillator circuits with two or three operational amplifiers based on integrators, which (for THD<0.1%) can work completely without separate amplitude control (diodes etc.).
The same also applies to the GIC oscillator (GIC_Resonator, Generalized Impedance Converter), see figure below.
This oscillator can be tuned with one grounded resistor only (R6).

GIC_resonator.jpg
 
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Audioguru again

Joined Oct 21, 2019
6,758
A CD4018 makes a good digital sinewave generator that I built years ago. Its clock also clocked the "switched capacitor" CMOS lowpass filter IC and the output had extremely low distortion.
 

Ian0

Joined Aug 7, 2020
10,021
The circuit using a CD4017 certainly does not come close to a low distortion sine wave.
If you read Don Lancaster's original paper it tells you that the circuit eliminates the first n harmonics, where n is the number of stages. The first harmonics is the clock freequency. Don Lancaster adds a Sallen & Key filter to remove the higher harmonics, which is great for a fixed frequency sinewave but not so good if you want it variable.
I think @Audioguru again uses the Lancaster circuit followed by a switched-capacitor filter so that the filter tracks the frequency being generated.
This is a good general paper on the topic of sinewave generation.
If the load is a solenoid or something else very inductive, then the steps won't matter.The inductor will be its own filter.
 

Pyrex

Joined Feb 16, 2022
291
I have some stable-amplitude, phase-shift oscillators designs, but they require one op amp IC
Why don't you want to use an op amp?

Below is the Ltspice sim of an example 20Hz phase-shift oscillator using a LM324 or LM358 op amp IC op amp package that is amplitude stabilized by diode clippers, and operates from a single 5V supply:
The harmonics are more than 40dB below the 20Hz fundamental, giving a harmonic distortion of <1%.

If you want the sinewave to be centered around zero volts, you can couple the output through a series DC block capacitor into a resistive load to ground.

View attachment 319954
Is the circuit sensitive to "C" and "R" tolerancies? Can I use a pot in series with one "R" to set the needed frequency, say +-10%? What for "Ref" is needed?
 
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