What is the best way to get a sine wave from a square wave?

Question

I have square waves of different frequencies (1KHz to 20KHz), and I need to convert them to a sine wave of the corresponding fundamental frequency.

A RC ladder Low-pass filter was the first thing which I tried, It gave good results but the output peak-to-peak voltage (Vpp) varied a lot with the frequency. The square wave has a Vpp of 3.6V. But after filtering, the output Vpp of the sine wave varies from 3V to 2V as the frequency increases.

Is there any other better way to get a pure sine wave from square wave of same frequency without this voltage drop?

Thank you

Answer 1

Is there any other better way to get a pure sine wave from square wave without this voltage drop?

Take your square wave and use a phase lock loop to generate a frequency that is maybe 50 times higher: -

Then use a clock tunable filter like this: -

Feed your square wave at the input (Vin) and you should get a pretty decent looking sinewave at the output.

It works by tracking the input frequency using the PLL - typically at an input frequency of 20 kHz the PLL output is 1 MHz and this is used by the LTC1066 to set its cut-off frequency to 20 kHz. Here's what LT say the frequency response looks like at the extremes of operation: -

Given that a square wave is composed of odd harmonics you need a steep filter that gives many dB attenuation at the most dominant harmonic (3rd). Look at the graph for 800 Hz low pass operation. At 800 Hz the response is approximately 0 dB and at 2.4 kHz (3rd harmonic) the attenuation id greater than 80 dB (10,000:1).

Answer 2

One simple way to approximate a sine : integrate to a triangle wave, and "soft clip" that. You'll find lots of circuits for "triangle to sine wave conversion" via the obvious technique.

Result isn't perfect but distortion can be under 1%.

Here are a few including this elegant JFET circuit.

One difficulty with this approach, if you are varying the input frequency, is that the integrator's gain varies inversely with frequency, while all of these triangle-sine converters require a constant input amplitude.

One possible solution is a variable gain amplifier after the integrator, or possibly even variable gain built into the integrator itself (e.g. using a VCA such as the LM13700) varying the gain such that the output amplitude is constant.

Answer 3

Consider a sine wave VCO and a phase-locked loop. You can use a CD4046 for the phase detector. Here, conceptually anyway, is a voltage-controlled oscillator that may be suitable with minor modifications:

Answer 4

You can't do this with a linear time-invariant filter. Such a filter doesn't know the difference between a harmonic and a fundamental, it just sees a frequency component at a given frequency.

To determine an acceptable soloution to this proeblem requires answers to two key questions.

• How pure does the output waveform need to be?
• What needs to happen when the input frequency changes? How much transisition period is acceptable? What outputs are acceptable during the trasnsition period?

One approach not yet mentioned would be to use a filter followed by an automatic gain control circuit. So as the voltage drops the AGC boosts it back up. For greater spectral purity (but at the cost of slower response times) a second filter and AGC could be added.

I would expect all analog approaches to have settling times of at least several and likely many cycles after an input frequency change. If that is not acceptable I would look to a digital approach where a FPGA (with a fast master clock) measures the input cycle period and uses those measurements to generate a waveform that is fed to a DAC.

Answer 5

Not sure of the circuit you are using, but is sounds like a joule thief to me. Those things change frequency with load in about the ranges you are talking about. To build something cheap that works, go get a 100 ohm (?) watt adjustable choke. connect one end of the choke to one side of the output and the other end to your load. Now wrap the other output wire in the same direction as the choke (x) times. This will create a simple, manually adjustable filter. If you are going to use this with a particular load, just tune it until it works the best with a light bulb of equal wattage. Tuning could require more wraps around the choke. I would start with about 10. If you have a variable capacitor, you can delete the wrapping of the choke. You could even make your own choke with small enameled iron wire and wrap it around a bolt in a few layers. Then just add a wire wound type resistor in series to bring total resistance to around 100 ohms. Again, this is certainly not a perfect solution, but would give fair results, it is inexpensive, and adjustable.

Answer 6

I would say that both PLL and triangle wave converstion are maybe just a bit of overkill. Try using active low pass RC filters (that is OP AMP with some C in feedback) to cut off the first harmonics (one for 2kHz and one for 40 kHz). They should ensure that your original frequencies are not attenuated (Gain=1).

Answer 7

I need to convert them to a sine wave of the corresponding fundamental frequency.

Sine wave of some frequency have zero constant "harmonics", it's only a sine(t), a ± wave.

You may get (visually) a pretty good sine by bandpass filtering your square wave (two filters sequently — lowpass and highpass probably with not very low Q). Actually, low border of bandpass (highpass) only needed to filter some constant amplitude.

After filtering, you may need to add some amplitude to your sine wave if you need it.

Also, you need a filter with 4th and more order. After all, i think it's worth to search some other methods.