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Is it good way to use LC circuit to convert Square Wave to sine wave

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moonnightingale

moonnightingale

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Hi
I am having different square waves and i am using LC (Inductor Capacitor) circuit to convert square to Sinewave
I am getting very good sine wave\

I just want to ask is it good practice to use LC circuit for this purpose

If such a cheap solution is available then why people make complex circuits for square to sinewave??

Can u suggest some site which can explain me working of LC circuit for making sine wave
 

It's a fine way to make a sine wave in certain circumstances. But the LC circuit can only be tuned to one frequency, so it won't work when you want a variable frequency sine wave. Also, if you want that sine wave to drive a load then a LC circuit will have trouble because its behavior can be impacted greatly by a load.
 
mtwieg said:
It's a fine way to make a sine wave in certain circumstances. But the LC circuit can only be tuned to one frequency, so it won't work when you want a variable frequency sine wave. Also, if you want that sine wave to drive a load then a LC circuit will have trouble because its behavior can be impacted greatly by a load.
Click to expand...

Thanks a lot mtweig
U r correct
Actually i am having 10 frequencies ranging from 250 Hz to 550 Hz and they are selected by push button

so they are all square wave.
I want to convert them into sine wave and then listen the sound on 8 Ohm 1/4 Watt Speaker

kindly tell me the circuit which i can make for square to sinewave for this freq range and then it can produce sound through speaker

thanks a lot

(I know i can also listen square wave on speaker but i have to convert it into sine wave)
 

I think i have a solution for your problem. Check this site: Complex Impedance Analyzer

The whole project had serious flaws IMO, but the method of generating variable frequency sine is interesting. Since you need low frequency sine, you can do this that way.

What they did was to use the MAX7480 ic, which is variable frequency, switched capacitance type, 8th order lowpass filter.
 

Yeah a switched capacitor bandpass filter is one neat possibility. You just have to generate a clock to select the center frequency.
 

We are having only capacitors and resistors in lab for this assignment
our teacher is saying to make sine wave from capacitors and resistors

how can i convert square wave to sine wave by capacitors and resistors
 

I'd say use a large electrolytic capacitor, take out the intestines and hide attiny13+max7480 inside.....

But seriously: does potentiometer count as resistor? That's would be a way to do that. In general if you want variable frequency RC filter then you need to vary either resistance or capacitance. Variable capacitors are out there, but of low value and big size, also usually much more expensive than potentiometers, so it's more convenient to vary resistance.

The general principle of convertine square to sine is that square consists of some fundamental frequency and it's odd harmonics. By using a filter with corner frequency equal to the fundamental frequency of square wave you filter out harmonics. This leaves only the fundamental frequency. And it so happend that frequency without harmonics means a sine wave.
 

By using a filter with corner frequency equal to the fundamental frequency of square wave you filter out harmonics. This leaves only the fundamental frequency. And it so happend that frequency without harmonics means a sine wave.

Yes, that´s one can learn in school.
However, the reality is not so simple - you cannot "filter out harmonics". You only can suppress harmonics down to a certain and acceptable level. And this determines the filter degree that is necessary to do the job. I think this forum is advanced enough not to simplify things to much.
And - the corner frequency should NOT equal the fundamental of the squarewave. It should be larger depending on the filter degree.
 

Kindly suggest me some simple circuit to meet my requirement
 

One possible way to solve the problem

If two stages of integrators are connected in series, the first one converts the square wave
to a triangle wave, and the second stage converts the triangle wave to near a sine wave.
If the input frequency varies widely then the output amplitude will vary.
If amplitude needed to vary less then an AGC stage must be added on the output.
Duty cycle of the square wave needs to be near 50%, or must modified so.
 

Simply integratng triangle wave doesn't produce sine wave, it produces quadratic function wave (parabolic shape). I mean simple math tells you that integrating a linear function doesn't get you a sine or cosine, but rather x^2. This may look similar to sine, but it's not.

There is a method of generating quite good quality sine from triangle wave, but it involves feeding the tirangle into a network consisting of diodes and resistors which creates sine exploiting the nonlinear regions of diode characteristics. This method is used in IC's like XR2206 and ICL8038.
 

poorchava said:
There is a method of generating quite good quality sine from triangle wave, but it involves feeding the tirangle into a network consisting of diodes and resistors which creates sine exploiting the nonlinear regions of diode characteristics. This method is used in IC's like XR2206 and ICL8038.
Click to expand...

Another simple solution for triangle-sinusiodal conversion is the following:
Feed the triangle wave into a classical diff. amplifier (long tailed pair) with an amplitude that uses some part of the non-linearity of the transfer characteristics (tanh-curve causes "soft clipping" of the sinus). This gives rather good and frequency-independent sinus signals:

THD app. 1% for a triangle amplitude around 80 mV.
 

mtwieg said:
Yeah a switched capacitor bandpass filter is one neat possibility. You just have to generate a clock to select the center frequency.
Click to expand...

Can u give me some site where it can show me the connectivity of switched capacitor bandpass filter with my square waveform
 

Here's a website showing many filter circuits. You'll see a couple bandpass type using just capacitors and resistors.

I don't see any switched capacitor type.

Lessons In Electric Circuits -- Volume II (AC) - Chapter 8

Edited to add:

This bandpass filter uses capacitors and resistors in a different configuration from the webpage at faqs.org (above):

'electronics-tutorials.ws'

Band Pass Filter

PDF article on switched capacitor filters. These circuits contain active components.

users.ece.gatech.edu/mleach/ece4435/sp07/dp03sp07.pdf
 
Last edited:

Switched capacitor is not something you usually do with discrete components. This is a way used in integrated circuits to obtain precise resistance. Manufacturing precision resistor in silicon is hard, while in case of capacitor it is not. By letting chatrge in and out of capacitor in controlled manner you gain controlled current flow which simulates resistance and can be very precisely controlled. In discrete circuits you usually don;t do that.
 

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