Square to Sine wave..........

[Ashish.chip]

i have an input square wave of 125khz...now i want to remove upper harmonics from this to get a sine wave of frequency 125khz......i used a low pass filter(L-C filter)...values (L=1mh,c=1.62nf)....but in simulation i observed that amplitude of sine wave keep on increasing.......i don't have any idea about RF components....is there any concept of RF modeling........
plzz suggest me any other circuit for this......
thanks a lot!!!!!

 

[aryajur]

The simulator is showing you a sine wave of increasing amplitude because you must be using an ideal inductor, capacitor and a ideal voltage source (i.e. infinite current sourcing capacity). Since 125KHz is your resonant frequency the impedance of the circuit seen by the source is 0 for a frequency of 125KHz and so the current will keep rising and that causes the voltage to keep rising. In the actual case however the source cannot supply infinite current and your sine wave output will stop rising after some level. To make sure the level does not reach very high voltages put a series resistor with the inductor. That will make the voltage level constant after some time.

 

[hr_rezaee]

Hi
use RC filter.
R is your load.
regards

 

[Borber]

Rising of amplitude short after simulation starts is normal. It starts from zero and increases exponentialy to it's maximum over transition time. If you disable initial conditions at transient analysis this won't happen or increase Tmax to look waveform after the end of transition time.

 

[IBO]

Use a resistance to damp oscillation, which are due flywheel effect produced by l and c.

 

[melc]

i have an input square wave of 125khz...now i want to remove upper harmonics from this to get a sine wave of frequency 125khz

125KHz square wave will have all the odd and even harmonics (x2, x3... to x10) in most of the circuit configuration (assuming the square is around 0.5 duty cycle)
If you need only the fundamental, then your filter must be able to cut everything above the 125KHz, and the filter must be better than first order (better than a simple RC).

I suggest you to search for the keyword "active filter". A third order Butterworth filter could gave you a clean sinus with a constant amplitude.