filter
so i basically have a low pass filtering that is filtering a square wave and giving me in output :
As far as i know the square wave can be made with the sum of multiple sine waves
So the over/undershoot i have is due to the frequencies that are not being filtered.
The triangle wave i have is this one, i want to know if the same logic can be used to understand the output of the filter
Here is a circuit which only lets through pulses with a repetition frequency of less than 1KHz.
When triggered by the leading edge of each input pulse, Monostable IC1 generates a 1ms pulse which clocks D F/F IC3's output high. IC3 is reset when the input pulse goes low again, so the output follows the input. If the frequency is higher than 1KHz then IC1 is continuously triggered and doesn't produce any clock pulses, so IC3 stays in reset.
R2, C2 and IC2B provide a short delay to ensure that IC3 is out of reset when it receives the clock pulse from IC1.
BTW this circuit can be simplified down to just one IC by configuring the other half of the CD4528 as a basic flip-flop. Connect pins 15 and 14 to GND, pin 11 to Vdd, apply clock input (from pin 6) to pin 12, and connect the input frequency direct to pin 4 (+TR) and pin 13 (/RESET). Output is on pin 10.
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).
Best Answer
No, you're not seeing the Gibbs phenomenon on your triangle wave (although it would happen, if it were slower and had a period that's an integer fraction of your sampling rate).
What it appears that you've done is to generate a sine wave that is quite fast with respect to your sampling rate. That is why it doesn't appear to be a perfect triangle wave. Moreover, it is not a low-order integer ratio of your sampling rate (i.e., it's not exactly 2/7, but it appears to be close). Because the ratio isn't exact, you're generating aliases of the higher-order harmonics -- this is why your "triangle" wave appears to be riding on a sine wave.
Your low-pass filter, acting in sampled-time, is filtering out the rapidly-moving "triangle" part, and passing the low-frequency sinusoidal, which is genuinely there as a consequence of the aliasing.