GPIO produce sine wave

Question

Hi

I need the Pi to generate a pulsing sine wave signal. I managed to generate wave signal from the GPIO pin but only square wave with High-Low waveform. So I am wondering if the GPIO can generate the sine wave signal that I need?

Also I have been using the function GPIO.PWM(pin,frequency) i was using it to generate 20kHz signal but when I check the output on oscilloscope it was only 862 Hz is this the maximum frequency of the PWM function or is there something wrong with my pi.

I read somewhere the using python Rpi.GPIO library I can output up to around 40 kHz, and I was able to actually output 50 kHz from it but don't know why the it wouldn't output 20 kHz using the GPIO.PWM ().

Can anyone tell me why or suggest another method that I could use?

Thank you!

michaelkellett · Answer

The Pi is very limited as a sine wave generator. What frequency of sinewave do you want to generate and what kind of pulsing. How much noise and distortion is acceptable. Depending on the answers it may be possible to generate the signal without extra hardware ot it may not. MK

michaelkellett · Answer

You still haven't defined the signal well enough to make a good suggestion. How long are the pulses, what rise/fall time, must the pulse be synchronous with the carrier (the 20kHz) ? You can't turn a square wave into a sine wave with an integrator - you can with a low pass or bandpass filter but the modulation would need to be applied after the filter otherwise you will get very slow rise and fall times. This is going to be tricky with an RPi without extra hardware. You can buy very cheap little DDS synthesisers on ebay (like New AD9850 DDS Signal Generator Module 0-40MHz Test Equipment | eBay ) - this could generate the 20kHz OK, you could use a second one to generate the modulation signal. One way you could do it with a Pi is to add a DAC to an I2S ouput and genrate suitable signals in software - this will need a kernel driver which you may have to write yourself. If you could expalin what the signal is for and how much you want to spend I could be abit more helpful. MK

Robert Peter Oakes · Answer

what ever method you choose you need to realize that in order to get a decent output waveform with minimal external circuitry you will need to output to a DAC or PWM controller at between 5-10* the frequency you want as output so if your looking to output 20Khz. At a minimum and with assistance with output filters you should follow the Nyquist rate which would specify 2* but that wont give a very clean output.

btw, a simple RC low pass filter will not give a sine wave it will be more like this

this may be good enough but far from Sine

this is more like what you need

details can be found here http://www.wenzel.com/documents/waveform.html

but beware this has a resonant frequency and will not work for a wide range of frequencies without varying the values.