FFT analysis : how to?

[Max Fazio]

Hi all , first post here
I'd like to make a fft and harmonics analysis on a synthesizer's sine wave
vco, given that the wave output is not pure and features some extra
harmonics: I want to detect them correctly and straight on the output of the
vco because I don't want to get artifacts: is there a way to sample a
waveform straightly from the circuit and make a FFT of it?
Thx for any reply
Max

 

[Lostgallifreyan]

Hi all , first post here
I'd like to make a fft and harmonics analysis on a synthesizer's sine
wave vco, given that the wave output is not pure and features some
extra harmonics: I want to detect them correctly and straight on the
output of the vco because I don't want to get artifacts: is there a
way to sample a waveform straightly from the circuit and make a FFT of
it? Thx for any reply
Max

Sound Forge.

That will let you look at the time domain too, which might be just as
revealing.

Tapping the VCO will be easy enough, but you'll need to identify the IC and
the pin it puts out the signal on, and then match the level to get the best
use of the 16 bit recording in Sound Forge. If you have a 24 bit 96 KHz
input, use it if you want to test frequencies above about 5 KHz.

 

[Max Fazio]

OK, I got SF
But the problem is: if I have to sample the wave from the pin of the VCO do
I have to use probes like for Oscilloscopes?
What I know is that I should grab the waveform straightly from a Yamaha
iG00150 which is actually a LFO whose highest frequency is 222Hz: the sine
is actually a waveshaped triangle, therefore there are some harmonics going
on the waveform
What is the tool I can use to grab the waveform? a microphone?a probe??
Let me know!
M

 

[Lostgallifreyan]

OK, I got SF
But the problem is: if I have to sample the wave from the pin of the
VCO do I have to use probes like for Oscilloscopes?
What I know is that I should grab the waveform straightly from a
Yamaha iG00150 which is actually a LFO whose highest frequency is
222Hz: the sine is actually a waveshaped triangle, therefore there are
some harmonics going on the waveform
What is the tool I can use to grab the waveform? a microphone?a
probe??
Let me know!
M

Yes, use a probe the same way as for a scope. One advantage to Sound Forge
is you just start it recording, then you don't have to watch anything but
where you put the probe tips. Connect the probes to the tip and sleeve
connections on the jack to a sound card input. take care with the VCO
output though, check with a voltmeter or scope first. Ideally, set to sine
wave at 50 or 60 Hz so you can measure the AC output voltage. Some VCO's
will put out 15 volts or so, and you'll need to attenuate that witha
variable resistor to match it to the sound card input. Just start low,
crank it up while watching the record level meter, till just short of
clipping.