High Frequency Generator

[UrbanCyborg]

Question for the devs in the forum: if the situation has arisen for you, what have you used to get frequencies at or near Nyquist? The average oscillator won't go near that. I'm talking about generating, say, pretty pure sine waves near Nyquist (or above Nyquist, assuming those frequencies will alias). The best I've been able to come up with is putting pure sine waves through ring modulators to get successively higher, but still simple, waveforms. This is for testing purposes.

Reid

[utdgrant]

You could put a single sine wave oscillator into both inputs of a ring modulator. This will generate a unipolar sinusoid at twice the input frequency. If you want to make it bipolar again, you can use ACE Constants and Multipliers to shift the level down by -2.5V and amplify it by x2. Alternatively, stick the signal through a high-pass filter (with low cut-off freq) to remove the DC component.

I'd recommend using the ACE Four Quadrant Multipliers or Simple Amplifier as a ring modulator, as they are completely linear in operation. The stock CA one has quite a high level of non-linear distortion, which will generate extra, unwanted sidebands, even from a pure sine wave input.

[ChR_is]

You can craft your own oscillator using the CA FORMULA module. you need a phasor and a waveshaper.
a phasor is basically a ramp up at the desired frequency and the waveshaper... well shapes the phasor into the respective output wave. if you need a sine wave use the sine function as a waveshaper. here's a basic setup:

Formula phasor

phasorSineFormula.png (1.16 MiB) Viewed 2698 times

the left FORMULA is the phasor. it's tuned to a base frequency of 65.4064hz which is roughly C2. it accepts exponential frequency cv (pitch cv) at the X input and can be tuned/played just like any other oscillator. you need the feedback from the output back into the Y input to increment the phasor. you can then route this phasor output into the sine waveshaper which is the right FORMULA. as you can see this way you can generate any frequency (according to the sample-rate limit of course) of sine wave. you can also generate other waveforms by using other waveshapers. i'm sure you're able to port this code into a test module with VMD as well

for easy copy-paste, here's the formulas:
1) mod(y+(2^x)*65.4064/48000,1)
2) 5*sin(2*PI*x)

PS: with a base tuning of 65.4064Hz you'll need log2(24000/65.4064)V ~ 8.52V CV for the exp FM (X input on the first FORMULA) to get to the nyquist frequency. you'll notice if you go above the frequency of the oscillator will fold back.

have fun experimenting!

[UrbanCyborg]

Thanks, Grant and Chris! Both suggestions are very interesting, and I'll certainly try them both out. I'm guessing that either one will do what I want, from the looks of them. Thanks mucho, mi amigos!

Reid