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control this LFO with pwm
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author control this LFO with pwm
Dimitree
I have this weird LFO (that is part of a bigger circuit) that alread has CV control over Depth and Frequency.
It needs 0V to 5V.
I'd like to control that using pwm from a 3.3V microcontroller.
I'm wondering if I can simply use a mosfet or a logic buffer (to shift from 3.3V to 5V) followed by a simple RC low pass filter.
Would it be enough, or do I need an opamp buffer?

jorg
Freq_CV is already being applied to an op amp buffer; you could easily just eliminate the 47k resistor there and send your PWM/RC filter directly to the (+) input of the op amp.

The Depth_CV input is seeing about a 16k load (parallel of the 27k to the Q14 emitter and 30k/10k to the IC24B op amp). On the op amp side, you could easily boost the impedance (e.g. use 300k/100k), but the transistor input is inherently low impedance. You probably still could adapt your RC filter to drive it; would need to use a relatively large capacitor.
Synthiq
As I think jorg meant, if you level shift to 5V and use a small filter resistor (much smaller than the load resistance) and a large filter capacitor, you may not need a buffer amplifier. This also means that you will charge and discharge the filter capacitor with a large current due to the small filter resistor so this is a disadvantage.

Since the LFO is powered by +/-15V, nothing prevents you from level shifting the PWM signal to the existing +15V and then use filter resistors equal to twice the load resistance so the filter/load combo will divide the signal by 3 to get an output range from 0 to 5V. So for the FREQ_CV a 100kohm filter resistor could be used and the DEPTH_CV would use a 32kohm resistor. The filter capacitor values depends on PWM frequency, maximum tolerable ripple and bandwidth of the modulating signals.
Dimitree
thank you!
are there solutions that do not imply modifing the existing circuit, and avoid using +/-15V, but only 3.3V and 5V supply? reason is this LFO is on another board that I'd like not to modify
Synthiq
Dimitree wrote:
thank you!
are there solutions that do not imply modifing the existing circuit, and avoid using +/-15V, but only 3.3V and 5V supply? reason is this LFO is on another board that I'd like not to modify

With those restrictions in mind, I would lowpass filter the 3.3V PWM signals with passive RC filters and then buffer them with non-inverting amplifiers with gains of 1.5 to scale the filtered PWM signals from 3.3 to 5V. The amplifiers would be powered by 5V and ground so they must be rail-to-rail input/output amplifiers like MCP6002 or similar.

If you want more signal bandwidth while maintaining low ripple you can use a second order Sallen-Key active filter with a gain of 1.5 instead.
guest
your original suggestion would be fine. use some level shifting logic to go from 3.3V to 5V, like 74LV14 or CD40109 and follow that with a 1k resistor and 0.1uF capacitor (depending upon the frequency response you want).
Dimitree
great! many thanks!
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