PWM phasers - has anyone scoped the output? (calling PWM phaser experts!!)

[StephenGiles]

You might want to look at the Eventide Instant Phaser circuits which did this 30 years ago  - was it? Where are they?

Mine are somewhere, and they might be buried in here
http://neo.eventide.com/marketing/ads/anthology%20ii%20cd/Documentation/Original%20Clockworks%20Manuals/

Mmmmm need a password to get into the schemo area

[StephenGiles]

A long discussion here

http://www.diystompboxes.com/smfforum/index.php?topic=76747.0

[Gurner]

Has anyone dabbled with switched capacitors in an all pass situation?

Apparently these things act as a pseudo resistor, with the resistance being related to the switching frequency &/or the actualt capacitor value.

I'm wondering here if altering the duty cycle is deemed 'altering the switching frequency'.....becuase obviously with pwm, the frequency is constant, but the duty cycle changes, but changing the duty cycle is techinically altering the switching frequency.... so PWM does indeed alter the switching frequency....

[StephenGiles]

There was a switched capacitor phaser unit published in Practical Electronics back in the April 1983 issue. If I lurch into our attic during the weekend, I'll see if I still have the issue containing that article.

I think it is on the net somewhere.

[puretube]

...
I'm wondering here if altering the duty cycle is deemed 'altering the switching frequency'.....becuase obviously with pwm, the frequency is constant, but the duty cycle changes, but changing the duty cycle is techinically altering the switching frequency.... so PWM does indeed alter the switching frequency....

If I understand your reasoning of: "changing the dutycycle = altering the switching frequency" right,
I`d like to take into consideration, that (starting from a "50/50" situation,)
increasing the "on-phase" means decreasing the "off-phase" for the same amount,
so that the "frequency/frequencies" seen from outside (your viewpoint?) seems to stay the same...

BTW.: the link that Stephen provided, contains some more (interesting) links in turn... 

[GFR]

I think switched capacitors could be more used in fx designs. Besides the phaser in PE magazine (it's two notch filters sweeped), I can imagine some other uses like:
- with a MF10 you build a steep 4th order low pass - what about a variable speaker emulator?
- If you use the capacitance from a pedal to a plate to control the switching frequency, you can make a wha like a dod fx-17, but you can make it 4th order (with a single mf10) if you want for synth-like filtering.
- If instead of a pedal you use the capacitance of your body to a plate... something like a wha probe...

[merlinb]

I'm wondering here if altering the duty cycle is deemed 'altering the switching frequency'.....becuase obviously with pwm, the frequency is constant, but the duty cycle changes, but changing the duty cycle is techinically altering the switching frequency.... so PWM does indeed alter the switching frequency....

If I understand your reasoning of: "changing the dutycycle = altering the switching frequency" right,
I`d like to take into consideration, that (starting from a "50/50" situation,)

Varying the frequency can be used in the same way as PWM, but it's called pulse-position modulation, PPM. See the 555 data sheet, for example.

[DrBoogey]

Maybe R.G. knows more 

http://www.geofex.com/Article_Folders/ASMOP/asmopphase.gif

Cheers

[earthtonesaudio]

(Approximately):
R_PWM = R_seies/(1-(1/duty)) , {0 < duty < 1}
R_SwCap = 1/fC


With PWM, the duty cycle acts as a multiplier for the resistance value.  BUT there's a caveat.  The resistance can go up near-infinite (duty = 0%) but can only go down to the resistor value (duty = 100%).  Your duty cycle : resistance graph will look like 1/x.

What I have found is that a smaller resistance value in series with the switch is more practical because it allows you to get the same swoosh from a smaller duty cycle variation.  This also makes the PWM more of a constant and therefore easier to design and tame (with respect to noise).


I've also used switched cap synthesized resistances, and while they can perform well, it's harder to get a good sweep because you need a wide range, high-frequency VCO (which by itself is not a trivial task).

[Hoffmann]

I was thinking of using a CD4046 to double the frequency on the output of the electric druid TAPLFO to clock some 4066.  

Could this do the job?

[Hoffmann]

Bump

[earthtonesaudio]

Doubling a PWM's frequency with a 4046 will turn it into a square wave, so no.

[Hoffmann]

Doubling a PWM's frequency with a 4046 will turn it into a square wave, so no.

I get it now..
If using Electrodruid's tap tempo uC (it's pwm is 19,3kHz) maybe it could be used with sucess if an appropriate LP filter(maybe double) is used at the end of audio signal chain?

[Morocotopo]

Guys, I don´t know s*ht about PWM, but here´s a post in a local forum I´m part of that discusses a PWM tremolo. Might be of some use to you, only thing is you have to read spanish. Or translate.

http://foro.cuartitodiyer.com/viewtopic.php?f=5&t=221&hilit=digitrem

Maybe it´s of some use.