[sdiy] ARGHHH - PIC detunes VCOs depending on RFI *only* if USB cable plugged in

[Eric Brombaugh]

Hmmm...

A few thoughts:

1) Wow! You've got a circuit based on a USB PIC! I've got an Olimex 
board with an 18F2250 that I'd like to play with, but there doesn't 
seem to be much in the way of documentation on how to drive the USB 
hardware. Care to share your code?

2) Your problem sounds like an EMC/EMI problem. Any time you get 
problems from touching the (insulated!) power cord, that sounds like 
some sort of radiated interference or ground loops between the synth 
and PC through the USB cables.

Some diagnostics:
a) Have you tried plugging both PC and synth into the same outlet?
b) try some big ferrite clamps on the USB cable to attenuate RF on the 
USB cable
c) Ferrite beads and small (high-freq) caps on the power into your 
USB->CV circuit to filter out possible RF leaking in from your PC.

That's the first thing I'd try.

Eric


On Feb 12, 2006, at 7:29 PM, Karl Ekdahl wrote:

> Hrrfggg... yes, so, i have a USB->CV based around a
> PIC18F2455. My VCOs detune when i lift or come close
> to the main powercable, but this *only* happends if
> the USB cord is plugged in to the PIC board. This
> happends wheter the actual v/oct from the pic board is
> connected to the VCOs or not, so it seems to be going
> through the powerlines... The firmware in the PIC is
> allways running, usb or not, hence any possible
> interference from the x-tal can be ruled out (i
> guess). The PIC was first running on USB-power, but i
> tried connecting it to the internal +5v rail instead
> and that seemed to fix the problem - until i actually
> soldered the cables in place. There might be something
> wrong with the internal 5v supply (since i "designed"
> it myself), it looks like this:
>
>                 43ohm 2W
> +9v o---------o---xxx-----oC
>         2.7K              /
> +9v o---xxx---o---->|---(  ) <- NPN 2N2219A
>               |           \__ 5.4v Out
>  6.2v Zener-> A            E
>               |
>              ---
>               -
>
> My thought was that 9/2700=3mA, down to 6.2, down
> about two voltage drops again through the diode and
> the NPN. And then given worst case hFE i can get
> 75*3=225mA though the resistor only allows 9/43=209mA
> which should be plenty for the pic to do its job.
> Right? Did i miss anything?
>
> I don't have a clue wheter it's the powersupply or
> not, but *something* weird is going on...
>
> thanks
>
> Karl
>