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

[Karl Ekdahl]

Eric,

see my earlier e-mail about the PIC USB
schems/firmware etc.

Putting the synthesizer and the computer / adding
ferrite ring to the USB cable didn't change the
behaviour.

I added RF-Beads to D+ and D- along with caps to gnd.
Any caps above 1nF made the USB not working anymore,
not too strange but i noticed that whenever i plug in
the USB cable the pitch jumps and starts wobbling.
This became more apparent while using too big caps
since the wobble quit as soon as the computer decided
it couldn't communicate with the device. This leads me
to think that the data being constantly transmitted to
the device (polling data) is causing the interference.
Nevertheless, the RF-Beads (i have two little black
dealies without any markings that i'm fairly sure are
beads... hehe) and caps didn't do anything. I am
currently running the device from the synth-psu so
that shouldn't be a problem. Should i put a bead on
the GND coming from the computer perhaps? As i
understand the USB ground and the PIC gnd (my internal
gnd) *has* to be connected for the USB to work at all.
Is there an alternative way of connecting the grounds
if that seems to be the problem??

thanks

Karl

--- Eric Brombaugh <ebrombaugh at earthlink.net> skrev:

> 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
> >
> 
>