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

[Karl Ekdahl]

Well, i've pinpointed the problem down to being the
+5v rail. The grounds are in star connection, this is
the only board running from the +5v. I placed the
module outside the synthesizer via a breadboard and
pow! The USB interface didn't work *at all*. I tried
putting a 1uF on the breadboard and it worked fine,
even the wobbling was gone. So i placed a 1uF on the
PIC board by the power connection, thinking it'd work.
When the board was back in place i powered up and
there it was again. So i placed another 1uF at the
connection on the PSU, same story, worked fine outside
the synthesizer but not in it. When the +5v supply
isn't connected to the PIC board it gives about 5.7v,
with the pic board it gives about 5.2v which is about
what i expect from a 6.2v zener and two diode drops.
My suspicion is that maybe that's *not* what's
supposed to happend, so i replaced the 2.7K resistor
before the zener with 470ohm, no luck. After a while i
put a 1uF *in* the power connector on the pic board,
just crammed it in. Now it works...! But i don't like
it.

Unfortunley i don't have a HF oscilloscope (this is
where my soundcard-scope is biting me in the ass) so i
can't see any >44khz interference...

Karl


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

> Karl,
> 
> Well, too bad those suggestions didn't work - my
> motto is to always 
> start with the easy stuff...
> 
> The fact that touching the line cord changes things
> really bothers me - 
> that still makes me think EMI/EMC which means you
> might need some good 
> test equipment to track this down.
> 
> Adding RF beads to the USB data lines isn't what I
> meant (sorry) - I 
> meant that you should filter the power supply lines
> into your USB-CV 
> module - remember to put caps on both sides of the
> bead - that way you 
> protect the module from the PS and the PS from the
> module.
> 
> Yes - the grounds between the PC and synth need to
> be hooked up to 
> eliminate common-mode junk on the USB lines. Putting
> filtering/beads on 
> the ground might help, but the impedance could mess
> things up too.
> 
> Are you using single-point (star) grounding in your
> synth supply 
> scheme? If not then ground loops within your synth
> can cause the 
> modules to talk to eachother.
> 
> I'm still wondering about ground loops, etc. If you
> hook your 
> multimeter between the synth and PC USB ground pins,
> do you see much 
> voltage difference? Hook a resistor between the two
> grounds - how much 
> does the voltage change? Use ohms law to figure out
> the ground loop 
> current.
> 
> Hang the multimeter on the synth power supply and
> plug in the USB - do 
> you see any change in the rails?
> 
> If the DC view doesn't help, the next trick is to
> look at it with a 
> good wideband 'scope. Check for HF noise on the
> supplies - does it 
> change when you plug in the USB? If you don't have a
> 'scope then try 
> changing the filtering on the supplies - does that
> change the amount of 
> junk on the VCOs?
> 
> That's all I can think of for now... Good luck &
> thanks for sending the 
> link to your site.
> 
> Eric
> 
> On Feb 12, 2006, at 10:31 PM, Karl Ekdahl wrote:
> 
> > 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
> 
=== message truncated ===