[sdiy] WNoise Gen Isolation

[harrybissell]

Sounds like good advice, Tim...

I'd suggest much the same.  The actual front end of the noise source
should be isolated with an RC network.  At this point, the impedance is
quite high so the resistor can be larger, and the cap smaller.

If you use a single opamp stage, and run the gain high enough to get
into
clipping you are asking for trouble.   I use two stages, and take a
modest
gain in each. This gives better bandwidth as well.  Maybe a gain of 10x
followed
by a gain of 10x.  AC couple those two stages so that you don't amplify
the
offset of the previous stage.

You can put RC filters on the opamps... but if you don't clip them they
should
not generate much noise on the rails.

The noise source should be kept away from sensitive inputs, and it might
even be a good idea to use shielded cable from the noise output to the
output jack. If that is a bananna jack, terminate the shield at the PCB
end only.

H^) harry

Tim Daugard wrote:

> From: "Michael Ruberto" <frankentron at hotmail.com>
> > On my old modular I used the circuit from the Minimoog but
> noise bleed was
> > very bad...
>
> The first noise module I built had the same problem - noise
> bleeding into everything.
>
> > I'm worried about the noise bleeding onto the power and ground
> rails. What
> > is an effective way to isolate this noise circuit from the rest
> of the
> > system?
>
> I leave the ground rails alone. I use a (semi-) star ground
> system and figure that the ground is robust enough to handle the
> noise. But then as I tried discussing earlier, I use current
> paths instead of voltage paths for signals.
>
> For the positive rail (or both if you use a three wire supply), I
> put a 100 ohm resistor in the module power supply path. This
> increases the impediance of any signals trying to bleed through.
> There are capacitors to ground at both ends of the 100 ohm
> resistor. I use my normal bypass cap at the power supply end of
> the resistor. At the circuit end of this resistor is lots of
> capacitance. I use a large electrolytic as a second power supply
> filter cap (and theorectily the actual voltage supply for the
> module). I also use a medium and small ceramic for further noise
> isolation (10 - 100 nF for the medium, 50 pF for the small). This
> reduces the noise signficantly.
>
> The next noise module I build will also have a mucher lower
> buffer gain. I've found that with other signals in the 10V ptp
> range, 1V of noise is enough. I though about adding inductors in
> to the power supply line, but have't tried that yet - inductors
> are to hard to find to use this way.
>
> Remember to use the right size resistor. The first resistor I
> used was 1/8 watt. It got hot! I bought some 100 ohm 1W resistors
> to use in these types of power supply filters.
>
> The same filter design is used in my low noise - instrument
> preamp module. The other thing to remember is that the modules
> with these filters will be charged and operating long after the
> other modules are powered down. These filters also require time
> to charge. I give the noise module a 3 to 4 minute warm up before
> making any adjustments.
>
> Tim Daugard
> AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
> http://home.sprintmail.com/~daugard/synth.htm
>
> not spell checked or proff read. Don't bother telling me wruds
> are mizspeled.