Hello PSN guys;

Just a small note about ferrite beads.
Back in the late seventies i worked with
A company named Omni Spectra Tuning Oscillators
And learned that the ferrite beads MUST be designed
to fit snugly onto any wire it is meant to shield.
This means it is made for a certain exact guage of wire.

These we called super beads and i think they
are more expensive and harder to aquire.

As for the noise of which you speak.

Assymetrical results about the baseline.

I have had this problem too and it seems to relate
to an unbalanced AC signal possibly power line
close by affecting the circuitry at the front end.
It is difficult to remove but proper grounding
shielding balancing between the differential lines
to ground within the front end seems to deal with this
kind of problem.

As for the 3 deg C ??
Hey Man where you all been.

Im talking about the Universal background temperature
as it relates to the big bang. It is actually
the mysterious accelerator force causing
an acceleration of universal expansion.
It represents a pressure spread out
over the entire universe, a very very big place.
Between it and entropy and a lack
of damping the entire universe makes some
kind of sense to me.
How do you seek equilibrium without any
friction or other resistances to help you. It sort of explains
everything. There really must be a great
nothing nothing into which we are expanding.
No friction no nothing to stop us.

You know... like Stephen Hawkings black hole,
When God rested he made a mistake.
He sat on a Black Hole, Now where is that God guy
when you need him ???
Too bad it was not the Devil who sat on the Black hole.

Best Regards,
geoff :-)


----- Original Message ----- 
From: "karlc" 
To: 
Sent: Wednesday, May 19, 2010 9:21 AM
Subject: Re: More on PSDs


> Hi Chuck,
> 
> Here are some thoughts.
> 
> It sounds like your noise may be due to grounding or power supply 
> issues. The comparator is going to take short current transients from 
> the power supply each time it switches. Unless these transients are 
> supplied from sources not shared with the analog circuitry there is 
> likely to be noise getting through. By not shared, I don't necessarily 
> mean different power supplies, but enough isolation in various part os 
> the shared power supplies that noise in one section doesn't get into 
> another.
> 
> Some op-amps have very poor power supply rejection at high frequencies. 
> Some actually have gain from one of the power supplies to the output at 
> some frequencies!
> 
> A traditional way to help this is with bypass capacitors from the power 
> supplies to ground at the parts consuming the transient currents. The 
> electrical path from each capacitor to the switching part it is 
> connected to should be short (5-10mm is good), and the capacitors should 
> preferably be multilayer ceramic of 0.1uF or more. There should also be 
> some solid tantalum capacitors in the 20-50uF range from the power 
> supplies to ground. Aluminum electrolytic capacitors, unless of a low 
> ESR type, are probably not much use in reducing high-frequency noise.
> 
> One source is when these switching current spikes travel along 
> conductors which are also used as analog grounds. The current spikes 
> cause voltage drop across the conductors which shows up as a signal. 
> This is more of a circuit layout problem than a schematic problem, but 
> the overall goal is not to have any transient currents travel through 
> conductors where a voltage drop along that path will affect the signal 
> output. This isn't always easy or even possible, but is a good place to 
> start. Sometimes isolating things with ferrite beads, as you mentioned, 
> can provide enough impedance to reduce the high-frequency currents. 
> Sometimes decoupling resistors (up to a few ohms) in series with power 
> supplies in strategic places can provide an impedance for bypass 
> capacitors to "break against" to improve filtering. These are often 
> installed as a PI section, with a bypass capacitor from each side of the 
> resistor to ground. Putting everything on a ground plane is a 
> brute-force approach that usually improves things.
> 
> Karl
> 
> 
> On 05/18/2010 03:11 PM, Chuck / Judy Burch wrote:
>> 
>> Thanks Matt, Karl and Chris for your responses.
>> 
>> I am currently using a modified version of the PSD described on page 4 
>> of Linear Tech's Application Note #3.  I use an amplitude stabilized 
>> Wien bridge oscillator (5000 Hz) for excitation.  The reference signal 
>> goes to an LF1011 comparator that drives an LT1034.  The amplified 
>> signal goes to the - input of  an LT1007;  the LT1043 switches the + 
>> input between ground and the signal so that the LT1007 acts as a 
>> synchronous detector.  (This circuit is also described in US patent 
>> #3940693.)  This is followed by a 2 pole LPF.
>> 
>> This arrangement works fine.  The LPF eliminates the excitation 
>> artifacts.  But the switching pulses still come through.
>> 
>> Shielding does not help the pulse problem, so I conclude that what I'm 
>> seeing on the downstream part of my boards is magnetic or EM propagated 
>> pickup.  Steel enclosures and/or ferrite beads might help, but I haven't 
>> tried either.
>> 
>> On the argument that eliminating a noise source is better than trying to 
>> filter or shield it, I wondered if other PSD designs might be 
>> intrinsically quieter.
>> 
>> I will try slowing the rise-time of the clock signal going to the LT1043 
>> and I have a Maxim DG419 switch on order to try as well.
>> 
>> I'll report in the event I have any success.
>> 
>> 
>> Chuck
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